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LTE Air Interface

Course Code: LT3602 Duration: 2 days Technical Level: 3

LTE courses include

LTE/SAE Engineering Overview

LTE Air Interface

LTE Radio Access Network

Cell Planning for LTE Networks

LTE Evolved Packet Core Network

4G Air Interface Awareness

Understanding Next Generation LTE

...delivering knowledge,

maximizing performance...

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LTE AIR INTERFACE

First published 2009

Last updated January 2012

WRAY CASTLE LIMITEDBRIDGE MILLS

STRAMONGATE KENDAL

LA9 4UB UK

Yours to have and to hold but not to copy

The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and

your employer to court and claim heavy legal damages.

Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs andPatents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior

permission in writing of Wray Castle Limited.

All of our paper is sourced from FSC (Forest Stewardship Council) approved suppliers.

© Wray Castle Limited



LTE Air Interface

II © Wray Castle Limited



Section 1 Introduction to LTE

Section 2 UE Bearers and Connectivity

Section 3 OFDM Principles

Section 4 Physical Layer Structure

Section 5 Layer 2 Protocols

Section 6 Radio Resource Control

Section 7 Lower Layer Procedures

LTE AIR INTERFACE

CONTENTS

III© Wray Castle Limited



LTE Air Interface

IV © Wray Castle Limited



SECTION 1

INTRODUCTION TO LTE

LTE Air Interface

I© Wray Castle Limited



LTE Air Interface




LTE Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.1

Broadband Access with LTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.2

Architecture Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.3

LTE Development and Design Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.4

LTE Standards Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.5

LTE Key Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.6

Access Networks and the eNB (E-UTRAN Node B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.7

X2 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.8

The EPC (Evolved Packet Core) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.9

S1 Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.10

Evolved Packet Core ‘S’ Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11

E-UTRA Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.12

CONTENTS

Introduction to LTE




LTE Air Interface




At the end of this section you will be able to:

outline the evolutionary process prescribed for GSM and UMTS networks and show where

LTE/SAE fits in

explain the significance of LTE in the continued progression toward converged

telecommunications and entertainment markets

outline the overall performance aims for LTE

identify the key air interface, radio access and core network technologies chosen for E-

UTRA

outline the basic architecture of the E-UTRAN and EPC (Evolved Packet Core) including the

eNB (evolved Node B), E-UTRAN interfaces and the EPC elements

explain the role of the X2 interface in the E-UTRAN

explain the role of the S1 interface and other possible S interfaces within the EPC

describe the E-UTRA protocol stack and assign layer functions to the correct network

entities

OBJECTIVES

Introduction to LTE

V© Wray Castle Limited



LTE Air Interface

VI © Wray Castle Limited



GSM/GPRS

GSM/EGPRS

50 kbit/s

150 kbit/s

UMTS

UMTS/HSPA

EDGEEvolution

UMTS/HSPA+

1 Mbit/s

10 Mbit/s

40 Mbit/s

400 kbit/s

LTE(4G)

100+ Mbit/s

LTE-Advanced

1+ Gbit/s

LT3602/v3 1.1© Wray Castle Limited

LTE Overview

LTE (Long Term Evolution) represents the next developmental step for the 3GPP (3rd Generation

Partnership Project) standards group. It provides for a continued evolutionary path from 2G GSM/GPRS,

beyond 3G UMTS/HSPA and ultimately towards a 4G solution.

UMTS (Universal Mobile Telecommunications System) has continued to build on the success of GSM

(Global System for Mobile Communications) and momentum is gathering behind its significantlyincreased capability with the introduction of HSPA (High Speed Packet Access). The classic fixed and

mobile telecommunications business models are undergoing enormous change with the move towards

all-IP switching and a total-communications service profile. Meanwhile, the last decade has seen the

Internet develop into a serious business tool and fixed broadband access is fast becoming a basic

commodity.

This market landscape is ready for a technology that combines broadband capabilities with an efficient

scalable switching infrastructure and a flexible service delivery mechanism. LTE provides just such a

solution and is designed to address growing global demand for anywhere, anytime broadband access

while maintaining efficient provision of traditional telecommunications services and maximizing

compatibility and synergies with other communications systems.

Although LTE most obviously represents an evolutionary path for UMTS networks, it has also been

designed to allow cost-effective upgrade paths from other technology starting points. For example, GSM

operators can now access 3G-like performance through EDGE (Enhanced Data rates for Global

Evolution) Evolution, and this in turn can be used as a direct pathway to LTE. Similarly, the interworking

capabilities of the EPC (Evolved Packet Core) make it possible for CDMA (Code Division Multiple

Access) to migrate radio access from 1x or 1xEV-DO (1x Evolution – Data Only) to LTE.

Evolution beyond LTE has been mapped out by 3GPP with the specification of LTE-Advanced, which

offers the possibility of downlink data rates (to stationary or low mobility users) of 1GBit/s or more.

Further Reading: 3GPP TS36.300 (LTE Radio Access), 23.401 (LTE Core Network)

Introduction to LTE



LTE RadioAccess

Former ‘Mobile’Operator

Former ‘Fixed’Operator

Broadcastcontentprovider

New market

opportunities

New marketopportunities

LTE RadioAccess

LT3602/v31.2 © Wray Castle Limited

Broadband Access with LTE

Wide-area LTE radio access combined with the EPC represents a complete adoption of an all-IP

(Internet Protocol) architecture, offering broadband delivery capability with the potential for bit rates of

several hundred megabits per second and QoS (Quality of Service) management suitable for real-time

operation of high-quality voice and video telephony.

LTE has a very important role in the overall telecommunications service convergence concept. LTE couldprovide a key to unlocking a truly converged fixed/mobile network for the delivery of quadruple play

services. Its potential bandwidth capabilities are sufficient for the support of services ranging from

managed QoS real-time voice or video telephony to high-quality streamed TV. Its flat all-IP architecture

means that it can act as a universal access network for a wide range of core network types.

LTE Air Interface



E-UTRAN EPCE-UTRA

UE

LTE SAE

EPS


Architecture Terminology

LTE is the term used to describe collectively the evolution of the RAN (Radio Access Network) into the

E-UTRAN (Evolved Universal Terrestrial Radio Access Network ) and the RAT (Radio Access

Technology) into E-UTRA (Evolved Universal Terrestrial Radio Access).

SAE (System Architecture Evolution) is the term used to describe the evolution of the core network into

the EPC (Evolved Packet Core). There is also a collective term, EPS (Evolved Packet System), whichrefers to the combined E-UTRAN and EPC.

Introduction to LTE



100 Mbit/s (downlink) and 50 Mbit/s (uplink)

Increased cell edge bit rate

2-4 times better spectral efficiency

Reduced radio access network latency

Scalable bandwidth up to 20 MHz

Interworking with 3G systems

LTE/SAE Design Aims


LTE Development and Design Goals

The debate about the structure and composition of LTE has been ongoing since at least 2004, with

many organizations promoting their preferred technological solutions.

3GPP brought some focus to the debate in June 2005 by publishing Technical Report TR 25.913 –

Requirements for Evolved UTRA and UTRAN.

TR 25.913 stated several objectives for the evolution of the radio interface and radio access network

architecture. Targets included a significantly increased peak data rate, e.g. 100 Mbit/s (downlink)

and 50 Mbit/s (uplink), and an increased ‘cell edge bit rate’ while maintaining the same site locations

as are deployed for R99 (Release 99) and HSPA.

Other objectives include significantly improved spectrum efficiency (two to four times that provided

by Release 6 HSPA), the possibility for a significantly reduced radio access network latency for both

C-plane and U-plane traffic, scaleable bandwidth with support for channel bandwidths up to 20 MHz,

and support for interworking with existing 3G systems and non-3GPP-specified systems.

LTE Air Interface



GSM900 GSM1800

GSM1900

GPRS

EGPRS

UMTS HSDPA

IMS

HSUPA HSPA+

GERAN enhancements

LTE/SAE

Phase 1 Phase 2 Phase 2+

Rel 96-98

Rel 99 Rel 4 Rel 5 Rel 6 Rel 7 Rel 8 Rel 9

LTE-Advanced

Rel 10 Rel 11 Rel 12

GSM

GPRS

EDGE

Rel 99 Rel 4 Rel 5 Rel 6 Rel 7 Rel 8 Rel 9 Rel 10 Rel 11 Rel 12

Rel 8 Rel 9 Rel 10 Rel 11 Rel 12

UTRAN enhancements


LTE Standards Development

Since the publication of the first GSM (Global System for Mobile Communications) specifications in the

late 1980s, the technologies and techniques employed by GSM networks have continually evolved and

developed. GSM itself underwent a series of changes, from Phase 1 to Phase 2 and eventually to Phase

2+. Phase 2+ progressed with a series of yearly releases, starting with Release 96.

The UMTS (Universal System for Mobile Communications) was introduced as part of Release 99 andfrom then onwards the 3GPP (3rd Generation Partnership Project) 3G network technology has also been

undergoing a process of evolution. The evolutions that particularly affect the air interface are mainly

contained in Releases 5, 6, 7 and 8. Release 5 and 6 introduced HSPA (High Speed Packet Access) –

HSDPA (High Speed Downlink Packet Access) in R5 and (HSUPA) High Speed Uplink Packet Access,

or Enhanced Uplink, in R6. Release 7 outlines the changes necessary to deliver HSPA+ and Release 8

specifications begin to describe LTE – the Long Term Evolution of UMTS. Specification of LTE, generally

described as 3.9G, was completed in Release 9. Specification of LTE-Advanced, a full 4G solution, is

detailed in Release 10.

Further Reading: www.3gpp.org/releases

Introduction to LTE



E-UTRAN EPC

E-UTRA

LTE Signalling LTE Traffic

SCTP

All-IP


LTE Key Technologies

Tests and evaluations carried out during 2007 led to the publication of the Release 8 36-series of

specifications, which began to detail the technological basis for LTE.

Of the original four candidate air interface technologies, two were chosen for the final version: OFDMA

(Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier FDMA).

OFDMA is employed on the LTE downlink and is expected eventually to provide peak data rates

approaching 360 Mbit/s in a 20 MHz channel. SC-FDMA is employed on the LTE uplink and may deliver

up to 86 Mbit/s. SC-FDMA is also sometimes known as DFT-FDMA.

In addition to the air interface technologies, LTE simplifies the range of technologies employed in other

parts of the network.

LTE is an ‘all-IP’ environment, meaning that all air interface, backhaul and core network interfaces will

carry only IP-based traffic. The need to support different protocols for different traffic types, as was the

case with R99, is therefore avoided.

In this all-IP environment, layer 4 transport layer functions for signalling connections are performed usingan alternative to the traditional choices, TCP (Transmission Control Protocol) or UDP (User Datagram

Protocol).

SCTP (Stream Control Transmission Protocol) was developed with the needs of IP-based signalling in

mind and is used to manage and protect all LTE signalling services.

LTE Air Interface



S1Evolved

Packet Core

S1

X2

eNB

(Evolved Node B)

eNB

E-UTRAN

LTE UE

PDCP

RLC

MACPHY

RRC

Uu

Inter-cell RRM

Radio bearer control

Connection mobility control

Radio admission control

Measurement control

Cell configuration

Dynamic resource allocation


Access Networks and the eNB (E-UTRAN Node B)

The basic building blocks of the E-UTRA access network are the eNB (Evolved Node B) plus backhaul –and nothing else. All layers of the air interface protocol stack, including the elements that previouslyresided in the RNC (Radio Network Controller) – RRC (Radio Resource Control), RLC (Radio LinkControl) and MAC (Medium Access Control) – have been moved out to the base station. As the eNB nowanchors the main backhaul link to the core network, it has also assumed responsibility for managing the

PDCP (Packet Data Convergence Protocol) service, which provides header compression and cipheringfacilities over the air interface.

HSDPA began the process of moving RRM (Radio Resource Management) functions, such as packetscheduling, from the RNC to the Node B. In LTE, all remaining RRC functions are devolved to the eNB,meaning that there is no longer a role for a device such as the RNC.

Among the RRM functions now devolved to the eNB are radio bearer control, radio admission control,connection mobility control and the dynamic allocation (via scheduling) of resources to UEs (User Equipments) in both uplink and downlink directions.

Following on from innovations in R4 and R5 networks, LTE also supports the concept of flexibleassociations between access and core network elements, meaning that each eNB has a choice of MME

(Mobility Management Entity) nodes to which to pass control of each UE. Dynamic selection of an MMEfor each UE as it attaches is therefore also an eNB responsibility. An eNB may be associated with MMEsbelonging to different PLMNs (Public Land Mobile Networks), allowing for the easy creation of multi-operator networks.

The eNB also receives, schedules and transmits control channel information in its cells, including pagingmessages and broadcast system information, both of which are received from the MMEs. It retains manyof the traditional roles associated with base stations, such as bearer management. It is responsible for routing U-plane traffic between each UE and its S-GW (Serving Gateway). The complexity of the eNBand of the decisions it is required to make are therefore much greater than for an R99 Node B

The complexity of the eNB and of the decisions it is required to make are therefore much greater than for an R99 Node B.

The broadening of the range of services offered by the LTE EPS over time has lead to the developmentof several specialised sub-types of eNB. Femtocell services, for example, are provided via HeNBs (HomeeNBs), whilst LTE Relay facilities are offered by Relay Nodes and controlled by DeNBs (Donor eNBs).

Further Reading: 3GPP TS36.300

Introduction to LTE



X2

IPData link layer

X2–AP

Physical layer

X2-C

SCTP

X2-U

UDP

IP

Data link layer

User planePDUs

Physical layer

GTP-U


X2 Interface

With the removal of the RNC from the access network architecture, inter-eNB handover is negotiated and

managed directly between eNBs using the X2-C interface. In LTE implementations that need to support

macro diversity, the X2-U interface will carry handover traffic PDUs (Protocol Data Units) between eNBs.

X2-C (control plane) signalling is carried by the X2AP (X2 Application Protocol), which travels over an

SCTP association established between neighbouring eNBs.

X2AP performs duties similar to those performed by RNSAP (Radio Network Subsystem Application

Protocol), which operates between neighbouring RNCs over the Iur interface in UMTS R99 networks.

X2-U (user plane) traffic is carried by the existing GTP-U (GPRS Tunnelling Protocol – User plane), as

employed in UMTS R99 networks. The facilities provided by the X2-U interface are only expected to be

required if macro-diversity handover is supported.

Both sub-types of the X2 interface travel over IP: SCTP/IP for the X2-C and UDP/IP for the X2-U.

LTE Air Interface



MME

S-GW

IP

Data link layer

S1–AP

SCTP

Physical layer S1-MME

Physical layer

UDP

IPData link layer

User planePDUs

GTP–U

S1-U


S1 Interface

Backhaul links to the core network are carried by the S1 interface. Following the general structure of the

Iub interface which it replaces, traffic over the S1 is logically split into two types.

S1-U flows carry user plane traffic and S1-MME flows carry mobility management, bearer control and

direct transfer control plane traffic.

Message structures for the S1-MME interface that operate between the eNB and the MME are defined by

S1AP (S1 Application Protocol). The S1AP performs duties that can be seen as a combination of those

performed by R99 RANAP (Radio Access Network Application Part) and GTP-C (GPRS Tunnelling

Protocol – Control plane).

To provide additional redundancy, traffic differentiation and load balancing, the S1-flex concept allows

each eNB to maintain logical connections to multiple S-GWs and MMEs – there may therefore be

multiple instances of the S1 interface per node.

The S1-U interface employs GTP-U to create and manage user-plane data contexts between the eNB

and the S-GW.

LTE Air Interface



Non-Access

Stratum (NAS)

Non-Access

Stratum (NAS)

RRC RRC

PDCP PDCP

RLC RLC

MAC MAC

Physical Layer Physical Layer

User Equipment eNB Evolved Packet Core


E-UTRA Protocols

In line with other aspects of E-UTRA, the air interface protocol stack has been designed to reduce

complexity.

Whereas an R99/HSPA-enabled Node B employs a protocol stack with a variety of RLC and MAC

instances, an E-UTRA eNB employs a protocol stack with just one instance of each layer.

The extent of the air interface protocol stack has also been reduced. In previous incarnations of UMTS

some layers operated between the UE and the Node B, while most extended all the way to the RNC.

With the elimination of the RNC, all air interface protocols in E-UTRA operate between the UE and the

eNB.

LTE Air Interface



SECTION 2

UE BEARERS AND CONNECTIVITY

LTE Air Interface




LTE Air Interface




The EPS as an IP-CAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1

EPS Quality of Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.2

EPS Bearer QoS Class Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3

ARP (Allocation and Retention Priority) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4

QoS Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.5

EPS Bearer Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.6

EPS Bearer Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7

EPS Bearer Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8

EPS Area Identities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.9

Subscriber Identities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.10

LTE State Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.11

EPS Service Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12

Device Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.13

EPS Initial Attach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.14

Default Bearer Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.15

IMS Registration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.16

CS Fallback Attach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.17

EPC Support for Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.18

TAU (Tracking Area Update) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.19

Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.20

CONTENTS

UE Bearers and Connectivity




LTE Air Interface





define an EPS bearer in terms of its endpoints and QoS characteristics

explain how the radio bearer, S1 bearer and S5/S8 bearer relate to an EPS bearer

explain the use of the QCI value in defining the QoS applied to EPS bearers

describe how different traffic types would be carried in different EPS bearers for a single UE

describe how uplink and downlink data packets are mapped into and carried through

appropriate EPS bearers relating to a single UE

explain the relevance and operation of the default EPS bearer

describe the procedure for establishing an EPS bearer

list and define the key identities for the EPS network hierarchical regions

name the identities that are used in respect of users and UEs

describe the procedure used by a UE to register for service on the EPS and the IMS

describe the key mobility management procedures

describe the principle of operation of CS fallback

explain how a Home eNB (femtocell) is connected into the LTE EPS

OBJECTIVES





LTE Air Interface




PDN-GW

E-UTRAN

PDN

EPC

MME

eNBUE

All-IP bearer with defined QoS

PCRF

S-GW


The EPS as an IP-CAN

Essentially, the EPS offers no services to the user other than connectivity to an external PDN (Packet

Data Network). In this respect the EPS can be considered to function only as an IP-CAN (IP Connectivity

Access Network). Any specific services that the user receives are provided through service platforms that

are implemented in the external PDN.

Connectivity is provided through one or more EPS bearers. The EPS bearer defines a transmission pathwith an associated set of QoS parameters between the UE and the PDN-GW.

The establishment of EPS bearers is managed by the PCRF (Packet Control Resource Function) in

conjunction with the MME and the eNB.

Further Reading: 3GPP TS 23.203




QCIResource

TypePriority

Packet

Delay

Budget

Packet

Error Loss

Rate

Example Services

1 2 100 ms 10-2 Conversational voice

2 4 150 ms 10-3 Conversational voice (live streaming)

3 3 50 ms 10-3 Real time gaming

4 5 300 ms 10-6 Non-conversational voice (buffered streaming)

5 1 100 ms 10-6 IMS signalling

Video (buffered streaming)

TCP-based (e.g. www, e-mail, chat, ftp, p2p,

file sharing, progressive video)

7 7 100 ms 10-3 Voice, video (live streaming), interactive gaming

8 8 Video (buffered streaming)

TCP-based (e.g. www, e-mail, chat, ftp,

p2p,file sharing, progressive video)9 9

GBR

Non-GBR

6 6 300 ms 10-6

300ms 10-6


EPS Bearer QoS Class Identifiers

In order that common levels of QoS can be provided in different operators’ networks, the 3GPP has

defined a limited set of QoS levels that are standardized for all operators. Each QoS level is identified

with a QCI value. The QoS targets for each QCI value are shown in the table. It can be seen that there is

a broad division between GBR and non-GBR services. There are also targets for the delay budget and

packet loss rate. Additionally, each QCI is allocated a priority level. This is used for prioritizing resource

allocations at the eNB.

Collectively, the parameters associated with each QCI determine the layer 2 and physical layer

configurations that are used on the air interface. For example, a GBR service with low delay

requirements would make use of the unacknowledged mode of RLC, while a non-GBR service with more

relaxed delay requirements but low packet loss tolerance would use the acknowledged mode of RLC.

The 3GPP has also provided guidance on the kind of services applicable to each QCI. It can be seen that

in several cases the same or similar services are associated with more than one QCI. This is to allow for

the possible differentiation of services as ‘standard’ and ‘premium’.





PDN-GW

HSS

eNB

PCRF

S-GW

MME

General ARP policyfor network

Specific ARPparameters per

subscription

Pre-emption Vulnerability

Pre-emption Capability

ARP Priority

yes/no

yes/no

1–15


ARP (Allocation and Retention Priority)

The EPS ARP facility allows the system to handle busy and overload periods. Each QoS class is

associated with an ARP level, which determines the relative priority of the EPS bearers established

according to those classes.

There are fifteen ARP levels ranging from Priority 1 (highest) to 15 (lowest).

The other components of ARP are simple ‘yes’ or ‘no’ parameters: a connection’s Pre-emption

Capability, which defines whether it is able to pre-empt other, lower priority connections; and Pre-emption

Vulnerability, which determines whether it can be pre-empted by higher-priority connections.

During busy periods when there is competition for scarce resources, the EPC will judge which of several

competing EPS bearer requests will be confirmed based on the ARP of the services requested. Those

with a higher ARP priority will be established and those with a low ARP priority will be rejected. In

overload situations the EPC will use the ARP levels of existing bearers to select those that can be

dropped – EPS bearers with a low ARP will be dropped first.

ARP does not have an effect on packet forwarding or prioritization decisions within EPC nodes, where

decisions are made based on a bearer’s QCI, or in the IP transport network, where these considerationsare handled by DiffServ (Differentiated Services).

Further Reading: 3GPP TS 23.401:4.7.3; 23.203:6.1.7

LTE Air Interface



UE

EPS Bearer with

GBR QoS

UE-AMBR for all

Non-GBR EPS

Bearers from UE

APN-AMBR for

Non-GBR EPS

Bearers to PDN-GW 2

PDN-GW 2

S-GW

PDN-GW 1

APN-AMBR for

Non-GBR EPS Bearers

to PDN-GW 1


QoS Levels

QoS in the EPC is currently defined by three levels: GBR, MBR and AMBR (Aggregate Maximum Bit

Rate).

GBR connections are assigned a guaranteed data rate and are therefore useful for carrying certain types

of real-time and delay-sensitive traffic. MBR connections are non-guaranteed, variable-bit-rate services

with a defined maximum data rate. If a connection’s data rate goes beyond the set maximum the networkmay decide to begin discarding the excess traffic.

GBR and MBR parameters are applied on a ‘per bearer’ basis, whereas AMBR is applied to a group of

bearers; specifically, a group of non-GBR bearers that terminate on the same UE. AMBR allows the EPS

to set a maximum aggregate bit rate for the whole group of bearers that can then be shared between

them.

The APN-AMBR parameter sets the shared bit rate available to a group of non-GBR bearers that

terminate on the same APN and can therefore be seen to be applied on a ‘per PCS’ basis; the UE-AMBR

parameter aggregates all non-GBR bearers associated with one UE.

Dedicated bearers can be established as GBR or non-GBR (i.e. MBR) as required. Default bearers, dueto the probable need to adjust their bandwidth after the initial Attach has taken place, must be non-GBR.

Further Reading: 3GPP TS 23.401:4.7.3




PDN-GW

eNBUE

S-GW

EPS Bearer (default)

EPS Bearer (e.g. VoIP)

EPS Bearer (e.g. VPN access)


EPS Bearer Types

Since an individual EPS bearer has only one definition for QoS and a user may be using more than one

service, the UE may have multiple EPS bearers active, each providing different levels of QoS for different

services. For example, a user may be web browsing and simultaneously engaged in a VoIP voice call.

This would require two different levels of QoS, thus requiring two EPS bearers, one for each of the

respective traffic flows. Additionally, signalling will be required to support the services and this in turn

may require a third EPS bearer.

In any case LTE will always allocate a default EPS bearer. This bearer is predefined and is allocated as

soon as the UE is successfully registered on the system. Context information describing bearer identities

and QCI for the default bearer is maintained in the EPC as long as the UE remains registered. However,

while the UE is in idle mode the default bearer will remain inactive. In this state there are no allocated

resources for the EPS bearer on the air interface or within the E-UTRAN.


