2013-05-06 Ove Edfors - ETI 051 1

Ove Edfors, Department of Electrical and Information TechnologyOve.Edfors@eit.lth.se

RADIO SYSTEMS – ETIN15

Lecture no: 11

GSM and WCDMA

2013-05-06 Ove Edfors - ETI 051 2

Contents

• (Brief) history of mobile ”telephony”• Global System for Mobile Communications (GSM)• Wide-band Code-Division Multiple Access (WCDMA)

2013-05-06 Ove Edfors - ETI 051 3

HISTORY OF MOBILE ”TELEPHONY”

2013-05-06 Ove Edfors - ETI 051 4

HISTORYThe short version

1959 - First automatic mobile telefony system in Stockholm. ’The Phone’ weighs 40 kg and costs as much as a car.

1981 - NMT (Nordic Mobile Telephony) starts in the nordic countries and Saudi Arabia.

1989 - First GSM-system (Global System for Mobile Telephony) starts in Germany.

2001 - First WCDMA-system (Wide-band Code-division Multiple Access) starts in Japan.

2013-05-06 Ove Edfors - ETI 051 5

HISTORYGenerations

Analog technology.Examples are NMT in the nordiccountries and AMPS in North America.

No data communication.

Digital technology.Examples are GSM (first in Europe) and CdmaOne in North America.

Slow data communication.New enhancements haveincreased datarate to50-100 kbit/sec. Still evolving!

Digital technology.Examples are WCDMA (Europe) and Cdma2000 (North America).

Focus on both speech and data/multimedia.Initially up to 2 Mbit/sec. Evolvingtowards higher data rates!

2013-05-06 Ove Edfors - ETI 051 6

GLOBAL SYSTEM FOR MOBILECOMMUNICATIONS (GSM)

2013-05-06 Ove Edfors - ETI 051 7

GSMSimplified system overview

BSS

BSS

AUC

HLR

EIR

VLR

VLRBSC

MSC

BTS

BTS

BTS

BTS

BTS

MSC

BSC

Interface toother networks

BTS Base Transceiver StationBSC Base Station ControllerBSS Base Station Sub-systemMSC Mobile Switching Center

VLR Visitor Location RegisterEIR Equipment Identity RegisterAUC AUthentication CenterHLR Home Location Register

2013-05-06 Ove Edfors - ETI 051 8

GSMSimplified block diagram

Speechcoder

Channelencoder

Burstformatting

Modulator/transmitter

ReceiverViterbi

equalizerViterbi

decoderSpeechdecoder

bits

quality info.

(Encryption not included in figure)

2013-05-06 Ove Edfors - ETI 051 9

GSMSome specification parameters

(initial specification)

2013-05-06 Ove Edfors - ETI 051 10

GSMGMSK modulation

2013-05-06 Ove Edfors - ETI 051 11

GSMPower spectrum

2013-05-06 Ove Edfors - ETI 051 12

GSMTDMA/FDMA structure

2013-05-06 Ove Edfors - ETI 051 13

GSMFrames and multiframes

Super frame 51 Multiframes

Timeslot 156.25 Bits

576.92 μs

Frame 8 Timeslots4.615ms

Multiframe 26 Frames120ms

6.12s

2013-05-06 Ove Edfors - ETI 051 14

GSMUp/down-link time slots

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

2 3 4 5 6 7 0 110

ARFCN

ARFCN

45

MH

z

Time slot index

Time slot index

Frame

Dow

nlink

Upl

ink

The MS transmits to the BS three time-slots after itreceives a transmission from the BS.

Using this strategy, the duplex scheme is a combinationof TDD and FDD, and the MS avoids simultaneous transmissionand reception.

2013-05-06 Ove Edfors - ETI 051 15

GSMSome of the time slots

RACH burst41 synchronization

bits36 data bits(encrypted)

3 stopbits

68.25 bits extendedguard period

SCH burst

FCCH burst

142 zeros

3 startbits

3 startbits

3 startbits

8 startbits

Normal26 training

bits

64 trainingbits

58 data bits (encrypted)

39 data bits (encrypted)

3 stopbits

3 stopbits

3 stopbits

8.25 bitsguard period

8.25 bitsguard period

8.25 bitsguard period

58 data bits(encrypted)

39 data bits(encrypted)

FCCH Frequency Correction CHannelSCH Synchronization CHannelRACH Random Access CHannel

2013-05-06 Ove Edfors - ETI 051 16

GSMViterbi equalizer

2013-05-06 Ove Edfors - ETI 051 17

GSMChannel coding of speech

The speech code bits are in three categories, with different levelsof protection against channel errors.

