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Section 1 · Module 1 ·

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All Rights Reserved © Alcatel-Lucent 2009

Module 1Introduction

LTE Basics




LTE Basics

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First editionMarius OLTEAN2009-1001

RemarksAuthorDateEdition

Document History




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Course Objectives

Upon completion of this course, you should be able to:

Understand the market trend that drives LTEKnow the basics about the LTE standardization List the main technologies employed at the PHY layer Describe the structure of the radio frame in LTE Explain the protocol stack corresponding to the LTE standardizationDescribe the ALU architecture for LTE




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1. LTE and the wireless market




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1 LTE and the wireless market

Market Trends

The trend of the market shows an increase and an acceleration of the data mobile traffic in the next years.

This has been possible thanks to the “3G improvements”:EV-DO with CDMA2000HSDPA and HSUPA with WCDMA

Mobile Data Traffic Evolution

With the recent introduction of HSDPA and EV-DO Rev A, there has been a significant increase in mobile data traffic, with some operators quadrupling their Packet Switched traffic in one year. At this growth rate, and with the proliferation of new applications on the network, cells in hot spots will be quickly saturated and the network will require densification in these overloaded areas. This can be delivered by using a higher capacity solution such as LTE. Mobile traffic growth is illustrated on this slide: mobile data traffic (in Gigabits per year), with a typical operator in a western country with a 60 million population.




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1 LTE and the wireless market

3G Limitations

The limiting factors of the 3G technologies are:

Data rates:Throughput of 14.4 Mbps in DL for the HSDPA (WCDMA)Throughput of 14.7 Mbps in DL for the EV-DO Rev B (CDMA2000)

Latency in the range of 50 to 100 ms

No straight connection in the IP domain3G radio access technologies are not natively based on IPThe current trend is to evolve towards end-to-end IP networks

No flexibility in spectrum management




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1.1 LTE objectives

3G LTE targets

LTE objectives:

Improved performance and capacityUser-plane latency: < 5 ms one way (UE to Core Network)Peak data rates of 100 Mbps in downlink and 50 Mbps in uplink

SimplicityFlexible carrier bandwidths (from 1.4 MHz up to 20 MHz)Both FDD and TDD allowedA wide area of choice for the spectrum (15 FDD bands and 8 TDD bands)Flat IP based architecture

Reduce transport network cost Natively based on IP, i.e. IP transport network for all the services. That brings flexibility and saves the cost

Wide range of terminals Not only mobile phones, but other electronic devices too (notebooks, smart phones)HO and roaming support to existing mobile networks




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2. Standardization




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2.1 Standardization bodies

What is the 3GPP?

The 3GPP, 3rd Generation Partnership Project is a collaboration between groups of telecommunication associations where all the telecom actors are involved:

ManufacturersOperatorsGovernments

The 3GPP has specified the following standards:

GSMGPRSGERANWCDMAHSDPA

The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunication associations, to make a globally applicable third generation 3G mobile phone system specification within the scope of the International Mobile Telecommunication-2000 project of the International Telecommunication Union (ITU). 3GPP specifications are based on evolved Global Systel for Mobile Communication (GSM) specifications. 3GPP standardization encompasses Radio, Core Network and Service architecture.

The groups are the European Telecommunications Standarts Institute, Association of Radio Industries and Businesses/Telecommunication Technology Committee (ARIB/TTC) (Japan), Alliance for Telecommunications Industry Solutions (North America) and (South Korea). The project was established in December 1998.

3GPP should not be confused with 3rd Generation Partnership Project 2 (3GGP2), which specifies standards for another 3G technology based on IS-95 (CDMA), commonly known as CDMA2000.




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2.1 Standardization bodies

3GPP2

3GPP2 stands for 3rd Generation Partnership Project 2.