LTE Air Interface



UE

Application/service layer

App. 1 App. 2 App. 1 App. 2BindingUL-TFTs

with RB-IDsBindingDL-TFTs

with TEIDs

TFT 1

TFT 2

TFT 1

TFT 2

BindingRB-IDs withS1-TEIDs

BindingS1-TEIDs withS5/S8-TEIDs

S-GWeNBUE PDN-GWUu S5/S8S1

Radio bearers

identified with a

RB-ID

S1 bearers

identified with a

S1-GTP-TEID

S5/S8 bearers

identified with a

S5/S8-GTP-TEID

RB-ID S1-TEID S5/S8-TEID

S5/S8-TEIDS1-TEIDRB-ID


EPS Bearer Composition

An EPS bearer represents the conjunctions of a radio bearer on the air interface, an S1 bearer and an

S5/S8 bearer. Each is defined in a slightly different way and binding between the different bearers is

provided at each node. In particular the binding between a radio bearer on the air interface and an S1

bearer on the S1 interface, which occurs in the eNB, defines an E-RAB (E_UTRAN Radio Access

Bearer). The E-RAB is then bound to an S5/S8 bearer in the S-GW and the combination of these two is

an EPS bearer.

Once an EPS bearer has been established, packets relating to that EPS bearer must be identified and

mapped to it. This function is performed by TFT (Traffic Flow Template) functions that are defined in the

UE for uplink traffic flows and in the PDN-GW for downlink traffic flows. The TFT for an EPS bearer will

identify packets relating to a particular service using TCP or UDP port numbers or DiffServ code points.





PDN-GWMMEeNBUE PCRFS-GWPCC decision

provisionCreate dedicated

bearer requestCreate dedicated

bearer requestBearer setup

requestRRCConnection

Reconfiguration

RRCConnection

Reconfiguration

Complete

Create dedicated

bearer response

Create dedicated

bearer response

Bearer setup

response

Provision

acknowledge


EPS Bearer Establishment

The establishment of a new EPS bearer is triggered from the PCRF. This is likely to be in response to a

service request from a service AF (Application Function) in an external PDN for the establishment of a

UE originated or terminated service or the establishment of the default EPS bearer when a UE registers

with the system.

The PRCF is responsible for PCC (Policy Control and Charging), and as such is responsible for decidingwhen an EPS bearer is required and what the QCI value should be used for the EPS bearer. This is

indicated to the PDN-GW using a PCC Decision Provision message.

The PDN-GW uses the QoS information to establish the appropriate bearer-level QoS parameters and

forwards these, along with the UL-TFT to be used in the UE, to the S-GW in a Create Dedicated Bearer

Request message. The message is then forwarded to the MME along with the S1 bearer ID.

The MME assembles the information in this message along with the EPS bearer ID and sends it to the

eNB in a Bearer Setup Request message. The eNB uses the QCI information to determine the

appropriate rules and configuration to apply for handling data relating to the EPS bearer and the creation

of the radio bearer. It then instructs the UE to set up the radio bearer using the RRC Connection

Reconfiguration message. This message contains all the information that the UE needs to configure theEPS bearer at all layers of the protocol stack.


LTE Air Interface



PLMN ID = MCC+MNC

MMEGI (MME Group ID)

Tracking Area ID (TAI)= PLMN ID + TAC

E-UTRAN Cell Global ID (ECGI) = eNB ID + Cell ID


EPS Area Identities

The EPS continues to use the PLMN identifier employed by legacy 3GPP systems, which consists of the

MCC (Mobile Country Code) and the MNC (Mobile Network Code).

The MMEGI (MME Group Identifier) is a 16-bit identifier assigned to an individual MME Pool. The

MMEGI only has to be unique within a PLMN.

The TAI (Tracking Area Identifier) is analogous to the LA (Location Area) or RA (Routing Area) identifiers

employed by the GERAN/UTRAN in that it is used to identify a group of cells in the access network. In

the E-UTRAN the TA (Tracking Area) is the granularity with which each UE’s location is tracked. It is also

the area within which a UE will be paged. The TAI consists of the network’s MCC and MNC followed by a

TAC (Tracking Area Code).

As in legacy systems it is necessary to be able to identify each cell uniquely in the network for call

establishment, paging, handover and billing purposes. 3GPP has devised an updated Cell ID known as

an ECGI (E-UTRAN Cell Global Identifier). The ECGI incorporates a unique eNB Identifier, which allows

the S1 and X2 interface protocols to discover and identify the target nodes for functions such as EPS

Bearer handover.

Further Reading: 3GPP TS 29.803, 23.401:5.2, 36.300:8.2




IMSI

IMEISV

MSISDN IP Address(es)

MCC MNC MMEI

24 bits

GUTI

M-TMSI

32 bits

32 bits

M-TMSI

M-TMSI

32 bits

MMEC

8 bits

S-TMSI

M-TMSI


Subscriber Identities

The main means of identifying EPS subscribers remains the IMSI (International Mobile Subscriber

Identity), which is permanently assigned to a subscriber account. The IMEISV (International Mobile

Equipment Identity and Software Version) and MSISDN (Mobile Station ISDN Number) also remain as

LTE identifiers.

Temporary and anonymous identification of subscribers is provided by the GUTI (Globally UniqueTemporary Identity), which is assigned by the serving MME when a UE has successfully attached and is

reassigned if the UE moves to the control of a new MME. The GUTI is analogous to the legacy TMSI

(Temporary Mobile Subscriber Identity) but with the additional feature that its structure uniquely identifies

not only the subscriber within the MME but also the MME that assigned it.

The GUTI is constructed from the GUMMEI, which consists of the network’s MCC and MNC followed by

a MMEI (MME Identifier), and the M-TMSI (MME Temporary Mobile Subscriber Identity). The M-TMSI is

used to provide anonymous identification of a subscriber within an MME once that subscriber has been

authenticated and attached. As with legacy TMSI use, the MME may elect to reissue the M-TMSI at

periodic intervals and it will be reissued in any case if the UE passes to the control of a different MME.

The M-TMSI allows a subscriber to be uniquely identified within an individual MME, whereas the S-TMSI(SAE TMSI) allows subscribers to be identified within an MME group or pool. To achieve this, the S-TMSI

also includes the one-octet MMEC (MME Code). The MMEC is the MME’s index within its pool.

Further Reading: 3GPP TS 23.203, 23.401:5.2

LTE Air Interface



MME

eNBUE

Registered

Deregistered

EMM EMM

EMM

Off Attaching Idle Connecting Active

Deregistered Registered

ECM ECM

Connected

Idle

Idle ConnectedECM

RRC Idle Connected Idle Connected

RRC RRC

Connected

Idle


LTE State Management

In order to offer effective service to UEs, the EPS needs to be able to define and keep track of the

availability and reachability of each terminal. It achieves this by maintaining two sets of ‘contexts’ for

each UE – an EMM (EPS Mobility Management) context and an ECM (EPS Connection Management)

context – each of which is handled by ‘state machines’ located in the UE and the MME.

A further state machine operates in the UE and serving eNB to track the terminal’s RRC state, which canbe either RRC-IDLE (which relates to a UE in idle mode) or RRC-CONNECTED (which relates to a UE

with an active traffic bearer).

EMM is analogous to the MM processes undertaken in legacy networks and seeks to ensure that the

MME maintains enough location data to be able to offer service to each UE when required. The two EMM

states maintained by the MME are EMM-DEREGISTERED and EMM-REGISTERED.

The ECM states describe a UE’s current connectivity status with the EPC, e.g. whether an S1 connection

exists between the UE and EPC or not. There are two ECM states, ECM-IDLE and ECM-CONNECTED.

Although the EMM and ECM states are independent of each other, they are related, and any discussion

of a UE’s reachability is best served by viewing these states in a combined fashion. There are three mainphases of UE activity, each of which can be described by a combination of EMM and ECM states. These

are with the UE powered off, with the UE in idle mode and the UE with an active traffic connection.





default APN may be stored in HSS subscriber data

IP address allocation by DHCP – UE address allocation preference

can be signalled

single MME, S-GW and PCRF per UE, multiple PDN-GWs per UE

piggybacking of dedicated bearers on default set-up


EPS Service Concepts

As part of the Attach process, the EPS will establish at least a default bearer for the UE. Details of a

‘default APN’ to use for the bearer may be stored in the subscriber’s HSS profile or may be selected

dynamically by the EPC.

HSS data may also indicate whether additional dedicated bearers need to be established along with the

default bearer during the Attach.

Each UE, irrespective of the number of EPS bearers it has established or PDN Connectivity Services it is

using, will only be served by one MME and one S-GW at any one time (except for the brief but inevitable

overlap that occurs during a relocation). Connection establishment and rating decisions for each UE will

be handled by the same PCRF in networks that employ dynamic PCC; this ensures that service-related

decisions are made by one device that has all details of the UE’s current service set to hand.

IP address allocation occurs as part of the Default Bearer establishment process; all linked Dedicated

Bearers will share the same IP address.

The UE will generally be assigned an IP address from within the range allocated to the default APN; it is

also possible for the UE to signal that it requires an IP address to be assigned by the external PDN towhich the APN connects. In either case the default IP address allocation method is DHCP (Dynamic Host

Configuration Protocol), although static IP address allocation (based on an address stored in the HSS

data) is also permitted.


LTE Air Interface



UE

eNB

UE selects eNB based on air interface

S and R criteria

MME

eNB selects MME from associated Pool

based on load-balancing parameters

S-GW

MME selects S-GW from the set

associated with UE’s current TA

PDN-GW

PDN-GW selected from the set

that supports the indicated APN


Device Selection

Each EPS Bearer will be carried between or controlled by a specific set of devices. For an EPS Bearer

established between the UE’s home E-UTRAN and home EPC, this set of devices will be an eNB, an

MME, an S-GW and a PDN-GW. As each network can be expected to have a number of devices of each

type deployed to it, the methods by which the devices involved in serving a bearer must be clearly stated.

Device selection for EPS connections operates as follows:

The UE selects the eNB to use based on air interface selection and reselection actions.

The eNB selects the MME to use from the MME Pool available based on load balancing principles and

any current overload notifications. Load balancing is managed using the MME ‘weighting factor’, which is

related to the MME’s capacity and is signalled to eNBs using the MME Relative Capacity information

element in the MME Configuration message during S1 set-up. An MME with a capacity of 0 is not

accepting connections; an MME with a capacity of 255 has the highest relative capacity level.

The eNB does not select the MME in the case of MME Relocation, when a target MME is selected by the

source MME. The SGSN is responsible for MME selection in the case of inter-RAT handover.

The MME selects the S-GW to use from the set associated with the UE’s current Tracking Area andtakes any current overload notifications into account. The MME may also take the UE’s current TA List

into account, by selecting an S-GW that serves one or more of the TAs included on the list.

The PDN-GW is selected by the MME based on APN details stored in the user’s HSS subscription data

or on the USIM.

It is common for network operators to ensure service resilience by deploying multiple instances of the

same APN to different PDN-GWs; this ensures that if one PDN-GW fails the APN service can continue.

Further Reading: 3GPP TS 23.401:4.3.8, 36.413:9.2.3




PDN-GWMMEeNBUE HSSS-GW EIR

RRCconnection

S1-APconnection

Attach Request

AKA/Security

Optional Stage

Identity Request/Response ME Identity Check

Ciphered Options Request

Ciphered Options Response

Update Location

Insert Subscriber Data

Insert Subscriber Data Ack

Update Location Ack


EPS Initial Attach

The UE’s objective when performing an attach is to register the subscriber’s identity and location with the

network to enable services to be accessed. During the attach procedure the UE will be assigned a

default EPS bearer to enable always-on connectivity with a PDN. The UE may be provided with details of

a local P-CSCF to enable it to register with the IMS.

A simplified view of the attach process – assuming that it is an initial attach with stored details from arecent previous context for a UE using its H-PLMN (Home Public Land Mobile Network) and accessing

via the Home E-UTRAN – is shown and the stages of the process are described below.

Once a suitable cell has been selected the UE employs the Random Access procedure to request an

RRC connection with the chosen eNB. With that in place an Attach Request message can be

transmitted. If the UE has previously been registered with the PLMN (Public Land Mobile Network), it

may include a previously assigned GUTI in the message, otherwise the Attach Request message

contains the subscriber’s IMSI and some other parameters.

On receipt of the Attach Request the eNB either derives the identity of the previously used MME from the

supplied GUTI or selects an MME from the pool available and forwards the message.

The MME contacts the HSS indicated by the subscriber’s IMSI and in response receives the relevant

elements of the ‘quintuplet’ that allows the EPS-AKA process to take place.

Optionally, at this point the MME may be required to check the identity and status of the UE via the EIR

using the ME Identity Check process. Ciphering may then be invoked over the air interface.

Once the AKA procedures have successfully concluded the MME transmits an Update Location message

to the HSS and receives the Insert Subscriber Data message in response containing the user’s service

profile . An Insert Subscriber Data Ack from the MME is followed by an Update Location Ack from the

HSS. The UE is now attached to the EPC.


LTE Air Interface



PDN-GWMMEeNBUE PCRFS-GW

Create DefaultBearer Request

PCCLookup

Optional Stage

Create DefaultBearer Response

Initial Context Setup

Request/Attach

Accept

RRCConnection

Reconfiguration

RRCConnection

Reconfiguration

CompleteInitial Context Setup

ResponseDirect Transfer

Attach Complete

Data flow


Default Bearer Establishment

A default bearer must then be established and the MME selects the S-GW that will handle it and a

PDN-GW that supports the requested APN. The MME issues a Create Default Bearer Request to the

selected S-GW, which assigns a GTP TEID to the EPS bearer and passes the request to the indicated

PDN-GW. If the network employs dynamic PCC the PDN-GW will query the PCRF assigned to serve

the UE for bearer parameters, otherwise the bearer will be established using local QoS parameters

stored in the PDN-GW.

A Create Default Bearer Response message passes from the PDN-GW to the S-GW, which contains

relevant parameters such as the EPS bearer’s IP address and possibly the IP address or DNS name of

a local IMS P-CSCF. The S-GW creates the bearer as specified and passes the Create Default Bearer

Response message to the MME. The details that define the S1-U service will also have been defined

during this stage.

The MME sends an Initial Context Setup Request/Attach Accept message, which contains the assigned

parameters for the EPS bearer context, to the eNB. That element in turn sends an RRC Connection

Reconfiguration message to the UE to inform it of the bearer details and the changed air interface

parameters. The UE returns an RRC Connection Reconfiguration Complete message to verify that the

radio bearer, which was initially established just to carry the attach message, has been reconfigured tosupport the new parameters. The eNB forwards an Attach Complete message to the MME.

The UE then sends a Direct Transfer message to the eNB, which confirms the details of the EPS

Bearer. Finally, the eNB sends an Attach Complete message to the MME to confirm that both the

Attach and the Default EPS Bearer processes have completed successfully. Uplink and downlink data

can now flow if required.





PDN-GWUE HSSI-CSCF S-CSCF

P-CSCF Discovery

Register

Cx Query

Cx Query

Response

Register

HSS updated

Service Control

200 OK

200 OK


IMS Registration

With a default EPS bearer in place the UE can register for services with the IMS, assuming that the

PLMN employs an IMS and that it is reachable via the default APN. However, before the UE can

register with the IMS it must first perform P-CSCF discovery. This may be achieved as part of the

attach process if the IP address of a local P-CSCF is returned by the PDN-GW when the default bearer

is set up. Alternatively, the UE may need to initiate a separate P-CSCF discovery enquiry. Once a local

P-CSCF has been discovered the UE can send a SIP Register message.

In non-roaming scenarios, the P-CSCF can interrogate the HSS directly to determine the subscriber’s

S-CSCF. In the roaming case the visited P-CSCF will have to pass the query to either an I-CSCF or an

S-CSCF in the subscriber’s home network for action.

The Register message, containing details such as the P-CSCF address/name, public user identity,

private user identity and UE IP address, is passed to the appropriate S-CSCF. ‘Appropriate’ in this

sense means an S-CSCF that has access to the AS (Application Server) or ASs that control services to

which the user has subscribed. If an individual S-CSCF does not support connectivity to all of the

required ASs it will be necessary to register the UE with more than one S-CSCF. The S-CSCF updates

the HSS with the UE’s changed status and the HSS responds with details of the service control

platforms associated with the user’s subscribed services.

The S-CSCF provides the required registration information to the set of service control platforms

indicated by the HSS, enabling the UE to be available for the user’s subscribed services. A SIP 200 OK

confirmation message is relayed from the S-CSCF to the P-CSCF and onwards to the UE, at which

point the UE is regarded as being ready and available for IMS services.

Further Reading: 3GPP TS 23.228:5.2

LTE Air Interface



UE MME MSC-S/ VLR

Attach Request

EPS Attach. UE indicates combinedEPS/IMSI Attach and CS Fallback required

MME determines VLRnumber and mapped LAI

Location Update

Request

SGs association created

Location Update

Accept Attach Accept


CS Fallback Attach

If a UE and a network supports CS Fallback, the UE will request CS Fallback registration during the

Attach process. The service will only be available in areas where there is overlapping E-UTRAN and

GERAN/UTRAN coverage.

An Attach Request message with the ‘Attach Type’ set to EPS/IMSI attach and a CS Fallback capable

flag set is sent to the MME. This will trigger the combined attach process in the MME. The MME willderive the number of the VLR responsible for the Location Area that maps to the UE’s current Tracking

Area. This information will form part of the MME’s databuild and will not be dynamically discoverable. The

MME uses the SGs interface, which is an evolved version of the Gs interface that provides connectivity

between legacy SGSNs and MSCs.

The MME forwards a Location Update to the MSC/MSC-Server causing the MSC/MSC-Server to create

an SGs association for the UE. The MSC/MSC-Server confirms the combined Attach with a Location

Update Accept and the MME confirms the Attach to the UE using an Attach Accept message.






EPC Support for Idle Mode

The MME currently serving each UE is responsible for ensuring its ‘reachability’. It achieves this by

monitoring the current TA in which the terminal is located.

The EPS allows a cell to be a member of more than one TA. This allows a UE to roam within a set of

contiguous TAs without being required to perform a TAU (Tracking Area Update), which reduces the

amount of location-related signalling that is required, although it may conversely increase the amount of paging required per UE connection request.

The MME reflects this extended mobility by maintaining a TA list for each registered UE within which the

list shows the set of TAs the UE currently registers.

During a TAU, and periodically in the event that a TAU does not occur within a set time-frame, the MME

is responsible for reauthenticating each registered UE and for reissuing the M-TMSI used to

confidentially identify it.

When a UE drops into the ECM-IDLE state its existing default bearer can be ‘parked’ and any dedicated

bearers can either be parked or released. To support this, the MME stores details of the UE’s current

‘bearer contexts’ ready to reactivate them in the event of a UE or network-triggered Service Request.

A TAU may result in the need to change the S-GW assigned to handle an idle UE’s bearer contexts or of

the MME with which the UE is registered, if the reselected cell is associated with a different S-GW

Service Area or MME Pool.


LTE Air Interface



MMEeNBUE HSS

TAU trigger event

TAU Request

TAU Request + TAI and ECGI

Authentication/Security

Optional Stage

TAU Accept

TAU Complete


TAU (Tracking Area Update)

A TAU takes place between a UE and the MME with which it is registered and is triggered by the UE

detecting a change in TAI after a cell reselection. A TAU is also used as part of the Initial Attach process

and may additionally be triggered by events such as the expiry of the periodic TAU timer or as part of

MME load balancing or rebalancing.

In the example message flow it is assumed that the UE is connected to its HPLMN and that an S-GWchange and MME relocation are not required. After detecting a change in TAI, the UE transmits a TAU

Request message to the eNB. The TAU Request includes the old GUTI, old TAI and EPS bearer status.

The eNB forwards the TAU Request (plus the new TAI and ECGI) to the MME indicated by the supplied

GUTI. If the MME indicated by the GUTI is not associated with the new eNB, an MME relocation will be

triggered and the base station will select a new MME to pass the TAU Request to. If the message

integrity check is successful the MME may elect not to reauthenticate the UE. If the MME is configured

always to reauthenticate or if the integrity check fails, then the EPS-AKA process must be followed and a

new GUTI (which includes the new M-TMSI) will be issued.

Once the MME is satisfied that the UE/USIM is authentic and assuming that the UE is allowed to roam in

the new TA, it transmits a TAU Accept message to the eNB, which relays it to the UE. The TAU Acceptmessage contains the new GUTI, if one was assigned, plus the current TA List associated with the UE.

The TA List enables the UE to determine the set of TAs within which it can roam without being required

to perform another TAU. The UE responds with a TAU Complete message, which finishes the process.






Paging

The main purpose of the TAU process is to ensure that the MME knows roughly where each UE is in the

event that there is inbound traffic to deliver. Paging will usually be triggered by the receipt of an S-GW

Downlink Data Notification at the MME, indicating that data has arrived at the S-GW on the S5/S8 portion

of a parked EPS Bearer.

If it becomes necessary to contact an idle UE (that is, a UE that has entered the ECM-IDLE state), theMME will employ the paging process. With no equivalent node to the RNC, EPS paging is managed

directly between the MME and eNBs. When a Paging message is to be sent, the MME checks the

current TA list stored for the target UE and inserts the paging data into the S1 paging messages sent to

all eNBs in the indicated TAs.

Each eNB inserts the UE’s NAS paging ID (IMSI or S-TMSI can be used) into the appropriate repetitions

of its PCH (Paging Channel). Paging groups may be established to reduce the number of repetitions of

the PCH that each UE is required to monitor; the operation of the paging reduction scheme is controlled

via cell-specific DRX (Discontinuous Reception) functions.

When a UE receives its paging ID on the PCH it initiates the service request process, which ensures that

any ‘parked’ EPS bearers are reactivated ready to carry traffic.

Further Reading: 3GPP TS 23.401:5.3.4; 36.300

LTE Air Interface



SECTION 3

OFDM PRINCIPLES

LTE Air Interface




Radio Carrier Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.1

Spectral Efficiency in OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2

Resilience to Time Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3

Resilience to Multipath Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.4

Concept Components for OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.5

Defining Orthogonality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.6

Principles of QAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.7

Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.8

The Fourier Transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.9

The OFDM Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.10

The OFDM Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.11

The Cyclic Prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.12

Sampling Factor Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.13

Subcarrier Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.14

Scaleability in OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.15

Scaleable OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.16

OFDMA Resource Allocation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.17

Subchannelization Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.18

Channel Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.19

OFDMA Allocation to Users . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.20

Turbo Coding for Error Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.21

Turbo Decoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.22

OFDM Peak to Average Power Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.23

SC-FDMA Principles of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.24

CONTENTS

LTE Air Interface




MIMO Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.25

The Benefits of MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.26

Multi-User MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.27

OFDM Principles




LTE Air Interface





outline the limitations of frequency division multiplexing

describe the conditions required to maintain adjacent radio carrier orthogonality

describe an OFDM transmission system as a set of closely spaced orthogonal radio

subcarriers

illustrate the potential performance benefit for the use of OFDM as opposed to single carrier

schemes

explain how the FFT (Fast Fourier Transform) is used to generate an OFDM signal

describe how QAM modulation and demodulation is achieved and apply this to OFDM systems

explain the use of the cyclic prefix in OFDM modulation symbols

explain how the sampling frequency and modulation rate relate to subcarrier spacing

identify typical performance characteristics of OFDM signals in multipath fading channels

list the options for multiplexing subchannels in OFDM systems

describe the key implementation considerations for OFDMA

describe how channel adaptation can be used to enhance the performance of OFDM systems

describe how scalability is achieved in OFDM systems

identify suitable error protection schemes for OFDM systems

identify how problems with power inefficiency can be dealt with in OFDMA systems

describe the basic principles of MIMO operation

identify the key benefits that can be gained from MIMO implementation

OBJECTIVES

OFDM Principles




LTE Air Interface




5 kHz

Spacing to next allocatedcarrier needs to be large

f 1

f 1 f 2


Radio Carrier Orthogonality

Consider a radio carrier being modulated by a 10 kbit/s bit steam using QPSK (Quadrature Phase Shift

Keying). It could be expected to see a spectral envelope following a (sin x)/ x function, as shown in the

diagram, with the first null located 5 kHz from the centre frequency.

In a classic FDM (Frequency Division Multiplexing) system, other radio carriers would be allocated and

spaced far enough away from the first to ensure minimal adjacent channel interference. The size of theguard band required would depend on the transmitter and receiver characteristics as well as the relative

powers.

