2010-05-11 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

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Contents

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

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HISTORY OF MOBILE ”TELEPHONY”

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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.

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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!

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GLOBAL SYSTEM FOR MOBILECOMMUNICATIONS (GSM)

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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

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GSMSimplified block diagram

Speechcoder

Channelencoder

Burstformatting

Modulator/transmitter

ReceiverViterbi

equalizerViterbi

decoderSpeechdecoder

bits

quality info.

(Encryption not included in figure)

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GSMSome specification parameters

(initial specification)

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GSMGMSK modulation

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GSMPower spectrum

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GSMTDMA/FDMA structure

ARFCN Absolute Radio Frequency Channel Number

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GSMFrames and multiframes

Super frame 51 Multiframes

Timeslot 156.25 Bits

576.92 μs

Frame 8 Timeslots4.615ms

Multiframe 26 Frames120ms

6.12s

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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.

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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

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GSMViterbi equalizer

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GSMChannel coding of speech

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

Uncoded

Block code

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GSMEncryption

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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

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GSMGPRS network

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

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GSMEDGE 8PSK modulation

3

8

2×3

8 3×3

8

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

We avoid transitionsclose to origin, thusgetting a lower amplitudevariation!

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WIDE-BAND CODE-DIVISIONMULTIPLE ACCESS (WCDMA)

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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!

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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.

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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.

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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.

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WCDMADownlink

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WCDMAUplink

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WCDMASpectrum mask

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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.

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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!

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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