Chapter 1

Air Interface Concept for GSM & UMTS

Topics

Radio Access Methods Radio Channels Handover & Power Control Cell Breathing Rack receiver

Multiple Access Techniques

Objective: To allow several users to share the available air interface resources.

Methods:

FDMA: Frequency Division Multiple AccessTDMA: Time Division Multiple AccessSSMA: Spread Spectrum Multiple Access

User-1User-1User-2User-2User-3User-3User-4User-4User-5User-5

Frequency Division Multiple Access

Each user has a unique frequencyDivide available frequency spectrum in to channels of same bandwidthUsed in analog systems like AMPS, TACS, NMT

All users can transmit at the same time

Limitation on : Frequency Re useNumber of subscriber per area

Freq

uenc

y

Time

Time Division Multiple Access

Each user has a unique time slots Each channel has a unique position with the time slots Frequency is allocated to the user for the duration of one time slot.

Several users share the same frequency i.e. IS-136, GSM

00 11 22 33 44 55 66 7 00 11 22 33 44 55 66 7

FRAME1 FRAME2

Time Slot

Freq

uenc

y

Time

Spread Spectrum Multiple Access

Each transmitter has a unique spreading code

Each data channel has a unique orthogonal code

Many users share the same frequency and time. i.e IS-95, CDMA2000, WCDMA

frequency f

time

t

Pow

er

P

Code 1Code 1Code 2Code 2Code 3Code 3

Spread Spectrum Multiple Access

SSMA: Summary

Duplex TransmissionTi

me

t

UL

DL

UL

DL

UL

Framewithn TS

TDD

DL

frequency fTi

me

t

duplex distance

UL

FDD

Orthogonality and Correlation of Signals

Correlation Values

Modulation

BPSK, QPSK & 8PSK: Amplitude is the same for every symbol

PSK:Phasedenominatessymbol

-1 1

(Q)

I

BPSK:

-1 1

-1

1

10

11

0 0

01

Q

I

QPSK:

8PSK :Q

I

000

001

101

100

110

111

011

010

Bit/symbol e.g.:

• GMSK: 1 (GSM)• BPSK: 1 (IS-95)• QPSK: 2 (UMTS)• 8PSK: 3 (EDGE)• 16QAM: 4 (HSDPA)• 64QAM: 6 (WLAN)

Bit/symbol e.g.:

• GMSK: 1 (GSM)• BPSK: 1 (IS-95)• QPSK: 2 (UMTS)• 8PSK: 3 (EDGE)• 16QAM: 4 (HSDPA)• 64QAM: 6 (WLAN)

Spreading Factor & Processing gain

Processing Gain Examples

spreadDatavia Air Interface

1 0 1 0User Data

Data(converted to +1/-1)

SpreadingCode

+1

+1

+1

-1

-1

-1

+1

+1

-1

-1

SpreadingCode

Data

1 0 1 0User Data

1 Bit (symbol)*

SF = Rchip / Rdata

SF: Spreading Factor

SF: Spreading Factor

Rchip: Chip Rate [cps]

Rdata: Data Rate [symbol/s]

(information rate on the air interface)

SF: Spreading Factor

Rchip: Chip Rate [cps]

Rdata: Data Rate [symbol/s]

(information rate on the air interface)

Spreading Factor

Channelization Codes in UMTS

DL & UL Channelization Codes

Radio Channels-GSM

GSM has mainly two channel types:Physical channels: Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.Logical Channels: Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types

• Common Channels• Dedicated Channels

GSM Physical Channels

GSM using both FDMA & TDMA on the air interface Each frequency supports 8 time separated physical channels A group of 8 timeslots is known as a TDMA frame.

Physical channel parametrsARFCNTime Slot Number

TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 6 TS 7

200 kHz

4.615 ms0.577 ms

1 TDMA FRAME

GSM Logical Channels

There are two types of logical channels are available• Traffic Channels• Control Channels

WCDMA Channels

In GSM, we distinguish between logical and physical channels. In UMTS there are three different types of channels:

Logical Transport Physical

Logical Channelscontent is organised in separate channels, e.g.

System information, paging, user data, link management

Transport Channelslogical channel information is organised on transport channel

resources before being physically transmitted

Physical Channels

(UARFCN, spreading code) Frames

Iub interface

WCDMA Physical Channels

WCDMA Physical Channels

WCDMA Downlink (FDD)

Power Control

Why Power Control is required?

Power Control

What is Slow power control? What is Fast power control?

Power control (PC) in WCDMA

Fast, accurate power control is of utmost importance – particularly in UL• UEs transmit continuously on same frequency Always interference between

users• Poor PC leads to increased interference reduced capacity

Every UE accessing network increases interference• PC target to minimise the interference Minimize transmit power of each link

while still maintaining the link quality (BER) Mitigates 'near far effect‘ in UL by providing minimum required power for

each connection Power control has to be fast enough to follow changes in propagation

conditions (fading) Step up/down 1500 times/second

Power Control

Open Loop Power Control

Closed Loop Power Control

Outer Loop Power Control

Power Control procedure in WCDMA

Radio Propagation

Multipath

Signaling Codes & Multi Path Propagation

RAKE Receiver

Handover Methods

Handover Methods

Soft Handoff Advantage

Soft Handoff Advantage

WCDMA with Soft Handover

WCDMA Softer Handover

Soft Handover Procedure in UMTS

Handover Example

Soft Handover & Micro diversity

Macro Diversity

Soft Handover & Macro-Diversity

Cell Breathing

Soft Handover & Cell Breathing handover

Softer Handover

Key Points for Handover

Key Points for Handover