The participating associations are: ARIB/TTC (Japan)China Communications Standards AssociationTelecommunication Industry Association (North America)Telecommunication Technology Association (South Korea)

The 3GPP2 specifies: CDMAOneCDMA200O, EV-DO

3GPP2 was born out of the International Telecommunication Union's (ITU) International Mobile Telecommunications “IMT-2000” initiative, covering high speed, broadband, and Internet Protocol (IP)-based mobile systems featuring:

network-to-network interconnection,

feature/service transparency,

global roaming,

seamless services independent of location.

IMT-2000 is intended to bring high-quality mobile multimedia telecommunications to a worldwide mass market by achieving the goals of increasing the speed and ease of wireless communications, responding to the problems faced by the increased demand to pass data via telecommunications, and providing "anytime, anywhere" services.




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2.2 3GPP standards evolution

3GPP Releases

The 3G LTE standard is defined by the 3GPP release 8.




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2.3 3GPP requirements for LTE

3GPP Requirements

The 3GPP requirements are:

Maximum throughput in DL: 326.4 Mbits/sec with 4 antennas (MIMO) and 172.8 Mbits/sec with 2 antennas for a bandwidth of 20 MHzMaximum throughput in UL: 86.4 Mbits/sec for a bandwidth of 20 MHz200 users per cell for a bandwidth of 5 MHz A latency below 5 ms (one-way path) for short IP packetsMore flexibility for the bandwidth allocation and usage (from 1.4 MHz to 20 MHz)

Current 3G systems used fixed 5 MHz bandwidth Cell range of 5 Km with optimal performance. 30 Km with reasonable performanceCo-existence with the current standards and smooth transition from the earlier technologies




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2.3 3GPP requirements for LTE

Carriers and Bandwidths for LTE

e-UTRAN is designed to operate in the frequency bands defined in the following table:




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3. PHY layer techniques




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PHY layer key points

OFDMAOFDM is improved by multiple access and scalabilityCyclic prefix added (two possible durations)In UL, SC-FDMA is employed, which can be regarded as a modified version ofOFDM, or, alternatively, as a single carrier transmission

Link adaptationThe transmission parameters are adapted in real time to the state of the channelAdaptive Modulation & Coding, time/frequency sensitive scheduling, interference management, power control algorithms

Channel coding done by turbo-codesMultiple antennas algorithms

Tx diversityBeam formingSpatial Multiplexing




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3.1 OFDMA and SC-FDMA

OFDMA Principles

The total amount of available resources (subcarriers) is shared between different users for the duration of the OFDMA symbolAt the same time, sub-carriers may carry data to/from different users

Frequency

Sub-carrier

Time

Data for several users

Symbols #1

Symbols #2

Symbols #3

User 1

User 1

User 1

User 2

User 2

User 2

User 2

User 3

User 3

User 3User 4User 4




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OFDMA (used in DL) has a high value of the PAR parameterimportant challenge for the Power Amplifiers PAR reduction required by complex signal processing

In the UL, a modified version of OFDM is employedTwo names are used: SCFDMA, or DFT-precoded OFDMLower PAR is achievedLess expensive amplifiers in the UE, increased battery life, less complex signal processing

eNode-B

DLUL

DL : OFDMA

UL : SC-FDMA

3.1 OFDMA and SC-FDMADifference between DL and UL




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3.1 OFDMA and SC-FDMASC-FDMA for UL transmission

The symbols to be transmitted are pre-coded by a M-point DFT operationM must be smaller than N !!!N is the total number of carriers used in transmissionSuch a signal has some single-carrier properties (even if IFFT is used like in multi-carrier)The receiver (eNodeB) preserves the basic structure of an OFDM receiver




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3.1 OFDMA and SC-FDMAWhy “Single-carrier” FDMA?