However, in such a system it is assumed that there is no synchronization between the potential

interferers. It is this that leads to the need for large frequency spacing between adjacent carriers. In fact,

if there was synchronization between adjacent channels, a much smaller frequency spacing could be

used. The key is to be able to make use of the complex nature of the instantaneously transmitted

spectrum. The modulation envelope is only an artificial way of indicating all possibilities over time; a

snapshot at an instant in time would look different.

Consider a second radio carrier allocated such that its centre frequency coincides exactly with the null in

the first carrier’s envelope. It is using the same modulation scheme and carrying the same data rate.The result is as shown. Note that the carrier spacing of 5 kHz is the same magnitude as the symbol rate

of 5 ksps. The spectra of the two carriers now overlaps, but as long as the carrier frequencies and the

baseband data remain accurately synchronized, both can be demodulated successfully. The reason is

that this relationship between centre frequency offset and symbol rate maintains a high level of

orthogonality between the two radio carriers.

OFDM Principles



f 1 f 2

15 kHz

2 QPSK subcarriers

10 kbit/s per subcarrier

15 kHz total bandwidth

Centre frequency

f 1

Centre frequency

1 QPSK carrier

20 kbit/s

20 kHz total bandwidth

20 kHz


Spectral Efficiency in OFDM

Considering again the two overlapping QPSK radio carriers, it can be seen that there is a relatively large

spectral efficiency gain. If the effective bandwidth of the transmitted signal is considered to be the

frequency separation of the first nulls then a single QPSK carrier modulated with 10 kbit/s would have a

null-to-null bandwidth of 10 kHz.

However, here there are two sub-carriers, each of which is carrying 10 kbit/s using QPSK. Their respective null-to-null spectra overlap by 5 kHz. This gives a collective null-to-null bandwidth for the pair of

sub-carriers of 15 kHz. Thus QPSK is being used to carry 20 kbit/s in a radio bandwidth of 15 kHz. Note

that a single QPSK modulated carrier carrying 20 kbit/s would result in a null-to-null bandwidth of 20 kHz.

The principle of independent reception of orthogonal radio carriers with overlapping spectrum can be

extended by using a large number of narrowband radio carriers within one wideband channel allocation.

This results in a very spectrally efficient channel that can carry high bit rates.

For example, if 1000 orthogonal radio carriers were modulated using QPSK, each carrying 10 kbit/s, the

net throughput for the channel would be 10 Mbit/s. This would require a total channel bandwidth of slightly

more than 5 MHz. Carrying the same bit rate with QPSK modulation onto a single radio carrier would

require a null-to-null bandwidth of 10 MHz. Thus OFDM (Orthogonal Frequency Division Multiplexing)almost doubles the spectral efficiency. Moreover, the resulting OFDM transmission is more resilient to

multipath effects in the channel.

LTE Air Interface



High-bit-rate serial stream

S to P

Low bit rate parallel streams

Guardperiod

Useful symbolperiod

Multipath 1

Multipath 1

Multipath 1


Resilience to Time Dispersion

Spectral efficiency is not the only benefit associated with using OFDM. It also exhibits good tolerance to

the effects of multipath propagation in the channel; both fading and time dispersion.

Because the data rate on individual subcarriers with the channel is very low, the symbol period is

correspondingly long. The resulting symbol period is typically significantly longer than the time dispersion

that occurs in the channel. This means that relatively simple equalization can be used to counteractmultipath even though the net rate in the whole channel is very high.

Furthermore, a guard period can be inserted in every symbol period that covers the expected time

dispersion for the channel. This removes most of the time dispersion distortion from the useful symbol

period.

This guard period is usually created by repeating a copy of the last part of the symbol at the start. In this

case it is referred to as the cyclic prefix.

OFDM Principles




Resilience to Multipath Fading

Tolerance to multipath fading effects comes from the overall wideband characteristic in the channel. A

narrowband channel tends to exhibit flat fading characteristics; that is to say, the fading characteristics

are coherent across the whole channel bandwidth. The effects of this can be seen in the diagram.

OFDM channels, on the other hand, are usually used to carry very high data rates and therefore require

many subcarriers occupying a relatively large bandwidth. In most cases the bandwidth will exceed thecoherence bandwidth by a large factor, so differing fading characteristics will be seen in different parts of

the channel. In effect, the wide channel provides a degree of frequency diversity with a resulting

improvement in performance.

However, it would be wrong to assume that this benefit for OFDM results solely because the channel

bandwidth is wide. A single carrier system with the same bit rate would also result in a wide radio

channel. Therefore, a single carrier system also benefits from this form of frequency diversity to some

extent.

In the single channel system, energy from each symbol will be spread across the whole radio channel

and each symbol will therefore suffer some distortion from any fading that may occur in any one part of

the channel. In an OFDM system only those symbols transmitted on subcarriers in the part of the channelaffected by a fade will be distorted. Symbols transmitted on other subcarriers will remain unaffected. It is

then possible to adapt the subcarriers in use according to the varying fading characteristics. This means

that only non-fading carriers will be used.

LTE Air Interface



OFDM Operation

M-ary modulation

(or QAM)

Orthogonality in

sine wavesThe Fourier transform


Concept Components for OFDM

OFDM clearly offers significant benefits as a modulation strategy for a wideband radio system.

Understanding how OFDM can be made to work in practice means first gaining a basic understanding of

three concepts:

orthogonality in sine waves

M-ary modulation (or QAM)

the Fourier transform

OFDM is achieved through the convergence of these three key concept components.

OFDM Principles



Signalpoint

mapping

I

Q

Serial data

-1, +1

-1, +1

Odd

Even

QPSK

0,0

1,0

0,1

1,1

+1

-1

-1 +1

-1

+1

cos(2 f ct)

csin(2 f t)

-1 +1

Signalpoint

mapping

I

Q

Serial data

-3,-1, +1,+3Oddpairs

Even

pairs

cos(2 f ct) -1

+1

-3

+3

sin(2 f ct)

-1 +1-3 +3

-3,-1, +1,+3

+1

-1

-1 +1

-3

+3

+3-3

16QAM


Principles of QAM

All OFDM systems use some form of QAM (Quaderature Amplitude Modulation). Note that QPSK is a

special case of QAM where there are just four phase states and one amplitude state.

The diagram shows a basic QPSK modulation train. Serial input data is symbol mapped into two parallel

(I and Q) streams. In this example a logic 1 bit is mapped to a symbol with value –1 and a logic 0 bit is

mapped to a symbol with value +1. One pair of symbols at a time is multiplied onto the respective I and Qversions of the radio carrier. The I and Q signals are then added to produce the resultant QPSK signal

and four-point constellation.

The only modification required to enable the same modulation train to produce a higher level QAM signal

is in the symbol mapping. For example, to produce a 16QAM signal two bits are now mapped into each

of the I and Q streams in each symbol period. Therefore, groups of four bits in total are inputted during

each symbol period; the first and third are mapped into the I stream while the second and fourth are

mapped into the Q stream. The following symbol mapping rule is then used in each of the I and Q

streams:

01 maps to +3

00 maps to +1

10 maps to –1

11 maps to –3

The rest of the modulation train remains the same. These symbols are again multiplied onto the

respective I and Q versions of the radio carrier. Then the I and Q signals are added, this time producing a

resultant 16QAM signal and 16-point constellation. Note that as it is, this new constellation would result in

a higher average power. To avoid this the signal would also be multiplied by an appropriate scaling

factor.

OFDM Principles



Serial dataReceived signal

0

T

0

T

Symboldecision

Symboldecision

I/Q symbolto data

mapping

sin(2 f ct)

cos(2 f ct)


Demodulation

The received signal is separated into its I and Q constituent parts by multiplying it by the same I and Q

signals used in the modulator. The result is that on the I branch the cosine component will integrate to

zero and conversely, on the Q branch, the sine component will integrate to zero.

The output of the integration on each branch will therefore be of a magnitude dependent on the

transmitted I and Q symbol mapping. In effect this is amplitude demodulation. A threshold detector isthen used to recover the I and Q symbols. Finally, symbol-to-bit mapping is used to reconstruct the

original serial data stream.

LTE Air Interface



Serial

data

S P

M-ary

symbolbit

grouping

{b0, b

1, b

2…b }

M-ary

symbol

mappingN

parallel

streams

N

complex

symbols

N -pointIFFT

I (real)

Q (imaginary)

N complexsamples in onesymbol period

sinecosine

Up-conversion

D/A

f c

OFDM signal with

N subcarriers

n


The OFDM Transmitter

The diagram shows a block representation of the transmitter that brings together the elements of symbol

mapping for QAM and the application of the IFFT in order to produce an OFDM signal.

The serial data to be carried on the radio link is first passed through a serial-to-parallel conversion process.

The number of parallel streams will be equivalent to the number of data-carrying subcarriers in the system.

This number will usually be a power of two since this makes best use of the efficiencies offered by the IFFT.

Bits on the parallel data streams will also be grouped as appropriate for the symbol constellation of M-ary

QAM scheme in use. For example, for QPSK bits are grouped in pairs; for 16QAM they are grouped in fours

and for 64QAM they are grouped in sixes.

The next process is symbol point mapping for the bit groups on each parallel data stream. The resulting

complex number symbols then form the input to an N-point IFFT where N will be a power of two equivalent

to the number of subcarriers in use.

The output of the IFFT will be a series of complex number digital samples representing the OFDM signal

during each symbol period. At this point the cyclic prefix is added by copying the last samples onto the

beginning of the symbol period. These complex real and imaginary sample values are used to form the Iand Q symbol streams. Next, the I and Q branches are subsequently multiplied onto sine and cosine

representations of the radio carrier. This generates a digital representation of the required multicarrier M-ary

QAM modulated transmit signal.

After digital-to-analogue conversion the resulting signal can be up-converted to the required channel centre

frequency before amplification and transmission.

LTE Air Interface



Serial

data

P S

{b0, b

1, b

2…b }

N

parallel

streams

N

complex

symbols

N -point

FFT

I (real)

Q (imaginary)

N complex

samples in onesymbol period

sinecosine

Down-conversion

A/D

f c

OFDM signal with

N subcarriers

n

Integration

and

symbol

decisions


The OFDM Receiver

The filtered OFDM signal is down-converted and then sampled for analogue to digital conversion. The

sampling rate at this point will be factored to allow for the inclusion of the cyclic prefix.

The cyclic prefix is removed and the sampled signal is separated into I and Q components. The result is a

series of complex samples that are used as the input to the FFT.

The FFT deconstructs the complex waveform in the symbol period to N complex values, each

representing a modulation symbol on one of the subcarriers. M-ary demodulation by integration and

reverse symbol mapping is performed to recover groups of bits represented by each of the received M-ary

modulation symbols.

Finally, parallel-to-serial conversion reconstructs that original serial bit stream.

OFDM Principles



Usable Symbol

Symbol Period

CP

T g T d

T s

Last x samples

of symbol


The Cyclic Prefix

The OFDM cyclic prefix is designed to combat the ISI (Inter Symbol Interference) effects caused by

multipath and other channel impulse response effects. Multipath causes ‘echoes’ of a previous part of the

signal that, having travelled via a longer path than the primary component of the signal, arrive later in

time.

The cyclic prefix eliminates or masks the effects of ISI, as long as the cyclic prefix period is longer thanthe maximum delay spread suffered by the signal.

The cyclic prefix is formed by taking a portion of the ‘useable’ part of each OFDM symbol and copying it

onto the beginning of the symbol period. As can be seen in the diagram the total OFDM symbol period

(Ts) is the sum of the useful symbol period (Td) and the cyclic prefix (Tg).

The copied samples in the cyclic prefix fill what would otherwise have been an empty guard period. This

ensures that any sample window within Ts and equivalent in duration to Td will always contain a whole

number of cycles for each of the received subcarriers, thus maintaining orthogonality.

The cyclic prefix ratio effectively reduces the bit rate carried relative to the total bandwidth required in the

radio channel, which has potentially significant consequences for the bandwidth efficiency of a channel.However, the consequences tend to be outweighed by the benefits in terms of minimized ISI.

Nevertheless, better performance can be obtained if the relative duration of the cyclic prefix and the

useful symbol period can be varied within a fixed total symbol period in sympathy with prevailing channel

conditions.

LTE Air Interface



10 MHz

512-point FFT

Subcarrier spacing approximately 19.5 kHz

Data symbol period approximately 51.3 μs

Cyclic prefix is set at 1/8

Example

512 samples in 51.3 μs

Nominal sampling frequency is 10 MHz512 samples

T d (51.3 μs)

585 samples in 51.3 μs

Required sampling frequency is 11.4 MHz

Sampling factor is 8/7

T s

T d T g

512 samples73

73


Sampling Factor Considerations

There are many interrelated figures that describe the characteristics of any given OFDM system. These

include the overall data rate, the number of subcarriers, the subcarrier spacing, the size of the FFT and

the cyclic prefix ratio. The sampling rate for the receiver is related, either directly or indirectly, to all of

these.

Ultimately, the data rate applied to the subcarriers and the M-ary QAM constellation in use willdetermine the data symbol duration Td. The number of samples required during this period is

determined by N, corresponding to the size of the FFT.

However, the requirement for cyclic prefix must be factored in and this results in a higher sampling rate

requirement. The ratio between the nominal sampling rate and the required sampling rate is generally

referred to as the sampling factor and is an important system design parameter. This is illustrated with

an example in the diagram. In this example an OFDM system is operating in a total channel bandwidth

of 10 MHz with a 512-point FFT. This requires a subcarrier spacing of approximately 19.5 KHz.

The bandwidth of each subcarrier will therefore be 39 kHz with a nominal data symbol period of 51.3 μs.

With a 512-point FFT it would be necessary to sample 512 times in each 51.3 μs symbol period.

Therefore the nominal sampling rate will be 10 MHz. However, this does not account for the inclusion of a cyclic prefix.

In practice the cyclic prefix is included not by extending the symbol period, but by reducing the useful

part of the symbol period Td in order to make space for the cyclic prefix Tg. In the example, the cyclic

prefix is set such that it occupies one eighth of the total symbol period Ts. In order to maintain

orthogonality it is still necessary to take 512 samples in the period Td. This requires a new higher

sampling rate, which is maintained through the cyclic prefix. Since the Tg is one eighth of Ts, it is now

necessary to sample 585 times in the total symbol period Ts. This corresponds to a required sampling

rate of 11.4 MHz.

The ratio of the required sampling rate of 11.4 MHz and the nominal sampling rate of 10 MHz is the

sampling factor. In this case the ratio is 8/7.

OFDM Principles



Data subcarriersReference/pilotsubcarriers

Upper

unused/guard

Subcarriers

DCsubcarrier

Lower

unused/guard

Subcarriers


Subcarrier Assignment

Different subcarriers from across the population of subcarriers created by an OFDM channel are

assigned to different functions. Most subcarriers will be assigned to carry modulated user data signals.

Each data subcarrier will be modulated to carry one part of the entire parallel signal being transmitted

across the multi-tone channel. The data rate of each data subcarrier is determined by a combination of

the symbol rate and the modulation scheme employed.

In some variants of OFDM (such as that employed by WiMAX), entire subcarriers are given over to

carrying ‘pilot signals’. Pilot subcarriers allow channel quality and signal strength estimates to be made

and allow other control functions, such as frequency calibration, to operate. Pilots are generally

transmitted at a higher power level than data subcarriers – typically 2.5 dB higher – which serves to

make them more easily acquired by receiving stations.

In LTE and other systems, including DVB (Digital Video Broadcasting), the same function is performed by

‘reference signals’. A reference signal, like a pilot, allows a receiving station to recalibrate its receiver and

make channel estimates, but instead of occupying an entire subcarrier it is periodically embedded in the

stream of data being carried on a ‘normal’ subcarrier.

There are also two types of ‘null’ subcarrier – guards and the DC carrier. Nothing is transmitted on nullsubcarriers.

Guard subcarriers separate the top and bottom data subcarriers from any adjacent channel interference

that may be leaking in from neighbouring channels and, in turn, serves to limit the amount of interference

caused by the OFDM channel. The more guard subcarriers that are assigned, the lower the amount of

adjacent channel interference that will be caused or detected, but this also has an impact on the data

throughput of the channel.

The centre subcarrier of each OFDM channel – the one that has a 0 Hz offset from the channel’s centre

frequency – is known as the ‘DC carrier’ and is also null.

LTE Air Interface



Initialbandwidth

Orthogonality lost

Increasedbandwidth

Increasedbandwidth

Orthogonality recovered


Scaleability in OFDM

An OFDM technology may need to be able to operate not only in different frequency bands but also in

different OFDM channel bandwidths.

If the number of subcarriers remains fixed but the bandwidth of the overall OFDM channel is increased,

then the separation between the subcarriers will also increase in proportion. Increased subcarrier

separation means that the symbol rate on each subcarrier must be increased in proportion, otherwise theorthogonality between subcarriers will be lost.

The top diagram shows a set of subcarriers operating orthogonally.

The centre diagram shows an example where the OFDM channel bandwidth has been increased while

the FFT size and subcarrier symbol rate have been maintained. Subcarrier spacing has increased as a

result and it can be seen that the subcarriers are no longer orthogonal.

In the lower diagram the OFDM bandwidth has been increased by the same proportion and again the

FFT size has not been changed. This time, however, the symbol rate on the subcarriers has also been

increased in proportion and orthogonality has been recovered.

Thus it is possible to design an OFDM system that can be reconfigured to operate in variable OFDM

channel bandwidths. Nevertheless, this is not the most efficient approach. The system would be more

flexible and perform better if the symbol rate on the subcarriers did not need to be adjusted for different

OFDM channel bandwidths.

OFDM Principles



5 MHz

256-point FFT

10 MHz

512-point FFT


Scaleable OFDM

It turns out that a simpler solution for operating in different bandwidths is to adjust the FFT size rather

than the subcarrier symbol rate. This means that most of the key system parameters remain the same for

different bandwidths and gives greater opportunity for taking advantage of channel adaptation as the

overall channel bandwidth is increased.

This approach is often referred to as scalable OFDM. The diagram shows a system operating in a 5 MHzchannel with an FFT size of 256 and then the same system operating in a 10 MHz channel with an FFT

size of 512. The symbol rate and therefore the required subcarrier separation is the same in both cases

and orthogonality is maintained.

LTE Air Interface



0 1 2 3 4 5 6 7 8 9

Symbol periods (time)

OFDM with

time

multiplexing

User 1 User 2 User 4

0 1 2 3 4 5 6 7 8 9Symbol periods (time)

User 2User 4

OFDM with time

and frequency

multiplexing

(OFDMA)

User 9

User 8

User 1User 3

User 7


OFDMA Resource Allocation Strategies

The simplest option for multiple access in an OFDM system is to use a form of time multiplexing on the

OFDM radio bearer. This is illustrated in the top part of the diagram. Each user is allocated the full

channel bandwidth and all data subcarriers exclusively for a defined number of symbol periods.

The greatest efficiency can be achieved if dynamic time allocation is applied so that users with higher bit

rate requirements are allocated a greater proportion of time. However, in such a system the minimumresource allocation is one OFDM symbol. Even with dynamic time allocation, such an arrangement can

still become very inefficient when there is strong demand for multiple lower bit rate connections, for

example when multiple voice circuits are active. Consider an OFDM system operating in a 10 MHz

bandwidth, with a 512-point FFT and using 16QAM. Allowing for null and reference subcarriers, such a

system could transfer in the order of 1,600 bits in a single OFDM symbol period. This may seem a

modest resource unit, but delay requirements must also be accounted for. For a real-time service such

as voice it is essential to avoid excessive round-trip delay. To meet the delay requirement for a voice

service, resources may need to be allocated, for example once every 20 ms. This would mean in a

minimum bandwidth allocation to one user of 80 kbit/s (or 120 kbit/s if 64QAM is in use). Even allowing

for the error protection overhead this minimum resource will significantly reduce system efficiency and its

ability to benefit from optimal techniques such as discontinuous transmission and channel adaptation.

Greater efficiency in resource allocation can be gained from the use of subchannelization. This involves

division of resource by time and by frequency. Thus a user may be allocated a subset of the subcarriers

available in the system, as illustrated in the lower part of the diagram. This approach allows much finer

granulation in resource allocation and therefore greater efficiency. OFDM systems that support this are

usually described as OFDMA (Orthogonal Frequency Division Multiple Access) systems.

OFDM Principles



OFDM channel bandwidth

Subchannels

0 1 n2

Localised

subcarriers in

one subchannel

OFDM channel bandwidth

Subchannels

0 1 n2

Distributed

subcarriers in

one subchannel


Subchannelization Organization

In practice, subchannelization is generally achieved through the structured grouping of all available

subcarriers into a defined set of subchannels. The smaller the group of subcarriers in each subchannel

the finer the granularity of resource allocation.

In some systems the subchannel definition may be fixed and resource allocation will simply involve the

identification of one or more subchannels for one or more symbol periods. However, system performancecan be enhanced if the subchannel structure can be made flexible.

The most significant aspect of subchannel structure is the way that grouped subcarriers are allocated

from across the channel bandwidth. There are essentially two options: they may be localized or

distributed.

The top part of the diagram shows a subchannelization organization where subcarriers within a

subchannel are localized. Since all the subcarriers in a subchannel are similar in frequency there is little

or no frequency diversity gain. The benefit in this strategy lies in the high correlation in propagation

characteristics within the subcarrier set. This high correlation enables more accurate channel estimation,

and therefore adaptation, to be applied. In particular this approach gives the best performance when

optimal techniques such as MIMO (Multiple Input Multiple Output) antenna systems are used.

The lower part of the diagram shows a subchannelization organization where subcarriers within a

subchannel are distributed across the channel bandwidth. This approach has the significant advantage

that it maximizes frequency diversity gain for the subchannel.

LTE Air Interface



Modulation (QPSK/16QAM/64QAM)

Error protection coding rate

Adaptive fast scheduling

Node BPoor Radio Path

Interference


Channel Adaptation

The quality of the radio link is affected by many factors including fading, interference and time dispersion.

Terrestrial mobile radio channels, which are usually assumed to be non-line of site, can be very poor.

Therefore most terrestrial cellular radio systems are designed with robust modulation schemes and large

error protection overheads.

However, close examination of real channel conditions shows them to be very variable in short timeframes, and much of the time any given channel will show good performance. Thus the standard

approach engineers the channel to deal with the worst case, which only occurs for a small amount of

time.

It is clear that if the channel could be adapted at a rate fast enough to track changing channel conditions

then the average performance of a channel could be significantly improved. This is the principle of

channel adaptation. Channel adaptation is a common approach in many broadband radio systems and in

most cases involves the adaptation of the modulation scheme and the error protection overhead applied.

Adaptive scheduling can also be very effective, enabling the cell to make the best use of the pool of

channels allocated to different mobiles, each of which will be varying independently.

OFDM Principles



Transmitpower

User ‘n’

DCsubcarrier

1 2 3 4 5 6 7 8 9 10 11

64QAM

16QAM

QPSK

Reference subcarriers


OFDMA Allocation to Users

In an OFDMA system groups of subcarriers will be allocated to multiple users’ devices from within the

total channel bandwidth. Thus multiple users can be served simultaneously at any given time. In a fast

scheduling system this allocation can change very rapidly. Additionally, at any given time a number of

subcarriers will be in use as reference signals. The position of these reference signals may also change

with time.

The diagram shows an example where a localized allocation of subcarriers is being used to form users’

subchannels. It would also be possible for a user’s allocation of subcarriers to be distributed through the

total channel bandwidth and for the distribution pattern to change on a symbol-by-symbol basis.

Where channel adaptation is being used different transmit powers, modulation scheme and error

protection overhead will be defined for each allocated resource as appropriate for service requirements

and radio channel conditions.

LTE Air Interface



Iteration ‘n’

Iteration 2Constituent decoder 2




Received

channel

data block

Error corrected

output

Partially decoded

data and confidence

information

Turbo decoder


Turbo Decoding

The specific operation of Turbo decoding algorithms is extremely complex, but the basic principle is

illustrated in the diagram. Turbo decoding is an iterative process that utilizes confidence information on

each iteration. Each alternate iteration is based on either a constituent coder 1 or coder 2 decode. The

output of each iteration provides reliability information for the subsequent decode.

This process is repeated until there is very little improvement in the reliability of the decode withsuccessive iterations, typically 18 to 20 iterations. Working in this manner provides a considerable

improvement in error correction performance when compared to a single convolutional coder. However,

the decode algorithms are more complex and the multiple iterations must be performed very rapidly to

avoid cumulative delay. Additionally, the improvement in performance is most noticeable for large data

block sizes.