Uncoded

Block code

2013-05-06 Ove Edfors - ETI 051 18

GSMEncryption

2013-05-06 Ove Edfors - ETI 051 19

GSMGPRS and EDGE

GSM has evolved into a high-speed packet radio system in two steps

GPRS General Packet Radio Serviceswhere empty time slots can be usedto transmit data packets.Four new coding schemes are used(CS-1, ..., CS-4) with different levelsof protection.

EDGE Enhanced Data-rate for GSM Evolutionwhere, in addition to GPRS, a new8PSK modulation is introduced.Eight new modulation and coding schemesare used (MCS-1, ..., MCS-8) withdifferent levels of protection.

Up to 115 kbit/sec

Up to 384 kbit/sec

2013-05-06 Ove Edfors - ETI 051 20

GSMGPRS network

SGSN Serving GPRS Support NodeGGSN Gateway GPRS Support NodeISP Internet Service Provider

2013-05-06 Ove Edfors - ETI 051 21

GSMEDGE 8PSK modulation

38

2×38 3×

38

Linear 8-PSK ... but with rotation of signal constellation for each symbol

We avoid transitionsclose to origin, thusgetting a lower amplitudevariation!

2013-05-06 Ove Edfors - ETI 051 22

WIDE-BAND CODE-DIVISIONMULTIPLE ACCESS (WCDMA)

2013-05-06 Ove Edfors - ETI 051 23

WCDMASome parameters

Carrier spacing 5 MHzChip rate 3.84 Mchips/secUplink spreading factor 4 to 256Downlink spreading factor 4 to 512

Like we discussed during Lecture 9, all cells usethe same frequency band!

2013-05-06 Ove Edfors - ETI 051 24

WCDMADirect-Sequence CDMA

What we learned during Lecture 9:

data

spreadingcode

spread spectrum signal

In WCDMA we do this a bit different:

data

spreadingcode

spread spectrum signal

scrambling

Users/channels are separated by different codes.

The total spreading is a combination of spreading and

scrambling.

2013-05-06 Ove Edfors - ETI 051 25

WCDMAChannelization and scrambling

data

channelizationcode

spread spectrum signal

scrambling

The different channelization/spreading

codes are orthogonal and have different spreading factors

between 4 and 512.

Scrambling makes thetotal spreading (spreading +

scrambling) uniquebetween different

sources.

2013-05-06 Ove Edfors - ETI 051 26

WCDMAOrthogonal Variable Spreading Factor

The OVSF codes used for variable rate spreading can be viewedas a code tree.

We can create several orthogonal channels by picking spreading codesfrom different branches of the tree.

2013-05-06 Ove Edfors - ETI 051 27

WCDMADownlink

2013-05-06 Ove Edfors - ETI 051 28

WCDMAUplink

2013-05-06 Ove Edfors - ETI 051 29

WCDMASpectrum mask

2013-05-06 Ove Edfors - ETI 051 30

WCDMAData rate and spreading factor

Data rate

TimeSpreading factor

Time

We always spread to the full bandwidth.

Transmit power

Time

Transmit power and generated interference to

others vary accordingly.

2013-05-06 Ove Edfors - ETI 051 31

WCDMAData rate and interference

In simple words, with a limited interference allowed, we can havemany low data-rate channels or a few high data-rate channels.

Interference

MS 1

MS 2

MS 3

Time

The interferencelevel also varieswith propagationloss, which makespower controlimportant!

2013-05-06 Ove Edfors - ETI 051 32

WCDMASoft handover

BS 1 BS 2

Since all base stations used the same frequency band, a terminalclose to the cell boundary can receive “the same” signal from more thanone base station and increase the quality of the received signal.

Cell b

ound

ary