The “original” symbols (x0, x1, x2 and x3) are retrieved in the vector obtained after IFFTThey will sequentially modulate a high-frequency carrier




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3.2 Multiple antennas techniques in LTE

Need for multiple antenna techniques

The highly-challenging requirements of LTE can only be met by using multiple antennas techniquesMultiple antennas can be used in both eNodeB and UE Multiantenna configuration determined by:

Multiantenna processing to be appliedNumber of Tx/Rx antennas

In LTE, the multiple antennas algorithm can be adapted on a “per-user-basis”




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Multiantenna schemes and benefits

Tx diversity achieved in LTE by Space-Frequency Block Coding (SFBC) or Cyclic Delay Diversity (CDD)Closed loop power control (SM, BF) versus open loop power control (Txdiversity)




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Cyclic delay diversity

Tx diversity relies on multiple antennas at the transmitter sideSpatial diversity can be achieved when transmitting one data streamCDD

The same signal, with different delays is transmitted by the different antennasA delay of the time-domain OFDM symbol can be perceived like a phase shift in the frequency domainTransparency to the UE, so it can be applied in conjunction with other techniques (SM)




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Space-frequency block coding

SFBC is a variation of the famous STBC, better adapted to OFDMAlamouti’s scheme is applied to the frequency-domain OFDM symbol (prior to IFFT)This technique mainly enhances the robustness of the link




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Spatial Multiplexing

Capacity enhancementDifferent streams are transmitted using the same time-frequency resource, but via parallel channels provided by spatially separated antennasFor M Tx antennas and N Rx antennas, L=min(M,N) independent channels can be theoretically provided Feedback required from the UE (close loop power control)BF can be regarded as a particular case of SM, where a single stream is transmitted by all the antennas




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4. OFDMA parameters for LTE




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4 OFDMA and LTE

OFDMA Parameters for LTE

The width of a Sub-carrier is 15 kHz, whatever the bandwidthThe bandwidths are: 1.4, 3, 5, 10, 15 and 20 MHz

5 MHz 10 MHz

… …15 kHz Sub-carrier

The symbol duration is always the same, whatever the bandwidthThere are 2 times more sub-carriers in 10 MHz than in 5 MHz

2 times more symbols can be sent or received at the same time.The capacity is multiplied by 2




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4 OFDMA and LTE

OFDMA Parameters for LTE [cont.]

1200 (1201)900 (901)600 (601)300 (301)150 (151)75 (76)Number of useful sub-carriers

204815361024512256128Number of sub-carriers (FFT size)

30.72 MHz(8 × 3.84)

23.04 MHz(6 × 3.84)

15.36 MHz(4 × 3.84)

7.68 MHz(2 × 3.84)3.84 MHz1.92 MHz

(1/2 × 3.84)Sampling frequency

15 kHz Sub-carrier spacing

20 MHz15 MHz10 MHz5 MHz3 MHz1.4 MHzSpectrum allocation

5 MHz7.68 MHz

For the 5 MHz, there are 512 sub-carriers of 15 kHz, whereas the total band is 7.68 MHz. It is larger than the 5 MHz band!But only 301 sub-carriers are used (Pilot, DC, data), the other ones are guard sub-carriers:

301 Sub-ca * 15 kHz = 4.515 MHz

Used bandwidth

FFT sizes are chosen so that sampling rates are a multiple of the UMTS chip rate (3.84 MHz).

It eases the implementation of dual mode UMTS/LTE terminals




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4 OFDMA and LTE

OFDMA Parameter for LTE [cont.]

The symbol duration depends on the sub-carrier bandwidth.

2 Cyclic Prefixes are defined:Long CP: 16.67 micro secondsShort CP: 4.69 micro seconds

The total duration of a symbol is:

Useful duration + CP = 66.6 + 16.67 Total duration = 83.33 µs (long CP)

1 1 66.615

UsefulSymbolDuration sSub CarrierBW kHz

= = = μ−

#4#3#2#1

User 1

User 1

User 1

User 2

User 2

User 2

User 2

User 3

User 3

User 3User 4User 4

Time




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Exercise 1

Let‘s assume the following 2 radio conditions:

Case 1: Radio conditions -> 16 QAM and coding rate ½Case 2: Radio Conditions -> QPSK and coding rate 2/3

How many symbols are required to transmit 100 bits when radioconditions are similar to:

case 1?

case 2?