LTE Air Interface



Peak

Average

Single carrier

Single carrier

Peak

Average

Multi carrier

Non-liner operationPeak power

Single carrier average

Multi carrier average

Power back-off

Power back-off


OFDM Peak to Average Power Ratio

A chief disadvantage of OFDM systems is the poor power efficiency achievable in the power amplifier.

There are two main reasons for this. The first relates to the use of QAM modulation, which necessitates

the use of a highly linear power amplifier in order to meet the ACLR (Adjacent Channel Leakage Ratio)

performance requirements. This would be true for any system using QAM modulation, UMTS for

example, but in OFDM systems this is compounded by a second factor relating to the multi-carrier nature

of the OFDM signal itself.

The OFDM signal is the sum of multiple sinusoids at different frequencies and each carrying different

modulation symbols. The sum of the sinusoids produces a complex waveform with a significantly higher

peak to average ratio than a single carrier signal would have. As different combinations of transmit

symbols occur on the concurrent subcarriers, the summed transmitted signal exhibits the periodic

occurrence of a high PAPR (Peak to Average Power Ratio). The larger the number of subcarriers the

more frequently this condition occurs. The result is that in order to maintain linear operation in the power

amplifier, larger power back-off must be provided. In turn this means either less efficient or more

expensive power amplifier solutions.

There are a number of strategies that can be applied in OFDM systems to mitigate the problems of the

high PAPR. Most relate to power amplifier design and signal conditioning. These techniques are used inthe LTE downlink, which is a pure OFDMA system. However, both cost and power efficiency are very

critical in the UE, so 3GPP has opted to use modification of OFDMA in the uplink known as SC-FDMA

(Single Carrier Frequency Division Multiple Access). As the name suggests this technique gives the

transmitted signal a single carrier characteristic, which in turn reduces the PAPR.

OFDM Principles



Serial

dataModulation

symbolmapping

andgrouping

M -point

DFT

M parallel

modulationsymbols

N

complex

symbols(inc. 0’s)

N -pointIFFT

I (real)

Q (imaginary)

N complexsamples in onesymbol period

sinecosine

Up-conversion

D/A

f c

OFDM signal with

N subcarriers

(inc. those at zeroamplitude)

0

0


SC-FDMA Principles of Operation

The typical layout of an SC-FDMA transmitter is shown in the diagram. Despite its name, the transmitted

radio signal for SC-FDMA is an orthogonal multicarrier transmission similar to OFDMA. The term ‘single

carrier’ refers to the preprocessing of the baseband data prior to its application to the IFFT.

The data stream is presented in a serial fashion to the radio transmission process. The first stage is

modulation symbol mapping, which produces parallel blocks of ‘M’ complex valued symbols. The number of bits represented by each of the ‘M’ modulation symbols is dependent on the modulation scheme in

use.

Each group of ‘M’ modulation symbols is then presented to an M-point DFT, which produces an output

effectively representing the frequency components of the group of modulation symbols. It is then these

frequency components that are mapped to the allocated inputs of the N-point IFFT, where N is the total

number of subcarriers available. The diagram shows localized mapping, but a distributed mapping to

allocated subcarriers would also be possible.

The output of the IFFT and modulator will be a multi-carrier transmission. However, unlike OFDMA there

is not a direct mapping of each baseband symbol onto individual transmitted subcarriers. Instead, the

frequency components of each baseband symbol are now represented across all the transmittedsubcarriers, it is this that gives the transmitted signal a ‘single carrier’ characteristic. The result is a

transmitted signal with an improved PAPR compared to an equivalent OFDMA transmission.

LTE Air Interface



Data

stream

mapping

Pre-

coding

matrix

Signal

generation

MIMOdecoding

and channelestimation

Stream 1

Stream 2

Layer 1

Layer 2

Powerweightings and

beamforming

Feedback

2x2 MIMO or Rank 2 4x4 MIMO or Rank 4


MIMO Concept

MIMO (Multiple Input Multiple Output) antenna arrays offer significant performance improvements over

conventional single antenna configurations.

The technique involves placing several uncorrelated antennas at both the receiving and transmitting ends

of the communication link. If there are four uncorrelated antennas at the transmitter and a further four

uncorrelated antennas at the receiver, then there will be 16 possible direct radio paths between thetransmitter and the receiver. Each of these is open to multipath effects, creating even more radio paths

between the transmitter and the receiver. These radio paths can then be constructively combined, thus

producing micro diversity gain at the receiver.

Since the receiver can distinguish between the various uncorrelated antennas, it is possible to transmit

different data streams in different paths. The stream applied to each antenna can be referred to as a

‘layer’ and the number of antennas available at the transmitter and receiver can be referred to as ‘rank’.

For example, a system operating with a 4x4 MIMO antenna array can be described as having four layers

and being of rank four. The way in which data streams are mapped to layers will change the specific

benefits offered by a particular MIMO implementation, and the specification of this is an important part of

system design. Pre-coding may also be used to improve the MIMO system performance. Pre-coding may

be adaptive and as such would be based on some source of channel estimation. This could be derived atthe transmission or the reception end of the link.

It is relatively easy to mount antennas on the base station in an uncorrelated manner. For a 2x2 MIMO

array a single cross-polar panel could be used. A 4x4 MIMO array would require two cross-polar panels

with suitable special separation. This is harder to achieve in a mobile. However, as for the base station,

2x2 MIMO could be achieved with cross polarization, but this could result in some undesirable directivity

in the antenna.

OFDM Principles



MIMO brings

Diversity gain Array gain Spatial multiplexing gain

Decorrelates fading

through different

transmission paths

Provides a beamforming

effect that focuses

radiated energy in the

direction of the receiver

Enables multiple data

streams to be transmitted

on the same

frequency/time resource


The Benefits of MIMO

MIMO is potentially a complex technology but it can provide very significant benefits in system capability.

There are three key ways in which MIMO improves system performance. Any given MIMO

implementation may make use of all these benefits or may be configured to take particular advantage of

one of them. Ideally, a system should be designed with sufficient flexibility in MIMO implementation to

allow a system operator to choose the most suitable implementation for different environments or system

goals.

Diversity gain arises out of the provision of multiple antennas at the transmitting and/or receiving end of

the radio link. This creates multiple transmission paths with decorrelated fading characteristics. The

result is an overall improvement in channel signal-to-noise ratio leading to increased channel throughput

and reliability.

Array gain refers to the beamforming capability of a multiple antenna array. With suitable signalling of

feedback from the receiver, or with measurements made on a return link, it is possible to direct radiated

energy toward the receiver in a steered beam. The result is improved channel performance and

increased throughput.

Spatial multiplexing gain arises out of the orthogonality between the multiple transmission paths createdby the multiple antenna array. Since the receiver can resolve independent transmission paths it is

possible to map different information streams into the transmission paths, identifiable by their spatial

signature. This results in a direct increase in the channel throughput in proportion to the number of

separate transmission streams used.

LTE Air Interface



SU-MIMO MU-MIMO

Multi-Cell MU-MIMO


Multi-User MIMO

The basic implementation of MIMO is generally referred to as SU-MIMO (Single-User MIMO).

The SU-MIMO concept can be developed into MU-MIMO (Multi-User MIMO). In this case the spatial

multiplexing capability of MIMO is used to multiplex a link to more than one mobile using the same

time/frequency resource. The order of multiplexing available depends on the number of antennas (or

rank) available at the transmitter and receiver ends of the link. For example, the diagram shows a 2x2MIMO arrangement being used for MU-MIMO with two mobiles. In this case, the rate available to each

mobile would be lower than that potentially available to a single mobile with an SU-MIMO configuration,

but both mobiles are allocated the same time/frequency resource and still have the potential for diversity

and array gain. Thus cell capacity is increased, but the resource can be shared between a larger number

of users. The use of more than one transmitting or receiving station in this way is sometimes called

virtual MIMO.

It is also possible to implement MU-MIMO in one direction only with just single antennas on each of the

mobiles. In this case, array and diversity gain would be reduced, but time/frequency resources can still

be reused in the cell.

MU-MIMO can be further developed into multi-cell MU-MIMO. In this case the data streams are mappedto the combined antenna resources of two or more base stations that provide a combined connection to

multiple mobiles in multiple cells. The scenario in the diagram is in effect 4x4 MIMO but shared between

two connections. Note that spatial diversity will be significant in such a scenario because of the

geographical separation of the base station and of the mobiles.

OFDM Principles




LTE Air Interface



SECTION 4

PHYSICAL LAYER STRUCTURE

LTE Air Interface




Physical Layer Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.1

Channel Bandwidths and Subcarriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.3

Frequency Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.4

Bandwidth Applicability in LTE Bands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.5

Radio Channel Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.6

OFDMA Parameter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.7

Modulation and Error Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.8

Physical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.9

The Physical Layer Timing Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.10

Type 1 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.11

Type 2 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.12

Resource Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.13

Downlink Reference Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.14

UE-Specific Reference Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.15

Uplink Demodulation Reference Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.16

Uplink Sounding Reference Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.17

Synchronization Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.18

PBCH Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.19

DC Subcarrier Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.20

Downlink Control Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.21

Downlink Control Channel Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.22

Downlink Resource Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.23

PUCCH Resource Allocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.25

Resource Allocation for PUSCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.26

CONTENTS

LTE Air Interface




Resource Allocation for PRACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.27

Summary of the Downlink Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.28

Summary of the Uplink Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.29

Physical Layer Structure




LTE Air Interface





outline the general structure of the E-UTRA physical layer

define the term ‘bandwidth agnostic’ in the context of E-UTRA

define the term ‘basic timing unit’ and its relevance in E-UTRA

outline parameters of the versions of OFDMA and SC-FDMA employed by E-UTRA

describe the configuration of downlink and uplink frames and list the range of frame types

employed

describe the resource allocation models employed by E-UTRA including the role of the

resource block, resource grid and resource element

list the modulation and error coding options made available in E-UTRA

outline the function of the reference signal and the purpose of UE sounding

describe the arrangements made for managing synchronization and timing control functions

describe the functions of the E-UTRA physical channels on both uplink and downlink

outline how control and traffic channels are mapped into the physical layer structure

OBJECTIVES





LTE Air Interface




RRC

MAC

Physical Layer

Transportchannels

Logical

channels

Physical channels

OFDM and SC-FDMABandwidth agnostic

TDD and FDDSFN for MBMSMIMO operation

Physical channel structure

Reference signalsModulation and coding

Synchronization and timingError coding and HARQ

Random accessPower control

Reporting and feedbackMeasurements

Handover


Physical Layer Functions (continued)

E-UTRA supports services in a variety of channel bandwidths. In fact, the specification explicitly labels

E-UTRA as ‘bandwidth agnostic’, meaning that it has no rigidly defined or preferred channel bandwidth

and can be scaled to channels of almost any size. Both FDD and TDD modes are supported, as is a

‘half duplex’ mode.

E-UTRA has also been designed to work as the bearer for Multicast and Broadcast Multimedia Services(MBMS) and as such includes support for SFN (Single Frequency Network) operation.

Support for advanced antenna configurations has also been designed into the specification with MIMO

and beam-forming adaptive antennas both being referenced.


LTE Air Interface



Channel bandwidths

(bandwidth/subcarriers)

1.4 MHz/72

3 MHz/180

5 MHz/300

10 MHz/600

15 MHz/900

20 MHz/1200


Channel Bandwidths and Subcarriers

E-UTRA/LTE is designed to work in a variety of bandwidths ranging initially from 1.4 MHz to 20 MHz. As

E-UTRA is described as being ‘bandwidth agnostic’, other bandwidths, ones that allow E-UTRA to be

backwards compatible with channel allocations from legacy network types, for example, could be

incorporated in the future.

The version of OFDMA employed by E-UTRA is similar to the versions employed by WiMAX or DVB, butwith a few key differences. In systems such as WiMAX, OFDMA schemes occupying different channel

bandwidths employ different subcarrier spacing, meaning that there is a different set of physical layer

parameters for each version of the system.

The E-UTRA scheme allows for two fixed subcarrier spacing options, 15 kHz in most cases, with an

optional 7.5 kHz spacing scheme, only applicable for TDD operation and intended for in very large cells

in an SFN. Fixing the subcarrier spacing reduces the complexity of a system that can support multiple

channel bandwidths.

Further Reading: 3GPP TS 36.211, 36.101:5.5, 36.104:5.5




FDD

Band UL Range (MHz) DL Range (MHz)

1 1920 – 1980 2110 – 2170

2 1850 – 1910 1930 – 1990

3 1710 – 1785 1805 – 1880

7 2500 – 2570 2620 – 2690

8 880 – 915 925 – 960

13 777 – 787 746 – 756

... ... ...

20 832 – 862 791 – 821

24 1626.5 – 1660.5 1525 – 1559

... ... ...

... ... ...

... ... ...

TDD

Band UL/DL Range (MHz)

33 1900 – 1920

34 2010 – 2025

35 1850 – 1910

36 1930 – 1990

37 1910 – 1930

38 2570 – 2620

39 1880 – 1920

40 2300 – 2400


Frequency Bands

There is considerable regional variation in the availability of spectrum for LTE operation and this is

reflected in the standards. Along with flexibility in bandwidth there is considerable flexibility for spectrum

allocation. There are no requirements for minimum band support nor for band combinations. It is

assumed that this is determined by regional requirements

The standards currently identify 19 bands for FDD operation, ranging from frequencies of approximately700 MHz through to frequencies in the range 2.7 GHz. There also eight bands identified for TDD

operation ranging from approximately 1900 MHz to 2.6 GHz. Considerable scope has been left in the

standards to add more frequency bands as global requirements evolve.

Further Reading: 3GPP TS 36.101; 5.5, TS 36.104; 5.5

LTE Air Interface



FDD

Band

1

2

3

4

56

7

8

9

10

11

12

13

14

...

17

1819

20

21

1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz

... ... ... ... ... ...

TDD

Band

33

34

35

36

3738

39

40



Bandwidth Applicability in LTE Bands

Not all bandwidths are mandatory in all bands. Those bandwidths that are mandatory for a UE supporting

each given band are shown in the table.

Further Reading: 3GPP TS 36.101; 5.6.1




Channel bandwidth (MHz)

Transmission bandwidth configuration (n x RB)

Transmission

bandwidth (n x RB)

12 subcarriers

EARFCN(100 kHz raster)


Radio Channel Organization

For both uplink and downlink operation subcarriers are bundled together into groups of 12. This grouping

is referred to as an RB (Resource Block). The RB also has a dimension in time and when this is

combined with the frequency definition it forms the basic unit of resource allocation.

The number of resource blocks available in the system is dependent on channel bandwidth, varying

between 100 for 20 MHz bandwidth to just six for 1.4 MHz channel bandwidth. The nominal spectralbandwidth of an RB is 180 kHz for the standard 15 kHz subcarrier spacing. Note that this means there is

a difference between the stated channel bandwidth and the transmission bandwidth configuration, which

is expressed as n x RB. For example, in a 5 MHz channel bandwidth the transmission bandwidth would

be approximately 4.5 MHz. This difference acts as a guard band.

OFDMA channels are allocated within an operator’s licensed spectrum allocation. The centre frequency

is identified by an EARFCN (E-UTRA Absolute Radio Frequency Channel Number). The precise location

of the EARFCN is an operator decision, but it must be placed on a 100 kHz raster and the transmission

bandwidth must not exceed the operator’s licensed spectrum.

Further Reading: 3GPP TS 36.101:5.6, 5.7; 36.104:5.6, 5.7

LTE Air Interface




Frame duration

Subframe duration

Carrier spacing

Number of subcarriers

Assumed FFT size

Sampling frequency (MHz)

Cyclic prefix duration

72 180 300 600 900 1200

128 256 512 1024 1024 2048

1.92 3.84 7.68 15.36 15.36 30.72

10 ms

1 ms

15 kHz (7.5 kHz)

Normal 5.2/4.7 μs, Extended 16.7 μs (33.3 μs)


OFDMA Parameter Summary

E-UTRA is based on a 10 ms frame structure, which is then divided into subframes of duration 1 ms. This

is common for all physical layer options. For most physical layer options the subcarrier spacing is 15 kHz.

This is a relatively large value compared to other OFDMA systems; however, it has been chosen to

provide a good compromise between the requirements for minimizing cyclic prefix overhead and

susceptibility to Doppler shift. This in turn means E-UTRA can operate for high-mobility connections.

The timings in the physical layer are defined around an assumed FFT size of 2048 and a consequent

sampling frequency of 30.72 MHz. This has been chosen because it is a multiple of eight of the UTRA

basic timing frequency 3.84 MHz, thus simplifying backward compatibility. However, practical

implementations may, if the manufacturer chooses, use smaller FFT sizes and proportionate sampling

frequencies for narrower bandwidth as shown in the table.

Note that in all cases the number of used subcarriers will be smaller than the FFT size.

Two different cyclic prefix sizes are defined for E-UTRA, known as ‘normal’ and ‘extended’. This is to

allow optimal configuration in different cell scenarios. In general, the longer cyclic prefix would be used in

large rural or suburban cells providing improved tolerance to time dispersion at the expense of overall

throughout in the channel.

For the TDD mode of operation only, there is an option for a reduced subcarrier spacing. This results in a

doubling of the symbol period, enabling the use of a doubled ‘extended’ cyclic prefix, which is designed

for use in broadcast single frequency networks.





Modulation

Schemes

Error Coding

Schemes

CRC

BPSK

QPSK

16QAM

64QAM

Signalling functions only

Optional on uplink

1/3 Turbo CodingTraffic and mostcontrol channels

1/3 CC BCH only

Transport Block 24 bit CRC


Modulation and Error Protection

The range of modulation schemes used in E-UTRA comprises BPSK (Binary Phase Shift Keying),

QPSK (Quadrature Phase Shift Keying), 16QAM (16-state Quadrature Amplitude Modulation) and

64QAM (64-state Quadrature Amplitude Modulation). BPSK is only employed for a limited set of

signalling and reference functions, while 64QAM is optional on the uplink.

The range of error coding options used in E-UTRA devices is far more limited than those available to, for example, a UMTS device. For most channels the only option is one-third rate turbo coding based on

convolutional coding.

Broadcast traffic channels are only permitted to use 1/3 Tail Biting convolutional coding. Various control

channels have been assigned either convolutional coding, block coding or simple repetition as their error

coding options.

In addition to error coding, transport blocks containing user and control traffic may also optionally have a

CRC block attached. Transport blocks on connections that have CRC (Cyclic Redundancy Check)

selected have a 24-bit CRC block appended to the end of the data container.

The familiar UMTS error monitoring levels of Bit Error Rate (BER), derived from the error coding service,and BLER (Block Error Rate), derived from CRC, continue to be available in E-UTRA.

Further Reading: 3GPP TS 36.211, 36.212, 36.300

LTE Air Interface



Physical signalsPSS/SSS

Reference signals

Physical

layer

MACBCCH PCCH CCCH DCCH DTCH

BCH PCH RACH DL-SCH UL-SCH

PBCH PDCCH PHICH PCFICH PRACHPUCCH PDSCH PUSCH

MAC

Control


Physical Channels

The physical layer involves the transmission and reception of a series of physical channels and physical

signals. The physical signals relate to the transmission of reference signals, the PSS (Primary

Synchronization Signal) and the SSS (Secondary Synchronization Signal).

The PBCH (Physical Broadcast Channel) carries the periodic downlink broadcast of the RRC

MasterInformationBlock message. Note that system information from BCCH is scheduled for transmission in the PDSCH.

The PDCCH (Physical Downlink Control Channel) carries no higher layer information and is used for

scheduling uplink and downlink resources. Scheduling decisions, however, are the responsibility of the

MAC layer, therefore the scheduling information carried in the PDCCH is provided by MAC. Similarly the

PUCCH (Physical Uplink Control Channel) is used to carry resource requests from UEs that will need to

be processed by MAC.

The PHICH (Physical Hybrid ARQ Indicator Channel) is used for downlink ACK/NACK of uplink

transmissions from UEs in the PUSCH. It is a shared channel and uses a form of code multiplexing to

provide multiple ACK/NACK responses.

The PCFICH (Physical Control Format Indicator Channel) is used to indicate how much resource in a

subframe is reserved for the downlink control channels. It may be either one, two or three of the first

symbols in the first slot in the subframe.

The PRACH (Physical Random Access Channel) is used for the uplink transmission of preambles as part

of the random access procedure.

The PDSCH (Physical Downlink Shared Channel) and the PUSCH (Physical Uplink Shared Channel) are

the main scheduled resource on the cell. They are used for the transport of all higher-layer information

including RRC signalling, service-related signalling and user traffic. The only exception is the system

information in PBCH.





c. 32.5 nsTs =

130,720,000

115,000 x 2048

Ts =

Ts (Time unit) =


The Physical Layer Timing Unit

Almost all numbers, durations and calculations related to E-UTRA are derived from a fundamental

parameter known as Ts or the basic ‘time unit’. Ts represents the ‘sampling time’ of the overall channel

and is itself derived from basic channel parameters. The definition of Ts is based on a 20 MHz channel

bandwidth with 15 kHz subcarrier spacing and an FFT size of 2048.

Ts is calculated to be the reciprocal of the subcarrier spacing multiplied by the total number of subcarriers in the FFT, or:

Ts = 1/(15,000 x 2048) seconds = 32.5 nsec

Frame, subframe and slot lengths, cyclic prefix durations and many other key parameters are defined as

multiples of Ts.

Crucially, the value of Ts does not vary between E-UTRA physical layer configurations. As Ts stays

constant, all of the key parameters used to define the E-UTRA structure also stay constant. This

consistency reduces the overall complexity of E-UTRA and enables system manufacturers to scale their

devices more easily to a variety of channel bandwidths and frequency bands.

Further Reading: 3GPP TS 36.211:4

LTE Air Interface



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 190

Frame – 10 ms (307200T s)

Slot – 0.5 ms (15360T s)

0 1 2 3 4 5 6

Subframe – 1 ms (30720Ts)

Normal cyclic prefix(total per subframe 2048T s)

0 1 2 3 4 5 6

OFDMSymbol

CP

CP (160/144Ts)

2048Ts

00 1 2 3 4 5 0 1 2 3 4 5

Extended cyclic prefix(total per subframe 6144T s)

OFDMSymbol

CP

CP (512Ts)

2048Ts


Type 1 Frame Structure

There are two basic frame types employed in E-UTRA, which are common to both uplink and downlink.

Type 1 frames are employed for FDD full- and half-duplex systems, while Type 2 frames are reserved for

TDD operation only.

The Type 1 frame duration is 10 ms and it is divided into 20 slots, each of 0.5 ms duration. More

significantly, however, for most information transmission, two slots are combined to form a subframe.Thus subframe duration is 1 ms, which corresponds to the TTI (Transmission Time Interval) for E-UTRA.

Type 1 slots contain either 7 or 6 symbols, depending upon which cyclic prefix type is in use.

Additionally, the length of the cyclic prefix prefixed applied in a particular symbol within a slot varies, also

dependent on which cyclic prefix length is in use. With the normal CP, symbol 0 in each slot has a CP

equal to 160 x Ts or 5.2 μsec, while the remaining symbols in the slot have slightly shorter CPs of just

144 x Ts or 4.7 μsec. When using the extended CP, all symbols are prefixed with a CP of 512 x Ts or

16.7 μsec.

Scheduling occurs across a subframe period. Up to the first three symbols in the first slot of each

subframe can be defined as a ‘control region’ carrying control and scheduling messages. The remaining

symbols of the first and all symbols in the second slot within the subframe are then available for user traffic.





1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 190

Frame – 10 ms (307200T s)

Slot – 0.5 ms (15360T s)Half-frame – 5 ms (153600T s)

2 3 4 5 7 8 90

Subframe

0 1 2 3 4 5 6


0 1 2 3 4 5 6

or

00 1 2 3 4 5 0 1 2 3 4 5


DwPTS GP UpPTS

UL/DL

Config.5 ms (half-frame) switching

0 D U

1 D U

2 D U

6 D U

10 ms (full-frame) switching

3 U

4 D U

5 D U

UL/DL Switching Options

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

D

U

D

U

D

U

U

U

U

U

U

U

U

U

U

U

U

U

U


Type 2 Frame Structure

Type 2 frames are used in TDD (Time Division Duplex) configured systems. They have a structure that is

generally similar to UMTS TDD LCR (Low Chip Rate), sometimes known as TD-CSCDMA. They share

the 10 ms frame structure and 1 ms subframe, but an additional demarcation known as a half-frame is

also defined.

Each half-frame carries five subframes, the second of which contains three specialized fields. DwPTS(Downlink Pilot Time Slot), UpPTS (Uplink Pilot Time Slot) and GP (Guard Period) appear in subframe 1

and optionally also in subframe 6 within a frame.

GP provides the downlink to uplink switching point for TDD operation, thus the system is configurable for

either 5 ms switching or 10 ms switching. The uplink to downlink switching points are variable within

either the 5 ms half-frame or the 10 ms frame, dependent on the configuration selected. Subframes 0

and 5, along with DwPTS, are always used for downlink transmission. UpPTS and the following frame

are always used for uplink transmission. The aim being to provide backward compatibility with UMTS

TDD mode and potentially also with WiMAX.

The terms DwPTS and UpPTS are inherited from UMTS, but in E-UTRA they can be used for normal

uplink or downlink symbol transmission carrying some control functions. Thus they really representfractional slot use leading into and out of a guard period.

Further Reading: 3GPP TS 36.211:4

LTE Air Interface



Subcarrier 1

Subcarrier 12

Resource block

1 ms subframe (2 slots)

Resource element

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 60 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6

0 1 2 3 4 5 6 0 1 2 3 4 5 6


Resource Blocks

A resource block consists of 12 subcarriers (in the frequency domain) for one slot period (in the time

domain). On both the uplink and downlink directions, 12 subcarriers correspond to 180 kHz of bandwidth.

The minimum possible capacity allocation period is the TTI of 1 ms. This equates to the allocation of two

consecutive resource blocks. Additionally, the sum of all the resource blocks in a single slot period is

known as the resource grid.

The theoretical minimum definable capacity allocation unit is the resource element, which is defined as

one subcarrier during one symbol period. Within each resource grid the resource elements that will be

carrying reference signals are assigned first; the remaining elements are then available to have user data

or control mapped to them.

In data transfer terms, one resource element is the equivalent of one modulation symbol on a subcarrier,

so if QPSK modulation was being employed, one resource element would be equal to 2 bits, with 16QAM

4 bits and with 64QAM 6 bits of transferred data.

If MIMO is employed on the downlink then separate resource grids are created for each antenna port –

each port maps to a different MIMO stream.





0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2R0 5 6 1 2 5 6

R0

R0

R0

R0

R0

R0

R0

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

0

0

4

4

0

0

4

4

SISO (Normal CP)

Cell-specific downlink reference signals

2x2 MIMO (Normal CP)

Antenna

port 0

Antenna

port 1

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2R0 5 6 1 2 5 6

R0

R0

R0

R0

R0

R0

R0

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3R1

R1

R1

R1

R1

R1

R1

R1


Downlink Reference Signals

In any mobile-radio system it is necessary to provide mobile devices with a means of measuring and

monitoring the strength and quality of the signal they receive and of calibrating their own output to ensure

that the correct frequencies are being employed.

E-UTRA’s version of OFDMA/SC-FDMA employs a physical reference signal, embedded in the main

body of the transmitted signal to provide an opportunity for channel estimation and frequency calibrationon the downlink. On the downlink, three types of downlink reference signals are currently defined: cell-

specific reference signals, MBSFN (Multicast/Broadcast Single Frequency Network) reference signals,

associated with MBSFN transmission, and UE-specific reference signals. In most circumstances only the

first of these reference signal types will be used. The reference signal takes the form of a modulated time

and frequency shifted of symbols generated from a Gold code of length 231–1.

Reference signal symbols are inserted into the transmitted resource grid following a predetermined

sequence, as shown in the diagram for cell-specific SISO (Single Input Single Output) and 2x2 MIMO

(Multiple Input Multiple Output) antenna arrangements and the normal CP. Modifications of this pattern

are also defined for 4x4 MIMO operation, use of the extended CP, and for MBSFN operation.

Cell-specific reference signals, as well as providing a ‘known signal’ upon which to base channelestimations, are modulated to identify the cell to which they belong. The sequence is related to the cell’s

physical layer identity in the set of 504 options.

Reference signals may have an applied power-boost over data symbols of up to 6 dB.

Further Reading: 3GPP TS 36.211:6.10, 36.300

LTE Air Interface



0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 0 1 2 4 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 1 2 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 0 1 2 4 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2R0 5 6 1 5 6

R0

R0

R0

R0

R0

R0

R0

3 3

3 3

3 3

3

3 3

3 3

3 3

3

3 3

3 3

3 3

3

0

0

4

4

0

0

4

4

R5 R5

R5 R5

R5 R5

R5 R5

R5 R5

R5 R5

SISO (Normal CP)

Antenna

port 5

UE-specific downlink reference signals

Beamforming antenna

for specific UE on

antenna port 5

Main cell coverage:

SISO – Antenna port 0

2x2 MIMO – Antenna ports 0 and 1

4x4 MIMO Antenna ports 0,1,2 and 3

MBMS – Antenna port 4


UE-Specific Reference Signals

UE-specific reference signals are used in addition to, not in place of, cell-specific reference signals. They

are intended for use when the cell supports beamforming antennas for individual UEs. The UE-specific

reference signals are only transmitted in PRBs that are scheduled to be received by the UE.

When beamforming is used the channel characteristic in the beam will be different than that for general

cell coverage. Additionally, the cell may be based on a MIMO transmission whereas there is only a SISOoption for UE-specific reference signals. Thus the UE-specific reference signals are required for accurate

channel modelling and CQI feedback for a UE with an allocated beam.

The diagram shows the arrangement for UE-specific reference symbols in the resource grid for cell SISO

operation and the normal CP. A second pattern is defined for the extended CP. UE-specific reference

signals are considered to be on port 5, ports 0 to 3 being for normal cell operation up to 4x4 MIMO and

port 4 being for MBSFN operation.





0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 60 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 60 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6


Uplink Demodulation Reference Signals

There are two types of reference signal used in the uplink, known as DRS (Demodulation Reference

Signals) and SRS (Sounding Reference Signals). DRS symbols are multiplexed with user data and

control transmissions. DRSs provide the receiving eNB with a ‘known signal’ element upon which to

perform channel estimations and from which it can calculate timing adjustments.

For the PUSCH one DRS symbol is transmitted per slot in the 4th symbol position (symbol number three). For PUCCH there may be either two or three DRS symbols per slot dependent on configuration

(not shown).

Because DRS symbols are multiplexed with user data they will always occupy the same allocated

bandwidth as the user data. This means that the length of the reference symbol sequence needs to be

the same as the number of allocated subcarriers in the transmission bandwidth (and always a multiple of

12). For each possible bandwidth allocation a number of base DRS sequences are defined. This is

organized such that there are 30 base sequences for 1, 2 and 3 resource block allocations and more

than 30, dependent on specific bandwidth, for allocations of more than three resource blocks. Thus there

are multiple DRS sequences in many different lengths. They are organized into 30 ‘sequence groups’.

Each sequence group contains one base DRS sequence of each length up to that suitable for bandwidth

allocations up to five resource blocks, and two base DRS sequences for bandwidth allocations above fiveresource blocks.

Each cell is allocated one sequence group. In addition, multiple orthogonal DRS sequences are then

created from a single base sequence using cyclic shifts; 12 are available for each base sequence. These

orthogonal sequences are used to multiplex signals from different UEs in the same cell.


LTE Air Interface



0 1 2 DRS 4 5 SRS

0 1 2 DRS 4 5 60 1 2 DRS 4 5

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 60 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6

SRS

SRS

SRS

SRS

SRS


Uplink Sounding Reference Signals

Channel estimations for received uplink signals are made by the eNB based on measurements taken of

the reference signal symbols embedded in uplink transmissions.

If there is no uplink transmission taking place, however, the eNB cannot take measurements. In these

circumstances a UE may be instructed to perform uplink sounding, which consists of the UE transmitting

a reference signal within an uplink resource allocation specifically set aside for the purpose. Sounding isperformed on the transmission of SRB signals. Resources for SRS are allocated over multiples of four

resource blocks and always transmitted in the last symbol of a subframe. SRS transmissions can be set

as periodic, can be frequency hopping and can have variable bandwidth; the configuration is set using

higher-layer signalling.

UEs may also be instructed to undertake sounding to enable the eNB to perform ‘frequency-specific

scheduling’. This term describes a procedure whereby the eNB measures the sounding signal

transmitted by a UE across some or all subcarriers and then chooses the resource block that consists of

the best performing set of frequencies. This is similar to the downlink process whereby scheduling can be

influenced by the UE’s CQI (Channel Quality Indication) reporting.





Radio Frame (10 ms)

DC

0 1 2 4 S P 0 1 2 4 5 6

R0 R0

3 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 S P 0 1 2 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

R0 R01 2 4 S P 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 6

R0 R0

3 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 S P 0 1 2 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

R0 R01 2 4 S P 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 6

R0 R0

3 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 S P 0 1 2 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

R0 R01 2 4 S P 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 6

R0 R0

3 3

0 1 2 4 0 1 2 4 5 63 3

0 1 2 4 0 1 2 4 5 63 3

0 1 2 0 1 2 5 63 3

0 1 2 4 0 1 2 4 5 63 3

R0 R01 2 4 1 2 4 5 63 3


Synchronization Signals

Synchronization signals are used for initial synchronization and for synchronization to neighbour cells

during handover measurements. There are two synchronization signals, the PSS (Primary

Synchronization Signal) and the SSS (Secondary Synchronization Signal).

The transmission structure for the PSS and SSS is independent of system bandwidth since they are only

transmitted on the centre 62 subcarriers. These are transmitted in the last two symbols of the first slot inthe first and sixth subframe within every frame. Note that the SSS is transmitted before the PSS. Note

also that the PSS and SSS sequences are of length 62, which means that the outermost subcarriers in

RBs occupied by PSS and SSS are not modulated. The DC subcarrier is also not modulated.

A defined set of Zadoff-Chu sequences is transmitted in the PSS and SSS, which are used to indicate

the cell’s physical layer identity. There are 504 physical layer cell identities divided into 168 groups of

three identities. The SSS identifies the group for the cell and the PSS identifies the identity within the

group. Generally, three cells on one eNB would have three identities from the same group.

The detected positions of the PSS and SSS provide frame and slot alignment for the UE, and since the

symbol positions are different for TDD mode, an FDD/TDD discrimination can also be made. Further, the

symbol duration is also different for the extended CP so normal/extended CP discrimination can bemade.

The SSS sequence is transmitted with two different shift values; one used in the first half of the frame

and the other in the second half. This means that the 10 ms frame alignment can be derived from the

reception of just one SSS.

Further Reading: 3GPP TS 36.211:6.11, 36.213:4

LTE Air Interface



Radio Frame (10 ms)

DC

0 1 2 4 S P 4 5 6

R0 R0

3

0 1 2 4 S P 4 5 63

0 1 2 4 S P 4 5 63

0 1 2 S P 5 63

0 1 2 4 S P 4 5 63

R0 R01 2 4 S P 4 5 63

0 1 2 4 S P 4 5 6

R0 R0

3

0 1 2 4 S P 4 5 63

0 1 2 4 S P 4 5 63

0 1 2 S P 5 63

0 1 2 4 S P 4 5 63

R0 R01 2 4 S P 4 5 63

B B B B

B B B B

B B B B

B B B B

B B B B

B B B

B B B B

B B B B

B B B B

B B B B

B B B B

B B B


PBCH Transmission

It is essential that the PBCH can be received without the UE having knowledge of the system bandwidth.

Therefore it is transmitted only on the centre 72 subcarriers.

The PBCH occupies the first four symbols of the second slot in each radio frame. However, it will not use

resource elements that are reserved for reference signals on the cell. This means that the cell’s antenna

port configuration affects the way in which the PBCH is encoded. The UE initially has no knowledge of the cell’s antenna port configuration, so blind decoding of the PBCH must be used until the UE identifies

which scheme applies.

The amount of information carried in the PBCH is very small, to reduce overhead. The RRC

MasterInformationBlock message is only 14 bits long and with error protection applied it is transmitted

across PBCH instances in four consecutive frames. Thus it is transmitted every 40 ms. However, the

error protection includes a very large overhead and it is possible that the UE can decode the message

before it has received all four transmissions in the four consecutive frames.





Radio Frame (10 ms)

Bandwidths with aneven number of RBs

DC

72 subcarriers(6 full resource blocks)

Bandwidths with anuneven number of RBs

72 subcarriers(5 full resource blocks

+ 2 half resource blocks)

DC

0 1 2 4 S P 4 5 6

R0 R0

3

R0 R0

B B B B

0 1 2 4 S P 4 5 63 B B B B

0 1 2 S P 5 63 B B B B

0 1 2 4 S P 4 5 63 B B B B

0 1 2 4 S P 4 5 63 B B B B

1 2 4 S P 4 5 63 B B B

0 1 2 4 S P 4 5 6

R0 R0

3

R0 R0

B B B B

0 1 2 4 4 5 63 B B B B0 1 2 5 63 B B B B

0 1 2 4 4 5 63 B B B B

0 1 2 4 4 5 63 B B B B

1 2 4 4 5 63 B B B

Half RB

Half RB

0 1 2 4 S P 0 1 2 4 5 6

R0 R0

3 3

R0 R0

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 S P 0 1 2 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

1 2 4 S P 1 2 4 5 63 3

0 1 2 4 S P 0 1 2 4 5 63 3

0 1 2 4 0 1 2 4 5 63 3R0 R0

R0 R0

0 1 2 0 1 2 5 63 3

0 1 2 4 0 1 2 4 5 63 3

0 1 2 4 0 1 2 4 5 63 3

1 2 4 1 2 4 5 63 3

Half RB

Half RB


DC Subcarrier Position

The DC subcarrier is only applicable in the downlink direction. It is always positioned as the centremost

subcarrier and always remains unmodulated. However, as the diagram shows, its position relative to the

RB alignment varies dependent on the configured system bandwidth.

For bandwidths with an even number of RBs (1.4, 10 and 20 MHz) the DC subcarrier is positioned

between the centre pair of RBs such that the highest frequency subcarrier of the lower indexed RB isspaced by two subcarrier steps from the lowest frequency subcarrier in the higher indexed RB. Thus the

placing of PSS, SSS and PBCH is aligned with the structure of the centre six RBs.

For bandwidths with an uneven number of RBs (3, 5 and 15 MHz) the DC subcarrier is positioned centre

of the centre RB such that there are six subcarriers either side of the DC subcarrier within the centre RB.

This RB therefore occupies a radio bandwidth equivalent to 13 subcarriers. Since the location of SSS,

SSS and PBCH is always over the 62 and 72 centre subcarriers, then in this case they will not be aligned

with the RB structure. As can be seen in the diagram they occupy the equivalent of five full RBs and two

half-RBs.

Further Reading: 3GPP TS 36.104; 5.6, TS 36.212; 6.6.4, 6.11.1.2, 6.11.2.2

LTE Air Interface



6 Resource Blocks

(72 subcarriers)

DC

Radio Frame (10 ms)

0 1 2 4 S P 4 5 6

R0 R0

3

0 1 2 4 S P 4 5 63

0 1 2 4 S P 4 5 63

0 1 2 S P 5 63

0 1 2 4 S P 4 5 63

R0 R01 2 4 S P 4 5 63

0 1 2 4 S P 4 5 6

R0 R0

3

0 1 2 4 S P 4 5 63

0 1 2 4 S P 4 5 63

0 1 2 S P 5 63

0 1 2 4 S P 4 5 63

R0 R01 2 4 S P 4 5 63

B B B B

B B B B

B B B B

B B B B

B B B B

B B B

B B B B

B B B B

B B B B

B B B B

B B B B

B B B

DownlinkControl

Channels

PCFICHPHICH

PDCCH

0 1 2 4 4 5 6

R0 R0

3

0 1 2 4 4 5 63

0 1 2 4 4 5 63

0 1 2 5 63

0 1 2 4 4 5 63

R0 R01 2 4 4 5 63

0 1 2 4 4 5 6

R0 R0

3

0 1 2 4 4 5 63

0 1 2 4 4 5 63

0 1 2 5 63

0 1 2 4 4 5 63

R0 R01 2 4 4 5 63

5 6

5 6

5 6

5 6

5 6

5 6

5 6

5 6

5 6

5 6

5 6

5 6

0 1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

0 1 2 3

1 2 3


Downlink Control Channels

The downlink control channels PHICH, PCFICH and PDCCH are mapped into the first symbols of every

subframe in every resource block. Dependent on configuration, this may be one, two or three symbols.

The only function of the PCFICH is to identify which option is configured.

Most of the available control channel capacity is used for PDCCH, but the resource is used to carry

multiple PDCCHs. A PDCCH is used to transmit uplink and downlink resource allocations for the PDSCHand PUSCH channels. Resource allocations are carried in messages called DCIs (Downlink Control

Information). Each PDCCH is mapped into a division of the radio resource called a CCE (Control

Channel Element). Each CCE is made up from nine sets of four resource elements known as REGs

(Resource Element Groups). The eNB may decide to map one PDCCH to one, two, four or eight CCEs,

depending on the channel conditions for the UE to which the DCI is addressed.

The PHICH is used for eNB HARQ ACK/NACK of transmissions in the PUSCH from UEs. The channel is

mapped onto REGs within the downlink control channel allocation in positions dependent on

configuration. Walsh codes are used to provide code multiplexing of multiple ACK/NACK indications

within one PHICH and multiple PHICHs will be configured on one cell.

The PCFICH carries the downlink control channel allocations with a parameter known as CFI (ControlFormat Indicator). Although this could potentially be determined though blind decoding in the UE, the

provision of a channel carrying this information reduces the processing overhead. The PCFICH is

mapped to REGs in the first symbol of each subframe and will not occupy resource elements that may be

allocated as reference signals. The specific positions and scrambling of the PCFICH is related to the

cell’s physical layer cell ID.





PRB 0RBG 0

PRB 1

PRB 2RBG 1

PRB 3

PRB 4RBG 2

PRB 5PRB 6

RBG 3PRB 7

PRB 8RBG 4

PRB 9

PRB 10RBG 5

PRB 11

PRB 12RBG 6

PRB 13

PRB 14RBG 7

PRB 15

PRB 16RBG 8

PRB 17

PRB 18RBG 9

PRB 19

PRB 10RBG 10

PRB 21

PRB 22RBG 11PRB 23

PRB 24 RBG 12

Type 0 Allocation Allocation by bitmap for

complete RBGs

UE 1 UE 2

Type 1 AllocationRBG subset size 11

Allocation by subsetselection, shift and bitmap

PRB 0RBG 0

PRB 1

PRB 2RBG 1

PRB 3

PRB 4RBG 2

PRB 5PRB 6

RBG 3PRB 7

PRB 8RBG 4

PRB 9

PRB 10RBG 5

PRB 11

PRB 12RBG 6

PRB 13

PRB 14RBG 7

PRB 15

PRB 16RBG 8

PRB 17

PRB 18RBG 9

PRB 19

PRB 10RBG 10

PRB 21

PRB 22RBG 11PRB 23

PRB 24 RBG 12

Type 2 Allocation

Localised Allocation by start point

and size in PRBs Always contiguous

PRB 0RBG 0

PRB 1

PRB 2RBG 1

PRB 3

PRB 4RBG 2

PRB 5PRB 6

RBG 3PRB 7

PRB 8RBG 4

PRB 9

PRB 10RBG 5

PRB 11

PRB 12RBG 6

PRB 13

PRB 14RBG 7

PRB 15

PRB 16RBG 8

PRB 17

PRB 18RBG 9

PRB 19

PRB 10RBG 10

PRB 21

PRB 22RBG 11PRB 23

PRB 24 RBG 12

DistributedPairs of PRBs frequencyhopped on a slot period

between two UEs

PRB 0RBG 0

PRB 1

PRB 2RBG 1

PRB 3

PRB 4RBG 2

PRB 5PRB 6

RBG 3PRB 7

PRB 8RBG 4

PRB 9

PRB 10RBG 5

PRB 11

PRB 12RBG 6

PRB 13

PRB 14RBG 7

PRB 15

PRB 16RBG 8

PRB 17

PRB 18RBG 9

PRB 19

PRB 10RBG 10

PRB 21

PRB 22RBG 11PRB 23

PRB 24 RBG 12


Downlink Resource Allocation (continued)

Type 2 allocation has two modes of operation known as localized and distributed . For localized operation

the allocation is defined in terms of an offset indicating the PRB in which the allocation begins and a size

in PRBs. Thus the allocation will always be contiguous, but it can be any size and in any part of the band.

For distributed type 2 allocation the PRBs within the TTI are shared between pairs of UEs. Additionally,

resources are always allocated in pairs of PRBs. The result is that the allocation is frequency hoppedbetween the two slots within the TTI. This method is particularly useful for offering frequency diversity

with very small bandwidth allocations since the paired PRBs can be in any part of the band. The most

likely application would be for VoIP.

Further Reading: 3GPP TS 36.213:7.1.6, 36.211

LTE Air Interface



PUCCH_1 PUCCH_0 PUCCH_1 PUCCH_0




Subframe

Slot

PUCCH region 0

PUCCH region 1

PUCCH region 2

PUCCH region 3

PUSCH demodulation

reference signals

Example,N (2)RB = 4


PUCCH Resource Allocation

The PUCCH is allocated to the outermost RBs in the allocated bandwidth. The amount of resource

allocated for use to carry PUCCH is indicated to UEs with the parameter NRBPUCCH . This parameter

indicates the number of RBs used for PUCCH per slot. However, PUCCH information is transmitted by a

UE using a PUCCH region, which occupies one subframe and utilises RBs on alternate sides of the

channel in alternate slots. This approach provides maximum frequency diversity in the PUCCH. Note that

since one PUCCH region equals two PUCCH slots, the parameter NRBPUCCH also describes thenumber of PUCCH regions per subframe.

The diagram shows an example based on NRBPUCCH equal to four. Note that uneven values may also

be used, in which case one RB will always be available for scheduled PUSCH use on alternating sides of

the channel.

A single PUCCH region can be submultiplexed using code sequences between up to 12 UEs.

Further Reading: 3GPP TS 36.211;5.3, 5.4




Frequency selective

Frequency diverse

inter-subframe hopping

Frequency diverse

intra-subframe hopping

PUSCH demodulation

reference signals



Subframe

Slot


Resource Allocation for PUSCH

Resource allocations for UEs transmitting in the PUSCH are made in terms of a number of RBs.

However, there are two main options for the way RBs are allocated within the available resource, and all

are based on a localized allocation. The allocation may be frequency selective (non-hopping) or

frequency diverse (inter-subframe hopping or intra-subframe hopping). Indication of which option is to be

used is delivered in higher-layer signalling.

The diagram illustrates the principles for each allocation option. For frequency selective scheduling the

eNB makes an allocation of RBs based on knowledge of the channel performance, most likely derived

though the use of UE sounding with SRS. The allocation does not hop in a predefined sequence but

would react to changing channel conditions.

However, some radio conditions may result in poor channel quality estimation, in which case frequency

diversity through frequency hopping can offer better overall performance. The diagram shows how this

may be performed on either slot or subframe boundaries.

Further Reading: 3GPP TS 36.213;5.4

LTE Air Interface



0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

R1

R1

R1

R1

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

Antenna port 1 Antenna port 1

R1

R1

R1

R1

FrameSubframe

Slot

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2 5 6 1 2 5 6

0 1 2 4 5 6 0 1 2 4 5 6

0 1 2 4 5 6 0 1 2 4 5 6

1 2R0 5 6 1 2 5 6

R0

R0

R0

R0

R0

R0

R0

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

3 3

Antenna port 0

Antenna port 0


Summary of the Downlink Structure

The diagram shows an example of a populated downlink FDD frame using the normal CP, 2x2 MIMO

and implemented in a 5 MHz bandwidth channel.

The PBCH is transmitted during subframe 0 of each 10 ms frame and occupies the centremost six

resource blocks. Alongside this and also in the sixth subframe in the frame are the primary and

secondary synchronization signals. Reference signal position for two resource blocks within a singlesubframe are shown for both antenna ports in the 2x2 MIMO system.

The diagram also shows the space allocated for downlink control channels, which includes PDCCH,

PCFICH and PHICH resources. A UE will be required to monitor some proportion of this dependent on

the connectivity state and the cell configuration.

The remainder of the allocation space will be used for scheduled downlink transmission in the PDSCH.

This includes common control signalling (system information and paging), dedicated control signalling

and traffic packets.