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Exercise 2

Let‘s assume the following:Guard time for symbol = 2 µsTotal symbol duration = 8 * guard timeTotal Bandwidth = 5 MHzBest modulation = 64 QAM and coding rate: 2/3

Taking in account the above assumptions, could you determine the maxbit rate this system can reach?

Note: The values are not LTE compliant (this is only an exercise !)

Hint: determine the number of sub-carriers.




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5. Radio Frame Structure




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5. Radio Frame Structure

Basic Frame Structure

FDD is commonly referred to as “paired spectrum” transmissionThe FDD RF frame is called Type 1 by the 3GPP.The RF length is 10 ms.

RF #3RF #2RF #1 ……

DL Carrier

UL Carrier

The radio frame is made up of 10 sub-frames of 1 ms each.Each sub-frame is made up of 2 slots of 0.5ms each.

RF #1

10 sub-frames

2 slots

RF #3RF #2RF #1

#9#8#7#6#5#4#3#2#1#0

#2#1




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5.1 Resource units on the radio interface

Slots

Each slot is made up of:7 symbols in case of short CP6 symbols in case of long (also called extended) CP

#9#8#7#6#5#4#3#2#1#0

RF #1

10 sub-frames

Tu= 66.7µs

Tu = Useful Symbol DurationTcp = Cyclic Prefix durationTecp = Extended Cyclic Prefix duration

#7

Tcp = 4.7 µs

#6#5#4#3#2#1

#6#5#4#3#2#1

Tecp = 16.7 µs

#2#1




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Resource Block

A slot can be represented as follows:

#7#6#5#4#3#2#1

time

Frequency

5 MHz (300 sub-ca)10 MHz (600 sub-ca)

1 sub-carrier/1 OFDM symbol is called a Resource Element

A group of 12 sub-carriers/7(or 6) symbols is

called a Resource Block (RB)

For data transmission, the minimum amount of resources allocable is 2 RBs.

#289

#149

#300




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Resource block: a different view

There are 84 resource elements (subcarriers) composing a RB (72 for the long CP)The way that the subcarriers are allocated is: 7 OFDMA symbols x 12 subcarriers/OFDMA symbol = 84 subcarriersThe 7 OFDMA symbols which are part of a RB span 0.5ms (1 slot, ½ subframe)The physical bandwidth of a RB is 180 KHz




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Resource Element Group

The Resource Block is not well suited for the control channel (the radio signaling).

The control channels are mapped to the Resource Elements Groups (REGs), which represent less radio resources

A REG is made up of 4 (or 6 if there are pilot sub-carriers) sub-carriers during 1 OFDM symbol.

Sub-carriers

Symbols

1 REG

#12

#11

#10

#9

#8

#7

#6

#5

#4

#3

#2

#1




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6. Protocol Stack




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6 Protocol Stack

Radio Interface Overview

An End-to-end service is mapped on the radio interface on a radio bearer.The radio bearer is managed by the radio protocols running in the eNodeB and the UE.

Radio S1

S1 and S5/S8 bearers refer to specific LTE interface within eUTRAN and EPC

EPS: Extended Packet Service (the “assembly” of the radio interface eUTRAN and the evolved Packet Core )




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6.1 Protocols architecture

Protocols on the radio interface

eNode-B

PDCP

Physical Layer

MAC Layer

RLC

RRC

NAS

Control PlaneUser Plane

PDCP

Physical Layer

MAC Layer

RLC

RRC

NAS

Control PlaneUser Plane

Transport Channel

Logical Channel

Radio Bearer

Physical Channel

Non-Access StratumSignaling between Core Network and UE

Radio Signaling

RRC = Radio Resource Control

PDCP = Packet Data Convergence Protocol

RLC = Radio Link Control

PDCP, RLC and MAC can be seen as layer-2 sublayers

L2 L2




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Higher layers

6.2 Protocols on the radio interface

RRC

The Radio Resource Connection (RRC) protocol is implemented in the eNodeB and the UE. In WCDMA, it is implemented in the RNC!