Further Reading: 3GPP TS 36.211, 36.300

LTE Air Interface



Frame

Subframe

Slot

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6

0 1 2 DRS 4 5 6 0 1 2 DRS 4 5 6


Summary of the Uplink Structure

The diagram shows an example of a populated uplink FDD frame using the normal CP and implemented

in a 5 MHz bandwidth channel. The overall uplink frame structure is simpler than that employed by the

downlink.

Symbol 3 in each slot carries the uplink demodulation reference signal, leaving the other six symbols

available to carry traffic.

A configurable number of outer resource blocks can be set aside to carry PUCCH messages. PRACH

resources are indicated in some of the remaining resource block as indicated to the UE in system

information.






LTE Air Interface



SECTION 5

LAYER 2 PROTOCOLS

LTE Air Interface




LTE Air Interface




Layer 2 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.1

L2/L1 Channel Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2

PDCP Functional Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.3

PDCP PDU Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.4

PDCP Sequence Numbers in Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5

PDCP Message Integrity Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.6

PDCP Ciphering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.7

RLC General Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.8

RLC Transparent Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.9

RLC Unacknowledged Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.10

RLC UM Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.11

RLC Acknowledged Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.12

RLC AM Frame Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.13

RLC Retransmission and Resegmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.14

MAC General Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.15

MAC Scheduling Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.16

Transmission Requirement Indications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.17

Use of Prioritized Bit Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.18

RACH Procedure for MAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.19

MAC PDU Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.20

RNTI Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.21

Downlink HARQ Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.22

N-Process Stop-and-Wait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.23

LTE HARQ Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.24

Management of DRX for Connected Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.25

CONTENTS

Layer 2 Protocols




LTE Air Interface





identify the three sublayers PDCP, RLC and MAC within layer 2 for E-UTRA

explain the key functions of each sublayer within layer 2

list the logical and transport channels defined for information interchange in layer 2 and layer 1

explain the function and multiplexing options for logical and transport channels

describe the functional architecture of PDCP

explain the operation of PDCP sequence numbers and how they operate during handover

explain the application of header compression in PDCP

describe how the security functions message integrity and ciphering are implemented and

applied in PDCP

describe the functional architecture of RLC

list and explain the three modes of operation for RLC: transparent mode, unacknowledged

mode and acknowledged mode

describe the RLC PDU formats for RLC unacknowledged and acknowledged mode

explain the retransmission mechanism for the acknowledged mode of RLC

describe the MAC functional architecture

list the key control functions of the MAC layer

explain MAC functions and procedures in respect of logical channel prioritization and

scheduling

explain the general operation of the random access process

describe how HARQ is implemented between the MAC layer and the physical layer

explain the need for and operation of ‘n’ stop-and-wait for HARQ in MAC

OBJECTIVES

Layer 2 Protocols




LTE Air Interface




RRC

PDCP

RLC

MAC

Physical layer

BCCH PCCH CCCH DCCH DTCH

BCH PCH RACH DL-SCH UL-SCH

PBCH PRACH PDSCH PUSCH

Logical

channels

Transport

channels

Physicalchannels


L2/L1 Channel Mapping

Logical channels are mapped by the MAC layer to transport channels on entry to the physical layer, and

then ultimately to physical channels within the physical layer.

The BCCH (Broadcast Control Channel) is used for system information broadcasting and carries three

RRC message types. The MasterInformationBlock message is mapped to the BCH (Broadcast Channel)

transport channel and then to the PBCH (Physical Broadcast Channel). All other system informationmessages are mapped to the DL-SCH (Downlink Shared Channel) and PDSCH (Physical Downlink

Shared Channel).

The PCCH (Paging Control Channel) carries paging messages and is mapped to the PCH (Paging

Channel) and PDSCH.

The CCCH (Common Control Channel), DCCH (Dedicated Control Channel) and DTCH (Dedicated

Traffic Channel) are all bidirectional channels and will be mapped to the DL-SCH and PDSCH for

downlink flows and UL-SCH (Uplink Shared Channel) and PUSCH (Physical Uplink Shared Channel) for

uplink flows.

The PRACH (Physical Random Access Channel) and RACH (Random Access Channel) are used only inthe uplink for initiating RRC connectivity. The random access process involves an interaction at the

physical layer under the control of MAC. There is no higher layer information in the random access

channels but the process will result in the allocation of resources for higher-layer message exchange.


LTE Air Interface



PDCP

PDCP

entity

PDCP

entity

PDCP

entity

RLC

UM-SAP UM-SAP AM-SAP

PDCP-SAP PDCP-SAP

Radio bearers

(SRB/DRB)

PDCP-SAP

Integrity protection

Ciphering

Add PDCP header

Sequence numbering

Header compression

Packets

from RBs

PDCP

control

packets

(U-plane only)

(C-plane only)


PDCP Functional Architecture

A PDCP enti ty is created for each SRB and/or DRB on a per-UE basis. Al l PDCP ent it ies are

bidirectional, thus when the AM mode of RLC is being used there is a one-to-one mapping between a

PDCP entity and AM SAP in RLC. However, for the UM mode of RLC one PDCP entity will be associated

with two UM SAPs, one configured for transmit functions and the other configured for receive functions.

Within a PDCP entity sequence numbering is applied for higher layer PDUs. This ensures in-order delivery at the receiving end. In the user plane PDCP control PDUs can be used to indicate missing

PDUs.

In the user plane, only IETF-defined ROCH (Robust Header Compression) is provided. Support for this is

only mandatory for UEs that have VoIP (Voice over IP) capability.

In the control plane, integrity protection is provided for RRC signalling messages.

Ciphering is then applied in both control and user planes, although separate cipher keys are applied for a

given UE in the two planes.


Layer 2 Protocols



Control Plane PDCP Data PDU

User Plane PDCP Data PDU

PDCP SN

(5 bits)

Data

(RRC message)I-MAC

(32 bits)

D/C

(1bit)

PDCP SN

(7 or 12 bits)

Data

(service signalling or traffic packet)

D/C

(1bit)

D/C

(1 bit)

PDU type

(3 bits)

FMS (First Missing

PDCP SN)

Bitmap

(optional)

PDCP Control PDUs

PDU type

(3 bits)

ROHC feedback

packet


PDCP PDU Formats

In the control plane PDCP will carry RRC signalling messages in the DCCH channel. The resulting PDU

includes a 5-bit PDCP SN (Sequence Number) and a 32-bit I-MAC (Integrity Message Authentication

Code).

In the user plane Data PDU are in one of two similar formats, the difference being the length of the PDCP

SN, which may be configured as seven or 12 bits. In addition the PDU begins with a D/C (Data/Control)bit, which differentiates a Data PDU from a PDCP control PDU. Note that message integrity is not used

for user-plane traffic.

There are two types of PDCP control PDU, both of which are applicable only in the user plane. The first

is used in support of the ROCH process. ROCH feedback confirms the initial static IP header values that

will be removed in compression.

The second PDCP control PDU is known as a status PDU and contains information about missing PDUs.

This takes the form of and FMS (First Missing PDCP SN) field and an optional supporting bitmap. The

status PDU is primarily used in conjunction with lossless handovers.


LTE Air Interface



4

d

Seamless Handover (non-delay tolerant, error tolerant)

MMES-GW

SourceeNB

TargeteNB

H/O

Unsent packetsforwarded

PDCP reset

4

c

Lossless Handover (delay tolerant, non-error tolerant)

MMES-GW

SourceeNB

TargeteNB

H/O

Unacknowledgedand unsent

packetsforwarded plusPDCP control

PDU sent via UEPDCP

sequencecontinued

3

dLP

UE forwardsPDCP control

PDU


PDCP Sequence Numbers in Handover

LTE offers two mechanisms for maintaining data flow to a UE as it moves from one cell coverage area to

another. Once a handover decision has been made based on measurement information the system may

transfer the data flow using either a seamless handover or a lossless handover.

A seamless handover is intended for non-delay-tolerant services such as VoIP. In general, seamless

handover is applied for radio bearers using the UM mode of RLC. In this procedure untransmittedpackets can be transferred from the source eNB to the target eNB, but no acknowledgment status is

supplied. Thus in the example shown the packet transmitted with PDCP sequence number 3, which has

not been acknowledged, will be lost. Only PDCP packet 4 is transferred to the target eNB since it has not

yet been transmitted. When transmission is resumed on the target cell, PDCP and other counters are

reset. This results in a faster handover, but may result in packet loss.

For data services that can tolerate some delay but for which packet loss is not acceptable, a lossless

handover is used. In general, lossless handover is applied for radio bearers using the AM mode of RLC.

In this procedure the PDCP context is transferred from the source eNB to the target eNB and the UE

uses a PDCP Status PDU to indicate missing PDUs. PDCP and other counters continue the existing

sequence. Thus, in the example, PDCP packet 3 is retransmitted on the target eNB since although it had

already been transmitted, it had not yet been acknowledged.


Layer 2 Protocols



COUNT

DIRECTION

RB-ID

EIA

KRRCenc or KUPenc

RRC algorithm ID KeNB

Trunc.

256/128

NAS COUNT K ASME

PLMN ID

RRC

PDCPLENGTH

Key stream block

Cipher text blockPlain text block


PDCP Ciphering

Ciphering is applied in the PDCP layer to both RRC signalling and traffic. The keys, known as K RRCenc and

KUPenc for the control and user planes respectively, are supplied by the RRC layer, which in turn has

derived it from KeNB, itself derived from CK and IK generated by a standard UMTS-based authentication

procedure performed in the NAS.

KRRCenc and KUPenc are truncated to 128 bits and then used in the ciphering algorithm along with a time-variable COUNT value, a direction indication (UL/DL), the radio bearer ID and a length indication

dependent on the PDCP SDU size in use. The output of the algorithm is a key stream block with a length

matching that of the plain text block (PDCP SDU). The key stream block and the plain text block are

modulo 2 added to create the cipher text block. The key stream block is recalculated for each

consecutive plain text block with incrementing COUNT values. The process is repeated at the receiving

end in order to return to the plain text block.


Layer 2 Protocols



RLC

Transmit

transparent

mode entity

Transmit

unacknowledged

mode entity

Receive

transparent

mode entity

Receive

unacknowledged

mode entity

Acknowledged

mode entity

Transmit side Receive side

Logical channels in MAC


RLC General Functions

RLC provides three levels of service: acknowledged mode, unacknowledged mode and transparent

mode. Radio bearers are mapped through RLC to logical channels and an RLC entity is created for each

active radio bearer.

For the transparent mode and the unacknowledged mode RLC entities are configured as either

transmitting or receiving entities. For acknowledged mode a single entity provides both transmit andreceive functionality for one side of the link. This configuration facilitates retransmission of failed RLC

PDUs.


LTE Air Interface



Transmitting

TM-RLC entity

PDCP PDUs

TM-SAP

PDCP PDUs

BCCH/PCCH/CCCH BCCH/PCCH/CCCH

RLC SDUs

RLC PDUs

Transmission

buffer

TM-SAP

TM-RLC entity

Receiving


RLC Transparent Mode

The transparent mode has no functions, only providing a buffer for higher-layer packets that are to be

transmitted over the air interface. Transparent mode entities are accessed via a TM-SAP.

The application of transparent mode is limited to the downlink transmission of system information and

paging messages as well as the exchange of RRC connection establishment messages associated with

the CCCH.

Further Reading: 3GPP TS 36.322:4.2.1.1

Layer 2 Protocols



Transmitting

UM-RLC entity

PDCP PDUs

UM-SAP

PDCP PDUs

DTCH DTCH

RLC SDUs

RLC PDUs

Transmission

buffer

UM-SAP

UM-RLC entity

Receiving

Segmentationand

concatenation

Add RLCheader

SDU

reassembly

Remove RLCheader

Reception bufferand HARQreordering

SDU SDU SDU

H H


RLC Unacknowledged Mode

Unacknowledged mode entities are accessed through a UM-SAP. Unacknowledged mode reorganizes

RLC SDUs into a size requested by the MAC layer. Unacknowledged mode also provides sequence

numbering for in-order deliver to higher layers at the receiving end. Reordering in the RLC layer is used

in support of the HARQ functions provides by the MAC layer.

Reorganization of RLC SDUs is provided by the segmentation and concatenation function. As shown inthe diagram, higher-layer SDUs can be fragmented and reassembled into the RLC PDU payload area to

produce a packet size suitable for scheduling by the MAC layer for transmission over the air interface.

The RLC header enables the receiving entity to reassemble the higher-layer SDU in the correct order.

The application of unacknowledged mode is limited to the user plane, where it would be utilised for

packet traffic flows with low tolerance to delay. The most common example would be VoIP connections.


LTE Air Interface



Transmission

buffer

DCCH/DTCH

RLC SDUs

RLC PDUs

Segmentation

and

concatenation

Add RLC

header

RLC Control

Retransmission

buffer

SDU

reassembly

Reception buffer

and HARQ

reordering

Remove RLC

header

Routing

DCCH/DTCH

PDCP PDUs

RLC SDUs

RLC PDUs

Transmitting

part of the

AM-RLC entity

Receiving

part of the

AM-RLC entity

AM-SAP


RLC Acknowledged Mode

The acknowledged mode of RLC is applicable in the control plane for RRC signalling messages carried

in DCCH and for user plane traffic carried in DTCH. Acknowledged mode entities are accessed through

an AM-SAP.

General transmission and reception functionality in terms of segmentation, concatenation, buffering and

HARQ reordering for AM mode are similar to those for UM mode. However, AM mode also providesretransmission of failed RLC PDUs. In this respect a number of enhancements in functional architecture

are provided. Firstly, a single entity for transmission and reception is required for interaction between the

transmitting and receiving side. Secondly a retransmission buffer is required in the transmit side. All

transmitted RLC PDU are retained in the transmission buffer until acknowledgement is received.

Additionally, control (status) PDUs are required in addition to data PDUs in order to manage the

retransmission process. These must be multiplexed with data PDUs at the transmission end and

demultiplexed (routed) from data PDUs at the reception end.


LTE Air Interface



Fixed part Extension part Data field elements

2 bits

1 bit

10 bits

1 bit

11 bits

FI E SN LI1 E LI2 E LIk Data FE1 Data FE2 Data FEk+1E

1 bit 1 bit

1 bit

D/C RF P

AMD PDU

Fixed part Extension part Data field elements

2 bits

1 bit

10 bits

1 bit

11 bits

FI E SN LI1 E LI2 E LIk Data FE1 Data FE2 Data FEk+1E

1 bit 1 bit

1 bit

D/C RF P

AMD PDU Segment

1 bit

15 bits

LSF SO

Status PDU

RLC control

header

Payload part

1 bit 12/42 bits1 bit

D/C CPT E1 ACK_SN E1

NACK_SN E1 E2 SOstart SOend

3 bits 10 bits 1 bit


RLC AM Frame Structures

There are three types of RLC PDU used for the AM mode of operation in RLC. The general structure of

each is shown in the diagram.

The basic PDU for delivery of RLC SDUs is the AMD PDU. It has a very similar structure to the UM PDU

but with three additional fields. The D/C bit distinguishes this PDU from a Status PDU. The RF

(Resegmentation Flag) distinguishes this PDU from an AMD PDU Segment. The P (Polling) bit is used totrigger the transmission of a Status PDU from its peer entity on the receiving end of the link.

AMD PDU segments are used for the retransmission of failed AMD PDUs where the PDU size indicated

by MAC is smaller than was used for the initial transmission of the AMD PDU. Thus the original AMD

PDU is resegmented into a number of AMD PDU segments. The general structure is the same as the

AMD PDU with two additional fields that are required for the reconstruction of the original AMD PDU.

These are the LSF (Last Segment Flag), used to indicate the last segment of a segmented AMD PDU,

and the SO (Segment Offset), used to indicate the position of the segment within the Segmented AMD

PDU.

Status PDUs are used to acknowledge received PDUs or trigger retransmissions of failed PDUs. They

contain only an RLC header part and a payload part. The header part is made up of a D/C bit, three CPT(Control PDU Type) bits and an E1 (Extension) bit. The Type field indicated the Control PDU type,

although in Release 8 of the standards only the type shown exists. The E1 bit indicates whether or not

any of the optional NACK fields are present in the payload part of the PDU.

The payload part of the contains one ACK_SN field, which indicates the SN of the next expected PDU.

Optionally it many also contain one or more NACK_SN fields, which indicate the SN of PDUs that have

been detected as lost. Each NACK_SN may be supplemented by SOstart and SOend if the NACK_SN

relates to lost segments of a PDU.

Further Reading: 3GPP TS 36.322:6.2.1.4, 6.2.1.5, 6.2.1.6

Layer 2 Protocols



AM RLC

Entity

AM RLC

Entity

MAC

Entity

MAC

Entity

Radio link

Size 600 octets

AMD PDU (600 octets)

Status PDU (NACK)

Size 200 octets

AMD PDU Segment (200 octets)




RLC Retransmission and Resegmentation

The diagram shows a potential sequence in which AMD PDU segments would be used.

The MAC layer instructs the RLC layer about the required size of RLC AMD PDUs; in the example it

requests a size of 600 octets. RLC assembles an AMD PDU of length 600 octets, transmits it and retains

it in the RLC transmission buffer pending acknowledgement. However, the AMD PDU is lost and this is

indicated to the transmitting end with Status PDU.

A retransmission of the lost AMD PDU is triggered, but in this case the MAC layer has reduced the RLC

PDU size to 200 octets. Therefore the original 600-octet AMD PDU must be segmented. Three AMD

PDU segments are used for the retransmission. Note that it would have been possible for MAC to modify

the RLC PDU size again during the transmission of the segments, in which case the AM entity would

have constructed further segments as instructed.


LTE Air Interface



MAC Downlink Assignment (PDCCH)

MAC Uplink Grant(PDCCH)

VoIP or otherconversational servicesBursty data

Dynamicscheduling

MAC (PDCCH)

Persistentscheduling

MAC (PDCCH)

RRC (DL-SCH)

Semi-Persistent

scheduling

Data

Data

Data

Data

Data

Data

Data

Data

Data

Data


MAC Scheduling Functions

The main function of the MAC is to manage the shared access to a common transmission medium by

multiple devices. This is achieved through the eNB’s scheduling function. Resource allocation will be

performed on the basis of a scheduling algorithm, the specifics of which are not defined by the standards.

However, channel performance, data buffer fill, UE power capability and traffic priority are likely to be

considered.

When a UE establishes an RRC relationship with an eNB it is assigned a C-RNTI, which will uniquely

identify that UE in that cell. The C-RNTI will be used to address any control and scheduling messages to

or from the UE. Each UE is capable of establishing multiple EPS bearers, which are the NAS traffic and

signalling connections that travel from the UE to the core network.

Resource allocations are defined in terms of one or more PRB (Physical Resource Block), which will be

populated using a specified MCS (Modulation and Coding Scheme). The allocations can be made for one

or more TTI (Transmission Time Interval) periods.

LTE offers three scheduling modes. The first, known as dynamic scheduling , involves the use of MAC

downlink assignment messages and uplink grant messages in the PDCCH to allocate resources as

required. Dynamic scheduling is intended for typical bursty packet data traffic.

For VoIP (Voice over IP) traffic where regular and reliable allocation of resources is required to meet

more demanding QoS (Quality of Service) requirements, LTE offers persistent scheduling . This is

achieved through a combination of RRC signalling in the DL-SCH, for the initial specification of the

resource allocation interval, and MAC signalling in the PDCCH for more specific PRB and MCS

information. The result is a lower overhead in the PDCCH for these regular resource allocations.

The third scheduling option, known as semi-persistent scheduling , is used specifically for the purpose of

resource allocation for the establishment or reconfiguration of a persistent scheduled resource, i.e. for the

transport of RRC messages relating to the persistent scheduled resource. In this case an SPS-C-RNTI

will be used to address the UE, which is different from the UE’s C-RNTI.


LTE Air Interface



Logical channels

eNB

Scheduling

Multiplexing andprioritization


Transmission Requirement Indications

The UE will neither receive nor transmit information unless it is scheduled to do so because there is no

dedicated radio resource in E-UTRA. Therefore, for every signalling message or data packet some

signalling activity must be performed and this must be preceded by a resource request.

Downlink resource allocation is triggered by need in the eNB. All resource allocations are indicated in the

PDCCH (Physical Downlink Control Channel).

For uplink transmission the UE must first indicate its need to the eNB. There are a number of

mechanisms that can result in a scheduled resource being indicated for a UE in the PDCCH. For initial

access, or where the UE has not been active for some time, the random access procedure can be used

for resource requests. When a mobile is continuously active it may be allocated a resource in the

PUCCH (Physical Uplink Control Channel) to use for resource requests needed for further data or

signalling transfer. Additionally, the eNB can request buffer status reports from UE that are currently

active. Based on this information the eNB makes scheduling decisions.

In the uplink direction it is the MAC layer within the UE that determines how an allocated transmission

resource should be demarcated between a number of different logical channels. This is based on

channel priority and channel PBR (Prioritized Bit Rate).


Layer 2 Protocols



MAC

Header

MAC PDU

PBR

PBR PBR

Wait

Available resource

LCID 1

Priority 1

LCID 2

Priority 2

LCID 3

Priority 3


Use of Prioritized Bit Rates

When a resource is allocated for transmission the MAC layer will assemble a MAC PDU corresponding to

the allocation size. The payload area of the MAC PDU may contain MAC SDUs (RLC PDUs) relating to

more than one concurrent logical channel. In order for MAC to arbitrate between the channels in terms of

the proportion of the potential resource that should be apportioned to each, two key values are used: the

channel priority and an associated PBR.

In the example shown there are three logical channels to be mapped in to the resource allocated in the

shared physical channel. The channels with LCID 1 and 2 are the highest priority, but the buffer sizes

currently exceed their respective PBRs. They are initially allocated resource proportions in the MAC PDU

up to their PBR values This allocation is done in order of priority; LCID 1 first and then LCID 2. Once their

PBR requirements have been met, resource can be allocated to LCID 3. In this case LCID 3

requirements are below its PBR. This results in some spare capacity in the MAC PDU. The spare

capacity can then be allocated higher priority channels above their PBRs. In this case this enables the

highest priority channel’s buffer to be cleared. However, the next priority channel, LCID 2, is left with data

to be sent in a subsequent resource allocation.


LTE Air Interface



MACEntity

MACEntity

Physicallayer

Physicallayer

RACH and preamble

instructions

L2/L3 Message

CCCH

Radio link

PRACH RACH indication

RAR (Random Access

Response)

DL-SCH

• Timing Advance

• UL Grant

• Temporary C-RNTI

RAR

DL-SCH/PDSCH

Resource allocation

for RAR

PDCCH

CRC scrambled

with RA-RNTI

MAC PDU [L2/L3 Message]

UL-SCH/PUSCH

CRI (Contention

Resolution Identity)

DL-SCH

L2/L3 Message

CCCH

Resource allocationfor CRI

PDCCH

CRI

DL-SCH/PDSCH

Contention check.

Temporary C-RNTIbecomes the allocated

C-RNTI


RACH Procedure for MAC

The RACH procedure is handled by the MAC and the physical layer and operates using a combination of

the PRACH on the uplink and the PDCCH on the downlink. UEs are informed of the range of random

access preambles available in system information, as are the contention management parameters. When

a random access event is required, the UE will perform the following functions:

review and randomly select a preamble

check the BCCH for the current PRACH configuration; this will indicate the location and periodicity

of PRACH resources in uplink subframes

calculate open loop power control parameters – initial transmit power, maximum transmit power

and power step

discover contention management parameters

Once the UE transmits an initial preamble it will wait a specified period of time for a response before

backing off and retrying. Open loop power control ensures that each successive retry will be at a higher

power level.

Upon receipt of a successful uplink PRACH preamble, the eNB will calculate power adjustment andtiming advance parameters for the UE based on the strength and delay of the received signal and

schedule an uplink capacity grant to enable the UE to send further details of its request. This will take the

form of the initial layer 3 message. If necessary, the eNB will also assign a Temporary C-RNTI (Cell

Radio Network Temporary Identifier) for the UE to use for ongoing communication.

Once received, the eNB reflects the initial layer 3 message back to the UE in a subsequent downlink

scheduled resource to enable unambiguous contention resolution. After this point further resource

allocations may be required for signalling or traffic exchanges; these will be addressed to the C-RNTI.