RRC is the highest protocol in the control plane on the radio side. The RRC protocol allows:

the 2 instances (eNodeB and UE) to exchange signaling messages.to forward signaling messages coming from the core network, called NAS signaling.

RRC

RRC

eNode-B

ePC

Radio signaling

NAS signaling

RRC acts exclusively on the control plane of the radio interface between UE and eNodeB




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2.3 RRC [cont.]

RRC protocol is in charge of:Broadcast of System Information related to the Non-Access Stratum (NAS) and Access Stratum (AS)PagingEstablishment, maintenance and release of an RRC connection between the UE and E-UTRAN including:

Allocation of temporary identifiers between UE and E-UTRAN.Configuration of signaling radio bearer(s) for RRC connection.Low priority SRB and high priority SRB.Security functions including key management.Establishment, configuration, maintenance and release of point to point Radio Bearers.

Mobility functions including:UE measurement reporting and control of the reporting for inter-cell and inter-RAT mobility.Handover.UE cell selection and reselection and control of cell selection and reselection.Context transfer at handover.

Establishment, configuration, maintenance and release of Radio Bearers for MBMS services.QoS management functions.UE measurement reporting and control of the reporting.NAS direct message transfer to/from NAS from/to UE.

The NAS layer performs:

Authentication

Security control

Idle mode mobility handling

Idle mode paging origination

MBMS = Multimedia Broadcast/Multicast Service

SRB = Signaling Radio Bearer




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2.3 RRC [cont.]

RRC uses the following states:

RRC_IdleThe UE is not connected. There is no radio link. The network knows that the UE is present on the network and is able to reach it in case of incoming call.The UE switches in idle mode when it is connected and there is no traffic. This allows to save radio resources and its battery.

RRC_ConnectedThe UE has an e-UTRAN-RRC connection.The network can transmit/receive data to/from the UE and knows its location at the cell level.The network manages the mobility with handover.




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PDCP

The main services provided by the Packet Data Convergence Protocol (PDCP) are:

PDCP

Physical Layer

MAC Layer

RLC

RRC

NAS

User Plane Control PlaneFor the user plane:

IP header compression and decompression with the Robust Header Compression (ROHC) method onlyCipheringTransfer of user dataIn-sequence delivery of upper layer PDUs at HO in the uplink

For the control plane:Ciphering and Integrity Protection to secure the transmission of core network signaling




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RLC

The Radio Link Control (RLC) protocol provides the following services:

PDCP

Physical Layer

MAC Layer

RLC

RRC

NAS

User Plane Control PlaneSegmentation of SDU according to the sizeRe-Segmentation of PDURadio Bearer to logical channel mapping

Transfer of data in 3 modes:

TM Transparent ModeWithout retransmission. For real-time service.

UM Unacknowledged ModeWithout retransmission, but error statistics (BLER)It can be used for the signaling.

AM Acknowledged ModeWith retransmission. For non real-time services, like web browsing.




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6.3 MAC Layer

Media Access Control functions

The MAC layer provides the following services:

PDCP

Physical Layer

MAC Layer

RLC

RRC

NAS

User Plane Control PlaneLogical Channel to Transport channel mapping

Scheduling:There is no dedicated channel allocated to a UE. Time and frequency resources are dynamically shared between the users in DL and UL.The scheduler is part of the MAC layer and controls the assignment of uplink and downlink resources.




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6.3 MAC Layer

MAC Scheduler

eNodeB

DL

UL

Frequency

Time

The scheduler (in the eNodeB) determines dynamically, each 1 ms (each subframe), which UEs is scheduled to transmit/receive data on the UL/DL shared channel, and what resources to be used

The basic time-frequency unit is the resource block (RB).

To select the adapted modulation and coding rate, the scheduler needs measurement reports in DL and UL.




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2.3 MAC Layer

MAC scheduler [cont.]

eNode-B

eNodeB

Scheduler

Buffer

Data

Multiplexing

Modulation, Coding

Transmission

eNodeB

UE

DL channel quality measurement

In downlink, the scheduler needs the following inputs to schedule data:

Amount of dataData typeRadio resource availableRadio Condition in DL

The UE reports regularly its measurement report, called Channel Quality Indicator (CQI).




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UE

2.3 MAC Layer

MAC scheduler [cont.]

eNode-B

eNodeB

Scheduler

Buffer

Data

Multiplexing

Modulation, Coding

Transmission

eNodeB

UL channel quality measurement

In uplink, the mechanism is similar but:

Measurements are made by the eNodeB

The eNodeB scheduler controls the UE transmission

Request to transmit




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7. ALU Solution for LTE




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Architecture overview

EPS = eUTRAN + ePC

Long Term Evolution (LTE) is the newest 3GPP standard for mobilenetwork technology.

P-GWS-GW

HSS

PDN

MGW

IMS

Transport

PCRF

PSTN

User and control

Control only

LTE

OFDM

SC-FDMA

eUTRAN

ePC

All-IP network carries all types of traffic, including VoIP.

Provides control functions for LTE access networks.

Routes and forwards user data packets to eNodeB.

Connects UE to external packet data networks.

AAA

Controls QoS policy for each service data flow that passes through the SGW and PGW.

IMS network provides services, including VoIP.

S1-MME

S1-U S5/S8

Gx

SGi

SWx

SGi

S6aS11

S6b

MMEx2

Gxc




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eUTRAN

eUTRAN comes from evolved UTRANThe interface between the user equipment (UE) and eNodeBeNodeB is the key element of UTRAN, implementing the air interface according to 3-GPP release 8 specificationsThe key points of the air interface are:

OFDMA for DL, SC-FDMA for ULSmart antenna algorithms (Tx Diversity, Spatial Multiplexing, BeamForming)Turbo-codes for error correctionAdaptive modulation and codingBoth FDD and TDD are described by the standard

The resources allocation and scheduling on the air it is entirely done by the eNodeBThe other physical device communicating over the eUTRAN is the User Equipment




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ePC

Comes from evolved Packet CoreBased on a flat, full IP architectureIt transports all types of traffic (voice, data video)SAM represents its management entity (ALU specific)




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ePC components

MME (Mobility Management entity)Control plane onlyResponsible for mobility and air link managementAuthentication, tracking, paging, bearer activation

Serving Gateway (SGW)Forwarding of user plane data packetsAnchor point for inter eNodeB and inter-technology handover (HO between LTE and earlier 3GPP technologies)

Packet data Gateway (PGW)Connection point between UE and external data networks Anchor point for inter-technology mobility (non 3GPP technologies)Traffic policy enforcement, charging support

Policy and Charging Rules Function (PCRF)Dynamic control of the QoS for the provided servicesGives to SGW and PGW the rules for the treatment of each service data flow




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External references (other than ALU docs)

G. Gomez, D. Morales Jimenez and oth., “Long term evolution: 3 GPP LTE radio and cellular access technology”, Taylor and Francis Group, 2009.E. Dahlman, S. Parkvall and oth., “3G Evolution: HSPA and LTE for mobile broadband”, Elsevier Academic Press, 2007.Ericsson’s white paper: “LTE: an introduction”, 2009.Motorola’s white paper: “Long term evolution (LTE): overview of LTE Air-Interface”, 2007.D. Astely, E. Dahlman and oth., “LTE: the evolution of mobile broadband”, IEEE Communications Magazine, April 2009, pp. 44-51.H.G. Myung, “Single Carrier FDMA”, 2008, available online at:http://hgmyung.googlepages.com/scfdma.

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