Further Reading: 3GPP TS 36.321:5.1, 36.213:6

Layer 2 Protocols



eNB

TB

MACPhysical

layer

MAC PDU

HARQ Entity

TBPDSCH

PDCCH

Scheduling and

HARQ control

ACK/

NACK

Propagation

delay

Processing time

UE

PUCCH or

TTI(1 ms)

PUSCH

TB

Propagation

delay

Retransmission

or next TB


Downlink HARQ Principle

MAC PDUs are mapped to physical layer transport blocks, which will be transmitted in one TTI (1 ms

subframe). In addition to the transport block MAC passes scheduling information and HARQ information

to the physical layer. One-third rate Turbo coding and puncturing is applied to the transport block before

transmission. The resource for the transmission of the transport block is indicated to the UE in the

PDCCH.

The UE receives the transport block and performs error correction through Turbo decoding. Once

complete, an ACK/NACK response is returned to the eNB in the PUCCH channel, or if it already has an

allocated uplink resource in the PUSCH, this may also be used. The ACK/NACK information is fed back

to the MAC layer. If the response is positive the MAC layer passes down the next transport block for

HARQ transmission. If a negative response is received then the MAC layer requests an HARQ

retransmission of the first transport block by the physical layer. Configuration parameters define how

many retransmissions can be used before the PDU is considered to have failed.

Further Reading: 3GPP TS 36.321:5.3.2, 5.4.2; 36.213

LTE Air Interface



Synchronous HARQ

Maximum retransmission configured

per UE

DL ACK/NACK sent in PHICH

Retransmission may be scheduled in

PDCCH or implicitly triggered by

PHICH (adaptive retransmission)

Asynchronous adaptive HARQ

UL ACK/NACK sent in PUSCH or

PUCCH

Retransmission always scheduled in

PDCCH

PDCCH indicates process number

and transmission/retransmission

indication

Uplink HARQDownlink HARQ


LTE HARQ Characteristics

The implementation of HARQ in the uplink direction differs slightly from the downlink. The key

characteristics of both implementations are summarized in the diagram.

The main difference relates to the use of a synchronous arrangement in the uplink direction. This means

that HARQ processes operate against a fixed timing structure, which results in a reduced overhead in

terms of HARQ signalling at the expense of some flexibility.

Further Reading: 3GPP TS 36.321; 36.213

LTE Air Interface



eNB

longDRX-Cycle shortDRX-

Cycle

shortDRX-

Cycle

longDRX-CycleReception period

Physical layersubframe

onDurationTimer

DL schedule forUE’s C-RNTI

drx-InactivityTimerstarted

drx-InactivityTimer expires or UE receives

a MAC CE

drxShotCycleTimer started

drxShotCycleTimer expires


Management of DRX for Connected Mode

In addition to DRX for UEs in idle mode, E-UTRA supports DRX for UEs in RRC connected mode. This

process is controlled collectively by MAC and RRC. The parameters are set by RRC but it is the MAC

layer the operates the process itself.

The onDurationTimer defines the length of time that the UE is active and monitoring downlink control

channels when DRX is running; in the example in the diagram this is set to two subframes (2 ms). Thisoperates in conjunction with a DRX cycle that defines the amount of time that the UE can be ‘off’. There

are two DRX cycles defined for a UE known as the longDRX-Cycle and the shortDRX-Cycle. As can be

seen in the diagram, the longDRX-Cycle is the default value.

When a period of activity is started through the scheduling of resources for the UE’s C-RNTI, the UE

starts the drx-InactivityTimer . If the UE remains active long enough for the drx-InactivityTimer to expire,

or if it receives a MAC CE on which it may have to act, then, when activity stops, the UE will use the

shortDRX-Cycle period and also start the drxShortCycleTimer . If no further activity takes place before the

drxShortCycleTimer expires then the UE reverts to the longDRX-Cycle period.

Further Reading: 3GPP TS 36.321:5.7; 36.331

Layer 2 Protocols




LTE Air Interface



SECTION 6

RADIO RESOURCE CONTROL

LTE Air Interface




LTE Air Interface




RRC Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.1

RRC States . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.2

RRC I-RAT State Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.3

Signalling Radio Bearers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.4

System Information Broadcasting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.5

System Information Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.6

Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.7

RRC Connection Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.8

RRC Connection Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.9

UE Capability Enquiry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.10

Security Mode Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.11

Data Radio Bearer Establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.12

Measurement Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.13

Intra-LTE Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.14

Handover from LTE (I-RAT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.15

NAS Information Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.16

CONTENTS

Radio Resource Control




LTE Air Interface





identify the functions of the RRC protocol

define the RRC protocol connected mode and idle mode states for a UE

describe inter-RAT state transitions between E-UTRA, UTRA and GSM/GPRS

explain the use of signalling radio bearers for the transfer of RRC signalling

describe the procedures and messages for the broadcasting of system information by RRC

outline the contents of RRC system information blocks

explain the functions and contents of RRC paging messages

explain the relationship between signalling radio bearers, data radio bearers and EPS

bearers

describe the operation of RRC connection establishment

describe the procedure for the identification of UE capability

describe how data radio bearers and EPS bearers are established, modified or removed

explain the measurement configuration and reporting procedures and the general options for

measurement and reporting in E-UTRA/LTE

describe RRC procedures relating to the operation of air interface security functions

explain RRC handover signalling flows

explain how RRC carries NAS signalling over the air interface

OBJECTIVES





LTE Air Interface




RRC

System information broadcasting

Paging

Connection management

Temporary identity management

Handover management

QoS management

NAS signalling direct transfer

DataTraffic

AS(Access Stratum)

NAS NASDataTraffic

AS(Access Stratum)

UE eNB EPC

RRC

EMM ECM

L1

L2

L1

L2

RRC

EMM ECM


RRC Functions

As with other E-UTRA protocols, the RRC layer, which previously resided in the RNC, has been

relocated to the eNB. In addition, the functionality and complexity of RRC has been significantly reduced

relative to that in UMTS. The main RRC functions for LTE include creation of BCH (Broadcast Channel)

system information; creation and management of the PCH (Paging Channel); RRC connection

management between eNB and UEs, including generating temporary identifiers such as the C-RNTI;

mobility-related functions such as measurement reporting, inter-cell handover and inter-eNB UE contexthandover; QoS management; and direct transfer of messages from the NAS to the UE.

The RRC is in overall control of radio resources in each cell and is responsible for collating and

managing all relevant information related to the active UEs in its area.

System information provides the main means of advertising the services available in a cell and the

means by which those services can be accessed. For E-UTRA the BCH carries only basic information

and acts as a pointer for broader system information related to the NAS, such as PLMN identity (network

code and country code) and AS details such as cell ID and tracking area identity; all of which is carried in

the downlink dynamically scheduled resource (DL-SCH).

E-UTRA has been designed with network sharing in mind and system information can carry details of upto six sharing PLMNs.

Each eNB is responsible for managing inter-cell handovers between all the cells it controls. When

handover to another cell site is required the eNB will pass details of the current UE context to its

neighbour. This includes details of identities used, historical measurements taken and active EPS

bearers.





Connectionestablishment/

release

IDLE

RRC CONNECTED

RRC IDLE

CELL_DCH

CELL_FACH

CELL_PCH

URA_PCH

RRC CONNECTED

GSM Dedicated

GSM_Idle/ GPRS

Packet_Idle

GPRS Packet

transfer mode

UMTS LTE GSM/GPRS

Reselection

Handover

Reselection

CCO, reselection


release


release


RRC I-RAT State Transitions

Both I-RAT (Inter-Radio Access Technology) handover and cell reselection are defined for E-UTRA,

which means that defined state transitions must also be defined for interworking to the radio resource

control states in other technologies.

The diagram shows the state transitions for UMTS and GSM/GPRS.

RRC E-UTRA idle mode transition for both UMTS and GSM/GPRS are primarily by reselection. However,

for GPRS operation an option for CCO (Cell Change Order) also exists.

The UMTS RRC connected state has a number of substates that are not a feature of E-UTRA. Therefore,

state transition between the two systems for RRC-connected UEs varies dependent on traffic activity.

Handover is supported to and from the UMTS CELL_DCH state. This would be most applicable for a UE

engaged in real-time traffic transfer. For intermittent packet activity or for signalling activity reselection to

and from the other UMTS RRC connected substates may be used.

Similarly, transitions for RRC-connected UEs to and from GSM/GPRS are also affected by the traffic or

signalling activity. Real-time traffic is most likely to be handed over between E-UTRA and GSM, but for

GPRS operation options for CCO or CCO with optional NACC (Network Assisted Cell Change) exist.





RRCSystem

Information

and paging

RRC

Connection

establishment

RRC

dedicated

control

NAS

direct

transfer

NAS

Layer 2

Logical Channels

Traffic data including

service related signalling

(e.g. IMS signalling)

Control Plane User Plane

SRB 0 SRB 1 SRB 2 DRB 0 DRB 1 DRB n

BCCH/PCCH CCCH DCCH DCCHDTCHs


Signalling Radio Bearers

RRC exists only in the control plane of the air interface AS (Access Stratum) protocol stack. RRC

receives information from functional entities in the NAS (Non Access Stratum) in the form of complete

messages for direct transfer, and also in the form of requests, information elements and parameters that

will trigger RRC activity and be used in RRC messages.

For broadcast functions over the air interface RRC messages are mapped directly to logical channels.This includes paging and system information broadcasting using the PCCH (Paging Control Channel)

and BCCH (Broadcast Control Channel) logical channels respectively.

For dedicated signalling functions between a UE and an eNB signalling flows are mapped into an SRB

(Signalling Radio Bearer). When a UE transitions to the RRC connected state a set of SRB instances is

created. SRB 0 is used only for the initial establishment of the RRC connection and is mapped to the

CCCH (Common Control Channel). Once the RRC connection is established the UE will be issued with a

C-RNTI (Cell-Radio Network Temporary Identity) and SRB 1 and optionally SRB 2 will be created. SRB 1

is used for all RRC specific signalling functions. SRB 2 is used for RRC direct transfer of NAS signalling

messages. However, NAS messages may also be piggybacked with RRC signalling in SRB 1. Both SRB

1 and SRB 2 are mapped to DCCH (Dedicated Control Channel) logical channels.

If required, one or more DRB (Data Radio Bearers) may be created during or subsequent to an RRC

connection establishment. These exist in the user plane and carry traffic. However, ‘traffic’ in this context

includes service-related signalling between service applications in higher layers, for example VoIP

connection establishment using the IMS. DRBs are mapped to DTCH (Dedicated Traffic Channel) logical

channels.


LTE Air Interface



MIB

BCCH

BCH

SIB 2-13

DL-SCH

SystemInformation message

Essential and

basic frequently

transmitted

parameters

All other parameters

with flexible scheduling

indicated in SIB 1

MasterInformationBlock

(40 ms periodicity)

SystemInformationBlockType1

(80 ms periodicity)

SystemInformation (Other SIBs)

eNB

SIB 1

IE


System Information Broadcasting

A ‘bootstrap’ approach is adopted for system information broadcasting on the E-UTRA air interface. The

physical layer is primarily a dynamically scheduled resource with very little permanently defined capacity.

Therefore, although a BCH (Broadcast Channel) transport channel and corresponding physical layer

resource exist, this is only used to carry the MIB (Master Information Block). The position of the MIB can

be determined by the UE as it performs initial synchronization with the cell.

The MIB contains only basic information enabling the UE to find and read the RRC message

SystemInformationBlockType1. This message in turn provides the scheduling information for the RRC

SystemInformationmessages being transmitted on the cell. SystemInformationmessages contain one or

more information elements, each of which will be a SIB (System Information Block). It is the SIBs that

provide the complete set of system information for a UE. The operator determines which SIBs are

transmitted, and how frequently, dependent on configurations, capabilities and services supported.





Type Key Information

MIB DL bandwidth, PHICH configuration, system frame number

SIB scheduling list, PLMN ID(s), TAC, cell barring info,

cell selection parameters, frequency band info

Detailed cell barring info, UL frequency allocation, UL bandwidth,

MBSFN information

SIB 3 Cell reselection information

SIB 4 Intra-frequency neighbour-cell descriptions

Inter-frequency E-UTRA neighbour-cell descriptions,

cell-specific reselection parameters

Inter-RAT UMTS neighbour-cell descriptions,

frequency-specific reselection parameters

Inter-RAT GSM/GPRS neighbour-cell descriptions,

frequency-specific reselection parameters

Inter-RAT CDMA2000 neighbour-cell descriptions,

frequency- and cell-specific reselection parameters

SIB 9 Home eNB name (text)

SIB 10 ETWS (Earthquake and Tsunami Warning System) primary notification

SIB 11 ETWS secondary notification

SIB 12 CMAS (Commercial Mobile Alert Service) notification

SIB 13 MBSFN information

SIB 1

SIB 2

SIB 5

SIB 6

SIB 7

SIB 8


System Information Messages

The table provides a summary of the contents of the MIB, SystemInformationType1 message and SIB

types 2–13 currently defined for E-UTRA/LTE operation.

Further Reading: 3GPP TS 36.331:6.2.2, 6.3.1

LTE Air Interface



MME

eNB

Paging

(PCCH/PCH)

Paging message

PR (Paging Record)

UE appliesDRX cycle

Paging triggered by:

Incoming call (establish an EPS bearer or CS Fallback)

Data activity (to establish/re-establish an EPS bearer)

Notification of system information modification

Notification of an ETWS indication

Notification of a CMAS indication

EPC

RRC

headerPR 1 PR 2 PR 3 ...

PRn

(nmax= 16)

Sys info

modification

ETWS

indication

CMAS

indication

Future

extension

CN domain

(CS/PS)

UE identity

(IMSI/S-TMSI)


Paging

The primary function of paging is to trigger the establishment of EPS bearers for incoming calls or to

establish or re-establish EPS bearers for new data activity relating to a UE that is already receiving EPS

connectivity service.

Paging requests are generated in the MME. The MME indicates a paging requirement to eNBs across a

tracking area. Each eNB compiles paging requests as one or more PR (Paging Record) informationelement within a single RRC Paging message. The maximum number of PRs in a Paging message is 16.

Paging messages are transmitted via the PCCH/PCH channel combination; however, the UE does not

need to monitor the PCH transport channel all the time since it is carried in the dynamically scheduled

downlink physical resource. This means that there is no paging channel as such at the physical layer and

the UE need only monitor scheduling information for paging occurrences. Scheduling instances for the

transmission of RRC Paging messages are identified with a specifically defined RNTI called the P-RNTI.

In addition UEs will apply a defined DRX cycle for the monitoring of the scheduling information.

Paging also has three secondary functions. Indications for system information change, ETWS and CMAS

are also carried in RRC Paging messages. This removes the need for the Idle mode UEs to monitor

system information continuously.

Further Reading: 3GPP TS 36.331; 5.3.2, 6.2.2




Serviceapplication

Serviceapplication

External PDN

S1-APconnection

EPS bearer

RRCconnection

PDN-GW

S1-APRRC

NAS

MME

eNB

Traffic

S1-APRRC

NAS

Traffic

DRBs

SRBs

NAS


RRC Connection Structure

The overall function of RRC is to create, maintain and clear DRBs as required to provide the radio link

segment of one or more EPS bearer relating to one or more EPS connectivity service. RRC receives

instructions on what EPS bearers are required from the NAS. The NAS activity in turn is driven by

instructions from service applications (via the PCRF on the EPC side).

In order to manage DRBs, RRC must exchange signalling with its peer entity and provide direct transfer for NAS signalling exchange. Connectivity for this comes from SRBs. However, signalling relating to

service applications, which are always external to the LTE/EPS, are treated as traffic flows and as such

are carried in DRBs within an EPS bearer. Note that an EPS bearer has only one set of associated QoS

characteristics, so if application signalling were to require different QoS treatment to the traffic that it

facilitates then a second EPS bearer would have to be defined. Multiple EPS bearers may or may not be

part of the same EPS connectivity service dependent on their respective connectivity requirements.


LTE Air Interface



eNB

UE Initial Identity (S-TMSI or 40-bit random value)

cause value (Emergency, high-priority access, MO

signalling, MO data, MT access)RRCConnectionRequest

CCCH/UL-SCH

(RACH/C-RNTI established)SRB 0

RRCConnectionSetup

CCCH/DL-SCH

RRC Transaction Identifier

dedicated radio resource configuration for SRB 1

SRB 1RRCConnectionSetupComplete

DCCH/UL-SCH


selected PLMN

registered MME (if applicable)

NAS signalling message


RRC Connection Establishment

The RRC connection establishment procedure is always initiated from the UE. It begins with the

transmission of the RRCConnectionRequest message containing an identity and a cause value. If the UE

has already registered with the network then it will use the S-TMSI as its identity. If this is a new mobile

needing to perform an initial registration then it will generate and use a 40-bit random value. The

message is carried in the CCCH/UL-SCH channel combination. This requires a scheduled resource

allocation, which is secured using the lower-layer random access procedure and the RACH (Random Access Channel). The lower-layer random access procedure also facilitates the allocation of a C-RNTI at

this stage.

The eNB responds with an RRCConnectionSetup message containing a transaction identifier, used to

relate future messages as part of this signalling sequence, and the radio resource configuration for SRB

1. Note that the exchange of the two messages to this point has involved the use of the implicitly

configured SRB 0.

The final part of this three-way handshake is the confirmation from the UE in the form of the

RRCConnectionSetupComplete message now using the defined SRB 1 and DCCH/UL-SCH

combination. For registered UEs this message contains identities of the PLMN and MME with which it is

registered. In any case the message will also piggyback the initial NAS message that triggered the RRCestablishment procedure, for example, a service request or registration message.

Further Reading: 3GPP TS 36.331:5.3.3, TS36.321:5.1




eNB

UECapabilityEnquiry

DCCH/DL-SCH

UECapabilityInformation

DCCH/UL-SCHSRB 1

SRB 1

RAT Type Capability Container

GERAN CS capability Concatenation of CSclassmark 2 and 3

GERAN PS capability Defined in TS 36.306

UTRA (UMTS) capability Defined in TS 25.331

CDMA2000 1x capability Defined in A.S0008

E-UTRA capability AS release (Rel-8 onwards)

UE-Category (TS 36.306)

PDCP parameters

Physical layer parameters

RF parameters

Measurement parameters

Feature group Indicators

IRAT parameters (support for...)

UECapabilityRATContainerList


UE Capability Enquiry

There are many possibilities for UE capability, both in terms of LTE feature support and the UE’s support

of one or more other RAT (Radio Access Technology). It is therefore important that at some point after

initial RRC connection establishment the eNB determines exactly what the UE capabilities are.

This is achieved through the relatively simple capability enquiry procedure. The diagram shows the

general information that may be contained in the UE’s UECapabilityInformation response message. Atpresent this covers the possibility for multimode operation with GSM/GPRS, UMTS, CDMA2000 1x and

1xEV-DO. It is for future study whether this may also include other RATs such as WiMAX or Wi-Fi. This

does not preclude the support of any other RATs in the device, but does mean direct interworking at the

eNB level is not possible. However, interworking via a GANC would still be possible.

Definitions for RATs other than E-UTRA/LTE are to be found in the relevant documents indicated in the

table. For E-UTRA/LTE the information elements in the UE capability container predominantly cover the

UE’s capability at layer 2 and at the physical layer. It includes the UE’s category, which describes the key

factors determining the maximum throughput that UE can achieve, for example, the UEs MIMO antenna

capability.

Further Reading: 3GPP TS 36.306, TS 36.331:5.6.3,6.3.6

LTE Air Interface



UE eNB MME HSS

NAS AKA procedure eNB obtains security

information from the

MME/HSS during the

AKA procedureIntegrity keyRRC cipher key

Data cipher key

Integrity keyRRC cipher key

Data cipher key

SecurityModeCommand

DCCH/DL-SCHSRB 1

Message sent using

Integrity Protection

UE checks

message integrity

before returning

response and

initiating the

required security

functions


ciphering algorithm to be used

integrity protection algorithm to

be used

SecurityModeComplete

DCCH/UL-SCHSRB 1



Security Mode Setting

RRC plays a largely intermediary role in the air interface security processes. Its main function is the

control of the available security functions, but the implementation of those functions is performed in lower

layers.

The RRC SecurityModeCommand message is used to start one or all of the three independent air

interface security functions, RRC message integrity, RRC message encryption and data trafficencryption. Each process uses a different key, each of which will have been generated in both network

and UE through NAS signalling interaction between the UE and the MME. Once started, the security

functions themselves, including RRC message integrity, are performed in the PDCP (Packet Data

Convergence Protocol) sublayer.





eNB

RRCConnectionReconfiguration

DCCH/DL-SCH

RRCConnectionReconfigurationComplete

DCCH/UL-SCHSRB 1

SRB 1

Information Element Comment

Measurement

configuration

Intra- and inter-frequency as

well as IRAT

Mobility control

information

Target cell configuration and

H/O parameters

NAS message(s) E.g. relating to an NAS

procedure that requires DRB

Dedicated radio resource

configuration

SRB or DRB add, modify or

remove

H/O security information Information regarding security

keys to be used after H/O

Future extension


Data Radio Bearer Establishment

A default EPS bearer and corresponding DRBs will be establ ished as part of the RRC connection

establishment procedure. For some services this may be sufficient, but if new services, or different levels

of QoS, are subsequently required then new DRBs and/or new EPS bearers may be needed to support

them. Additionally, existing DRBs may require reconfiguration because of service change or addition, or

because a handover is required. All of these things can be performed using the

RRCConnectionReconfiguration message. In this respect the RRC message contains details of anySRBs or DRBs that are to be added, modified or removed.

The RRCConnectionReconfiguration message is also a key part of the RRC handover control,

procedures. It is used as an intra-E-UTRA handover command and it is used to configure the

measurement processes used by active RRC connected UEs.

Further Reading: 3GPP TS 36.331:5.3.5, 6.2.2

LTE Air Interface



Measurement Parameters

MeasurementReport

DCCH/UL-SCH


DCCH/UL-SCH


DCCH/DL-SCH

Measurement

processes started

(Periodical or

event-based)

Report triggered

SRB 1

SRB 1

SRB 1

Measurement objects

Measurement quantities

Reporting configuration

Gap configuration

Measurement identities

Future extension


Measurement Configuration

The RRCConnectionReconfiguration message is used to set up, modify or remove measurements to be

made by UEs. In principle, the measurement and reporting process is similar to that used in UMTS. The

UE is instructed in some detail what it should measure, when it should measure it, how it should interpret

the results and under what circumstances it should report measurement results to the eNB. As for UMTS,

measurement reporting can be configured as either periodical or trigger-based.





Source

eNB

Target

eNB

MeasurementReport

X2 – Handover preparation


MAC – random access procedure



Intra-LTE Handover

The diagram shows the air interface signalling for an intra-E-UTRA/LTE handover. The handover is

triggered after the reception of a MeasurementReport message containing measurements and identity for

a valid target cell. Negotiation for resources takes place directly between source and target cell over the

X2 interface. A change in S1 interface resource allocation is also required and involves a negotiation

between the eNBs and the MME. Once all of this is in place the handover instructions, including a

description of the new SRBs and DRBs on the target cell, is transmitted to the UE using anRRCConnectionReconfiguration message.

The UE uses the lower-layer random access procedure to obtain an uplink resource to transmit on the

target cell and a new C-RNTI. The uplink resource is then used to transmit an

RRCConnectionReconfigurationComplete message to the target eNB.


LTE Air Interface



Source

eNB

Target

RAN

Target

CN nodeMME

MeasurementReport

Handover

preparation

MobilityFromEUTRACommand

Handover Complete (or as required for target RAT)


Handover from LTE (I-RAT)

When a measurement report indicates that an I-RAT handover is required, the eNB cannot negotiate

directly with the target cell. Instead, the mobility procedures are handled by interactions via the MME.

Once suitable resources are allocated on the target cell, handover information is forwarded to the source

eNB, which forwards them to the UE in an RRC MobilityFromEUTRACommand message.

On reception of this message the UE changes RAT mode and implements the new channel asinstructed. Handover acceptance and confirmation after this point is dependent on the RAT concerned.

However, for GSM or UMTS this will involve the transmission of a RR or RRC Handover Complete

message.





SECTION 7

LOWER LAYER PROCEDURES

LTE Air Interface




LTE Air Interface




Cell Search Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.1

PLMN Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.2

Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.3

Cell Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.4

Cell Reselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.5

DRX Operation in Idle Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.6

E-UTRA Radio Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.7

Measurements for RRC Connected Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.8

Measurement Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.9

Trigger Events for E-UTRA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.10

Uplink Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.11

Timing Advance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.12

CQI Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.13

CQI Reporting Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.14

MIMO Options for LTE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.15

CONTENTS

Lower Layer Procedures




LTE Air Interface





list the stages through which a UE moves in order to find and synchronize with a cell

define the idle mode state for an LTE UE

identify all the key functions an procedures associated with idle mode operation

describe the cell selection process and identify the key parameters involved

describe the cell reselection process and identify the key parameters involved

explain the operation of DRX for idle mode

define the key radio measurements that are applicable to E-UTRA

explain the options for measurement configuration in the UE

describe how measurements are configured for the UE in connected mode

explain the need for and configuration of measurement gaps

explain the options to event-triggered reporting in LTE

explain the operation of uplink power control for a UE in connected mode

describe timing advance adjustment for LTE

describe the process and application of CQI reporting

list and explain the MIMO options for LTE

OBJECTIVES





LTE Air Interface




UE eNB


Cell Search Procedure

Before the UE can obtain service from an LTE system it must perform a cell search procedure.

Essentially this procedure is used to find and then determine timing and frequency parameters that

enable successful transmission and reception from the cell in question.

The cell search procedure is used for initial synchronization when a UE has just been switched on or has

just entered LTE coverage, and it is also used for new neighbour cell identification for the overall mobilitymanagement processes that control both the idle and connected modes of operation. For initial search

and for new neighbour identification the UE is likely to need to be able to search other RATs in addition

to LTE. In this case the cell search and synchronization procedures are as specified for the technology

being searched.

After detecting an LTE signal the UE searches for the primary and secondary synchronization signals on

the cell. Acquisition of the primary synchronization signal provides slot alignment and identifies the cell ID

(one of three) within the cell ID group. The secondary synchronization signal provides frame alignment,

identification of the cell ID group (one of 168) and identification of CP length. This is sufficient information

for the UE to be able to read the MIB (MasterInformationBlock ) message in the PBCH.

In turn, information in the MIB and the cell ID enable the UE to find and decode the downlink signallingchannels. Ultimately scheduling based on the SI-RNTI in the PDCCH enables the UE to find and read all

the system information for the cell, which is transmitted in the PDSCH.





PLMN selectionand reselection

Cell selectionand reselection

Location

registration

Support for

manual CSG IDselection

Locationregistrationresponse

Servicerequests

Indicationto user Manual

mode

Automaticmode

AvailablePLMNs

SelectedPLMN

Registrationarea changes

Locationregistrationresponse

NAS

control

Radio measurements

CSG IDselected

Available CSGIDs to NAS


Idle Mode

Idle mode represents a state of operation for the UE where it has successfully performed the following:

PLMN selection, cell selection and location registration (by tracking area).

Once in idle mode, the UE will continue to reassess the suitability of its serving cell and, in some

circumstances, its serving network. In order to do this it will implement cell and PLMN reselection

procedures. A UE in idle mode will be monitoring its current serving cell in terms of radio performanceand signalling information. The radio performance measurements are done on the basis of a quality

measure. This is an assessment of radio signal strength and interference level, and it can be made for

both the serving cell and its neighbours. The aim will be to ensure that the UE is always served by the

cell most likely to give the most reliable service should information transfer of any kind be required.

The UE will also be monitoring two key types of signalling from the serving cell system information

messages and paging or notification messages. System information messages convey all the cell and

system parameters. The UE will record changes in these parameters that may affect the service level

provided by the cell, or access rights to the cell. Changes in these parameters could provoke a cell

reselection, or a PLMN reselection. Paging or notification messages will result in connection

establishment.

All of these procedures are performed through communication between the AS and the NAS. In general,

instructions are sent from the NAS to the AS; the AS then performs the requested procedure and returns

a result to the NAS.

If CSG (Closed Subscriber Group) is supported then these procedures are modified such that a cell’s

broadcast CSG ID forms another level of differentiation between cells. CSG is intended for use with

HeNBs (femtocells).





Based on priority ofRAT/Frequency layers

and thresholds

Based on priority ofRAT/Frequency layers

and thresholds

Based on measurements,offsets, parameters and

mobility status

1 sec since last reselectionCell is suitable

I-RATInter-frequency

E-UTRAInter-frequency E-UTRA

Intra-frequency

Low

Medium

High

Measurement rules

Evaluation

Ranking

Reselection


Cell Reselection

Cell reselection in LTE both reuses many principles that were are well established in legacy technologies

and introduces new strategies. A key addition for LTE is the use of RAT/frequency prioritization. Each

frequency layer that the UE may be required to measure, either E-UTRA or any other RAT, is assigned a

priority. The cell-specific priority information is conveyed to UEs via system information messages.

Additionally, UE-specific values can be supplied in dedicated signalling, in which case they take priority

over the system information values. Any indicated frequency layers that do not have a priority will not beconsidered by the UE for reselection.

In general, the measurement rules are used to reduce unnecessary neighbour cell measurements. The

UE always measures cells on a higher priority E-UTRA inter-frequency or IRAT frequency. The UE will

only measure E-UTRA intra-frequency cells if the Srexlev value for the current selected cell falls below

an indicated threshold (Sintersearch). Similarly, the UE only measures E-UTRA inter-frequency or IRAT

frequency cells on equal or lower priority layers if the Srexlev value for the current selected cell falls

below an indicated threshold (Snonintrasearch).

Measurements are then evaluated for potential reselection. Again, the frequency/RAT priority level is

used along with system-defined threshold for this assessment. A UE will always reselect a cell on a

higher priority frequency if its value of Srxlex exceeds Threshx,high for longer than TreselectionRAT. It willonly select a cell on a lower priority frequency when the Srxlev of the serving cell falls below

Threshserving,low and Srxlev of the neighbour is above Threshx,low for TreselectionRAT and there is no other

alternative. For neighbour cells on intra-frequencies or on equal priority E-UTRA inter-frequencies, the

UE uses a ranking criterion ‘Rs’ for the serving cell and ‘Rn’ for the neighbour cell. Ranking is based on a

comparison of the respective Srxlev values with a hysteresis added to the serving cell value and an offset

added to the neighbour cell value. The UE will select the highest ranked cell if the condition is maintained

for TreselectionRAT.

In addition to all of this, the UE will apply scaling to Treselection, hysteresis values and offset values

dependent on an assessment of its mobility state, which may be high, medium or low. This is based on

an analysis of resent reselection frequency.





Paging cycle (32, 64, 128, 256 radio frames)

PF

PO


DRX Operation in Idle Mode

Once successfully registered on the system and camped on a cell in idle mode, the UE will need to

monitor paging messages. Paging messages are transmitted in the PDSCH and scheduled in the

PDCCH with the P-RNTI identity. This means that the UE need only monitor the PDCCH channel.

However, to provide further battery saving, a DRX cycle is established such that the UE only has to

monitor the PDCCH at defined UE-specific intervals.

Idle mode DRX parameters are broadcast by default in system information, but higher-layer dedicated

signalling may be used to give the UE a UE-specific paging cycle. In the case that the UE has both

system information and dedicated paging cycles, it will use whichever is the lowest value.

The standards define the formula that the UE uses to calculate its paging occasion. The calculation is

based on the DRX information received from the cell and its IMSI value. The output of the calculation

defines the frames within a paging cycle, known as the PF (Paging Frame), and the subframe within an

PF, known as a PO (Paging Occasion) that the UE needs to monitor for scheduling relating to paging

messages.


LTE Air Interface



(Reference Signal Received Power) (Received Signal Strength Indicator)

Total received power in RS OFDMsymbol periods including the serving

cell, all co-channel and adjacent channelinterference and thermal noise

Linear average power ofthe reference signalresource elements

The ratio of the reference signal power,calculated as N x RSRP, to the RSSI, where

N is the number of RBs in the RSSImeasurement bandwidth

(Reference Signal Received Quality)

RSRP RSSI

RSRQ

Serving cellServing cell


E-UTRA Radio Measurements

There are three key measurement values used in E-UTRA, the RSRP (Reference Signal Received

Power), the RSSI (Received Signal Strength Indicator) and the RSRQ (Reference Signal Received

Quality).

The standards define RSRP as:

‘The linear average over the power contributions of the resource elements that carry cell-specific

reference signals within the considered measurement frequency bandwidth’.

The standards define RSSI as:

‘The linear average of the total received power observed only in OFDM symbols containing

reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource

blocks by the UE from all sources, including serving and non-serving cells, adjacent channel

interference, thermal noise, etc.’

The standards define RSRQ as:

‘The ratio Nx RSRP/(E-UTRA carrier RSSI), where N is the number of RBs of the E-UTRA carrier

RSSI measurement bandwidth’.

Note that the measurement of RSRP is based on reference signals from antenna port 0, but where

antenna port 1 can be received reliable, reference signals from that port may also be included.

Additionally, the values of RSRP and RSSI used to calculate RSRQ must have the same measurement

bandwidth.





UE

RRC Connected

eNBServing cell

Gap configuration

Quantity configuration

Measurement identities

Reporting configuration

Measurement objects

Measurement Parameters

Neighbour

cells


Measurements for RRC Connected Mode

When the UE becomes RRC connected, the measurement and reporting process as well as mobility

decisions becomes the responsibility of the eNB. The required measurement and reporting settings are

signalled to the UE in the RRCConnectionReconfiguration message.

The measurement object defines what the UE is to measure. This is defined as a frequency and

measurement bandwidth; optionally it may also contain a list of cells. If it does contain a list of cells thenthey will be indicated as either white list or black list. The UE will measure any cells it detects but will not

report black list cells. Frequency- or cell-specific offsets will also be included in this field.

The reporting configuration sets what quantities the UE is to measure, what quantities the UE is to report

and under what circumstances a measurement report is to be set. Reporting may be set as either trigger-

based, periodic or triggered periodic. This field also defines the other contents of the measurement report

message.

Measurement identities provides a reference number such that some part of this identified measurement

can be modified or removed in future.

The Quantity configuration sets the filtering to be used on the measurements that are taken.

The gap configuration defines periods when the UE can take measurements of neighbour cells.


LTE Air Interface



eNBServing cell

Transmission gap(6 ms)

Transmission gap repetitionperiod (N x 10 ms)

Neighbour cell Neighbour cellNeighbour cell


Measurement Gaps

When the UE is in RRC connected mode it will be engaged in data transfer in the uplink or downlink

directions or both. In order to simplify the design of the UE it is not required to be able to take neighbour

cell measurements and transfer data with the serving cell at the same time. This requires defined periods

where the UE is able to take neighbour cell measurements and is not required to communicate with the

serving cell.

Transmission gaps perform this function and are very similar in concept to compressed mode for UMTS.

The transmission gaps have a duration of 6 ms since this allows sufficient time to take measurements

and gain basic synchronization with most RATs in a single transmission gap. For GSM, however, 6 ms

remains a sufficient gap, but multiple transmission gaps are required to take measurements and

determine a cell’s BSIC.

The transmission gap period is variable, but will be a multiple of 10 ms.

The transmission gap pattern to be used by a UE is included in the measurement parameters.





Absolute threshold

Serving cell

Event A1

E-UTRA Trigger Events

Absolute threshold

Serving cell

Event A2

Offset

Serving cell

Event A3Neighbour cell

Absolute threshold


Serving cell


Absolute threshold1

Absolute threshold2

Absolute threshold

Event B1

neighbour cell

IRAT Event Triggers

Serving cell

Event B2

Absolute threshold1

Absolute threshold2

neighbour cell


Trigger Events for E-UTRA

The trigger events defined for E-UTRA are as follows:

Event A1 – The serving cell becomes better than absolute threshold

Event A2 – The serving cell becomes worse than absolute threshold

Event A3 – A neighbour cell becomes better than an offset relative to the serving cell

Event A4 – A neighbour cell becomes better than absolute thresholdEvent A5 – The serving cell becomes worse than absolute threshold1 and a neighbour cell becomes

better than absolute threshold2

There are also two more events for IRAT mobility:

Event B1 – A neighbour cell becomes better than an absolute threshold

Event B2 – The serving cell becomes worse than absolute threshold1 and a neighbour cell becomes

better than absolute threshold2

Events can be modified with time-to-trigger values and hysteresis values if required. Triggers may be

used to cause the transmission of a single measurement report or may be used to trigger a session of

periodic reporting. This is defined by setting the parameters reportAmount (which includes an infinitevalue) and reportInterval.


LTE Air Interface



P PUSCH (i)

Power control

headroom

mBd } )i ( f + )i ( F T +LP . ) j ( a+ ) j ( H C SU P _OP + ) )i ( H C SU P M ( 0 1g ol 0 1, X AM C P { ni m=

UE Power Class

1

2

3

4

...

...

23 dBm

...

Bandwidthdependent element,

MPUSCH(i) is thenumber of allocated

RBs

ssolhtapr of r otcaf noitasnepmoc

asiLP.) j(dnalevelesabdenif ed-llecehtsi) j(HCSUP _ OP

,tniopgnitar epopool-nepocitats-imeS

sdnammocCPTf onoitalumuccaehtsi)i(f dnadnammocCPT

enosi)i(FT,tesf f opool-desolccimanyD


Uplink Power Control

Even though uplink transmissions from LTE UEs in a cell are orthogonal, uplink power control is still

important if maximum throughput efficiency is to be achieved for individual UEs and for the cell as a

whole.

The UE calculates the transmit power to be used in each subframe in which it has a resource allocation

according to the formula shown in the diagram. Maximum power is limited by the UE power class, whichwill correspond to 23 dBm. The calculation for power to be used below this level is based on three

elements: a bandwidth-dependent element, a semi-static open-loop operating point and a dynamic

closed-loop offset.

The bandwidth element is based on the number of scheduled RBs in the UE’s uplink transmission.

The semi-static control point is itself made up from two elements. The first, P O_PUSCH( j ) , is a cell-

defined offset between –126 dBm and +23 dBm. The second part is a compensation factor based on the

UE’s estimate of downlink path loss. The value α can be varied between 0 and 1. Variation of

P O_PUSCH( j ) and α provide a trade-off between absolute cell performance and overall system

performance.

The dynamic closed loop offset is based on TPC (Transmit Power Control) commands transmitted to the

UE in the PDCCH and identifies using a TPC-RNTI. The closed loop mode of operation can operate in

two modes, one in which absolute power control commands are sent and one where corrections on a

accumulative value are sent. It is in the latter case that is referenced by the parameter f (i ).

If a UE were to be allocated an uplink bandwidth that resulted in a calculated power higher than 23 dBm,

then the UE would be unable to use the full resource. To avoid this the UE will send power headroom

reports to the eNB. These represent the UE’s estimate or its power control requirements in the current

subframe, and based on this, the eNB will be able to schedule resources efficiently between UEs in a

cell.

Further Reading: 3GPP TS 36.101:6.2.2, 36.213:5.1




eNB measures propagationdelay from PRACH preamble

TA step size is 16T s (0.52 μs)

Correction is included in theRAR as a value of steps in the

range 0 to 1282 (0 to 0.67 ms

TA adjustments are madeusing MAC controlmessages in the PDSCH

Correction is a value in therange 0 to 63 interpreted as+/– 31 steps (+/– 16 μs)


Timing Advance

In order to maintain orthogonality between uplink transmissions from multiples UEs in a cell, timing

adjustment must be applied to compensate for variations in propagation delay.

Initial timing advance is calculated at the eNB from a UE’s preamble transmission on the PRACH. The

timing advance correction is given as an 11-bit value although the range is limited to 0–1282 timing

advance steps. Granularity is in steps of 16Ts (0.52 μs) so timing advance can be varied between 0 and0.67 ms. One timing advance step corresponds to a distance change of c.78 m and is significantly

smaller than the normal CP. The maximum timing advance value corresponds to a range of c.100 km.

The maximum specified speed for a UE relative to an eNB is 500 km/h (139 m/s), which would require

slightly more than one timing advance change every two seconds. Consideration also needs to be given

to the possibility of more extreme changes in the multipath characteristics of a channel, for example the

sudden appearance or disappearance of a strong reflected path from a distant object or delay through a

repeater. However, these are extreme examples and, in any case, timing advance update commands

can indicate up to +/– 16 μs in a single step. Thus the rate at which timing advance commands need to

be sent in practice is typically much less than one every two seconds.

Timing update commands are transmitted to UEs as MAC control messages and as such are included inMAC PDUs carrying data for the UE on the PDSCH. The command itself is a six-bit value giving a

number range from 0–63. Values less than 31 will reduce timing advance and values greater than 31 will

increase timing advance.


LTE Air Interface



Efficiency

(bits/symbol)

0 No TX ... ...

1 QPSK 0.076 0.1523

2 QPSK 0.12 0.2344

3 QPSK 0.19 0.377

4 QPSK 0.3 0.6016

5 QPSK 0.44 0.877

6 QPSK 0.59 1.1758

7 16QAM 0.37 1.47668 16QAM 0.48 1.9141

9 16QAM 0.6 2.4063

10 64QAM 0.45 2.7305

11 64QAM 0.55 3.3223

12 64QAM 0.65 3.9023

13 64QAM 0.75 4.5234

14 64QAM 0.85 5.1152

15 64QAM 0.93 5.5547

CQI Index ModulationApprox. code

rate

Downlink channel

adaptation based on

UE CQI reporting

Uplink channel adaptation based on

eNB measurements of UL data

transmissions and SRS if requested


CQI Reporting

Link adaptation is a crucial part of the LTE air interface and involves the variation of modulation and

coding schemes to maximize throughput on the air interface.

Link adaptation for scheduled uplink resources can be can be calculated by the eNB from a number of

different inputs based on measurements of a UE’s uplink transmissions. Additionally the eNB may

request that UEs transmit sounding reference signals, the measurement results of which can also beused for link adaptation.

For downlink transmissions the eNB needs information about the success or otherwise of the UE in

receiving its downlink transmissions. The UE assesses the quality of the downlink signal through

measurements of the received signal and consideration of the error correction scheme. It then calculates

the maximum modulation and coding scheme that it estimates will maintain an error rate better than 10%.

This is indicated to the eNB as a CQI (Cell Quality Indicator) value. The table in the diagram (taken from

the 3GPP standards) shows how the CQI values are interpreted as modulation and coding schemes. The

table is also useful for estimating the likely physical layer throughput in a given radio configuration.





CQI Reporting

AperiodicReports on request inPUSCH

PeriodicReports regularly inPUCCH

UE-selected sub-bandfeedback

CQI across total system bandwidth

+Preferred sub-band positions

+

CQI average for preferred sub-bands

Wideband feedback

CQI across total systembandwidth

eNB-configured sub-bandfeedback

CQI across total system bandwidth

+CQI offset for each sub-band


CQI Reporting Options

CQI reporting can be configured for a UE in several different ways. Firstly, the type of reporting is

instructed as either periodic or aperiodic. For periodic reporting the CQI is carried in the PUCCH at

regular intervals that can be configured between 2 ms and 160 ms. For aperiodic reporting the CQI is

carried in the PUSH only after a specific request from the eNB, which is included in the PDCCH

scheduling information.

Additionally, the CQI feedback mode may be configured as wideband feedback, eNB-configured sub-

band feedback or UE-selected sub-band feedback. All three options are applicable for aperiodic reporting

but only wideband feedback and UE-selected sub-band feedback can be configured for periodic

reporting.

For wideband feedback the reported CQI value is based on an assessment across the whole system

bandwidth. For both sub-band feedback modes, sub-bands are defined across the system bandwidth as

groups of consecutive RBs. The size and number of sub-bands is fixed, dependent on the total system

bandwidth and the feedback mode in use. For the eNB-configured sub-band feedback mode the UE

reports the wideband CQI and then each sub-band CQIs as relative offset values. For the UE-selected

sub-band feedback mode the UE selects a set of preferred sub-bands from the total available sub-bands

and indicates their positions to the eNB. Then it reports an average CQI value for these preferred sub-bands along with and wideband CQI value.


LTE Air Interface



Transmit Diversity Beamforming

Closed loop with PMI feedback

SU-MIMOMU-MIMO (virtual MIMO)


MIMO Options for LTE

In its first release, LTE is specified with several options for SU-MIMO implementation and a more limited

option for MU-MIMO operation. The specification include descriptions of operation up to rank 4 (4x4

MIMO).

The simplest option is not MIMO, as such, but uses the multi antenna array at an eNB to provide transmit

diversity. The standards allow configuration with up to four antennas at the base station. It is likely thatcross-polar antennas would be used as part of the antenna array, so a two-antenna array could be

implemented using a single cross-polar panel, with a four-antenna array requiring two cross-polar panels.

Transmit diversity involves the transmission of a single data stream to a single UE, but makes use of the

spatial diversity offered by the antenna array. This can increase channel throughput or increase cell

range.

There are also two beamforming options available. These are based on the use of a single layer with

rank one pre-coding but make use of a multi antenna array for beamforming to a single UE. The two

options for this are a closed loop mode, which involves feedback of PMI (Pre-coding Matrix Indicators)

from the UE, and an open loop mode, which involves the transmission of UE-specific reference signals

and the eNB basing the pre-coding for beamforming on uplink measurements.

Full SU-MIMO configurations are available in LTE in the downlink direction with ranks up to four.

However, a maximum of two data streams is used, even when four antenna ports are available. In SU-

MIMO the UE can be configure to provide PMI feedback as well as RI (Rank Indicators), which indicates

the rank that the UE calculates will give the best performance.

In the first release of the LTE specification there is only a limited implementation of MU-MIMO specified.

It is applicable in the uplink direction and allows two UEs to use the same time frequency resource within

one cell.

Further Reading: 3GPP TS 36.211:6.3.3, 6.3.4, 36.213:7.1





LTE Air Interface



LTE AIR INTERFACE

GLOSSARY OF TERMS

LTE Air Interface




LTE Air Interface




LT3602/v3G.2 © Wray Castle Limited

FDM Frequency Division Multiplexing

FFT Fast Fourier Transform

FI Framing Information

FMS First Missing PDCP SN

GAN Generic Access Network

GANC Generic Access Network Controller

GBR Guaranteed Bit Rate

GGSN Gateway GPRS Support Node

GMM GPRS Mobility Management

GMSC Gateway MSC

GSM Global System for Mobile Communications

GT Guard Time

GTP-C GPRS Tunnelling Protocol – Control plane

GTP-U GPRS Tunnelling Protocol – User plane

GUTI Globally Unique Temporary Identity

HARQ Hybrid ARQ

HeNB GW Home eNB GatewayHLR Home Location Register

HNB Home Node B

H-PLMN Home Public Land Mobile Network

HSDPA High Speed Downlink Packet Access

HSPA High Speed Packet Access

HSS Home Subscriber Server

HSUPA High Speed Uplink Packet Access

IAM Initial Address Message

IDFT Inverse Discrete Fourier Transform

IFFT Inverse Fast Fourier Transform

IK Integrity KeyI-MAC Integrity Message Authentication Code

IMEISV International Mobile Equipment Identity and Software Version

IMS IP Multimedia Subsystem

IMSI International Mobile Subscriber Identity

I-RAT Inter-Radio Access Technology

ISI Inter Symbol Interference

LCID Logical Channel Identity

LCR Low Chip Rate

LSF Last Segment Flag

LTE Long Term Evolution

MAC Medium Access Control

MBR Maximum Bit Rate

MBSFN Multicast/Broadcast Single Frequency Network

MCS Modulation and Coding Scheme

MIB MasterInformationBlock

MIMO Multiple Input Multiple Output

MME Mobility Management Entity

MMEC MME Code

MMEGI MME Group Identifier

MSISDN Mobile Subscriber ISDN Number

M-TMSI MME Temporary Mobile Subscriber Identity

MU-MIMO Multi-User MIMO

NACC Network Assisted Cell Change

NAS Non Access Stratum

LTE Air Interface



S1AP S1 Application Protocol

SAE System Architecture Evolution

SAP Service Access Point

SC-FDMA Single Carrier Frequency Division Multiple Access

SCTP Stream Control Transmission Protocol

SDP Session Description Protocol

SFN Single Frequency Network

SGSN Serving GPRS Support Node

S-GW Serving Gateway

SIB System Information Block

SI-RNTI System Information RNTI

SISO Single Input Single Output

SN Sequence Number

SO Segment Offset

SRB Signalling Radio Bearer

SRI Send Routing Information

SRS Sounding reference Signals

SRVCC Single Radio Voice Call Continuity

SSS Secondary Synchronization SignalS-TMSI SAE TMSI

LTE Air Interface

lt3602 lte air interface - v3

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