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GSM Physical Layer

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Advanced Technology Center Chung-Wei Ku


Outlines

■ Overview of PHY

■ Source Coding

■ GMSK Demod and Equalization

■ Baseband Process

■ Burst Structure


References

■ M. Mouly and M. Pautet, The GSM System forMobile communications, 1992.

■ A. M. Kondoz, Digital Speech Coding for LowBit-Rate Communications Systems, WileyPublishers, 1994.

■ ETSI GSM related standards documents.


GSM Physical Layer

■ Involved techniques:• Source coding/decoding

• Error correction code

• Interleaving/De-interleaving

• Channel equalization

• GMSK modulation/demodulation

• Synchronization

• TDD with combined TDMA and FHMA


Block Diagram

digitizing

and source coding

channel coding

interleaving

burstformatting

ciphering

modulation

source decoding

and D/A

channel decoding

de-interleaving

burstde-formatting

deciphering

demodulation

speech speech


Source Coding

■ source sampling, A/D conversion• A-law, u-law

■ full-rate speech coding• RPE-LTP (CELP-based coding): 13 Kbps

■ half-rate speech coding• VSELP (CELP-based coding): 5.6 Kbps


General CELP Codec

W(z)

1/Aw(z)

1/Aw(z)1/P(z)

find D, β

1/Aw(z)1/P(z)

weighted LPC

weighted LPC

Zero excitation

Zero excitation

code-book

find index and G

G

originalspeech


CELP Parameters

■ LPC: short-term prediction• LPC is the envelope of spectrum

• LPC can be effectively expressed by LSF

■ Pitch: long-term prediction• pitch represents the periodic part; tone of the

speaker

■ Excitation:• near white noise


RPE-LTP Speech Coding

■ Regular Pulse Excitation - Long TermPrediction.

■ Encoding is much time-consuming thandecoding.

■ Complexity: around 2 to 3 MIPS.

LTPFilter

InverseLPC Filter

De-emphasis

ExcitationSignal

SynthesizedSpeech


RPE-LTP Codec

LPC InverseFilter

Pitch InverseFilter

WeightingFilter and RPEGrid Selection

ADPCMQuantizer

LPCAnalysis

PitchAnalysis

MUXLPC Parameter

Pitch Parameter

Grid Position

Inputspeech

ResidualDecoder

Up-Sampling

PitchSynthesis

Filter

LPCSynthesis

FilterDEMUX

Pitch Parameter

Grid Position

LPC Parameter

DecompressedSpeech


Process for Blocks

■ 1. Information bits are coded with a systematicblock code: info+parity bits.

■ 2. info + parity bits are encoded with aconvolutional code, building coded bits.

■ 3. Reordering and interleaving the coded bits,adding stealing flag, gives the interleaved bits.


Channel Coding

■ Convolutional Coding• depends on the channels

■ Fire Code (Cyclic coding)• (X23+1)(X17+X3+1)

■ Parity Coding

■ Interleaving


Convolutional Codes

■ Punctured convolutional code:• Ex: D3+D+1 and D2+D+1 (goal: 2/3)

Source block (12 bits)Addition of tail bits

Delay = 1 bitDelay = 2 bitsDelay = 3 bits

1st conv. seq. (15 bits)2nd conv. seq. (15 bits)

punctured 2nd seq. (8 bits)transmitted block (23 bits)

1 0 0 1 0 0 1 1 0 1 0 10 0 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0 0

0 0 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0 00 0 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0 0

0 0 0 1 0 0 1 0 0 1 1 0 1 0 1 0 0 01 1 0 0 1 0 0 0 1 0 0 1 0 0 11 0 1 0 0 1 0 1 1 1 1 0 1 1 1 1 1 0 0 1 1 1 1 11101010000011001101011

Eventual: 12/23


Summary of Convolutional Codes


Fire Code

■ Generation function:• g(D)=(X23+1)(X17+X3+1)

• add 40 coded bits

■ Basically, Fire Code is used for controlchannels.

■ Traffic channels are only convolutional codedwith parity bits then interleaved.


Interleaving

■ To avoid burst errors which are fetal forconvolutional coding, interleaving is necessary.

■ Implementation: transpose memory


GSM Interleaving

■ 456 (4 x 114) bits can be divided into• 4 parts of 114 bits, each for one burst

• 8 parts of 57 bits, each for half a burst

• 24 parts of 19 bits, each for 1/6 burst

• 76 parts of 6 bits, using 1/19 burst– 16 pieces of 24 bits, 2 pieces of 18 bits, 2 pieces of 12 bits

and 2 pieces of 6 bits

– a burst includes 4 pieces of 24 bits plus either one pieceof 18 bits or two pieces of 12 and 6 bits.


8bit A-law to 13bitUniform Converter

RPE-LTP Encoder

Low-Pass Filter A/D Converter RPE-LTP Encoder

Mobile StationMobile Station

MSCMSC

Analog Signal

Digital Signal

13×8000=104 kbps

13 ×8000=104 kbps

13 kbps

13 kbps

ToChannel Encoder

ToChannel Encoder

• Source (Speech) Coding– Mobile Station (Analog Signal)

• Low-pass filter, then A/D converter, then RPE-LTP speech encoder

– MSC (Base Station) (Digital Signal)• 8-bit A-law to 13-bit Uniform converter, then RPE-LTP speech encoder

Speech Coding


bits per 5 ms Bits per 20 ms

Linear Prediction Coding (LPC) filter 36Long Term Prediction (LTP) filter 9 36Excitation Signal 47 188Total 260Class I 182

(class Ia=50, class Ib=132)Class II 78

• Source (Speech) Coding– Regular Pulse Excited Long-Term Prediction (RPE-LTP) Encoder

• Input has bit rate of 104 kbps

• Has net bit rate of 13 kbps

• Output from RPE-LTP 260 bits every 20 ms

Speech Data Formatting


RPE-LTP Speech EncoderRPE-LTP Speech Encoder

Cyclic Redundancy EncoderCyclic Redundancy Encoder

1/2 Convolutional Encoder1/2 Convolutional Encoder

260 bits

20 msClass I: 182 bits

Class II: 78 bits

50 bits

132 bits

53 bits

185 bits4 tail bits all equal to zero

189 bits

378 bits

456 bits

20 ms

Speech and Channel Coding


• Structure of Interleaver– interleaving speech frames onto TDMA frame

Interleaver


TCH/F9.6• 9.6 Kbps refers to the user’s transmission rate, the actual rate is brought up to 12 Kbps through channel

coding in the terminal equipment; that is, 12 Kbps is the rate delivered to the MS.

User InformationUser Information

1/2 Convolutional Encoder1/2 Convolutional Encoder

240 bits

20 msAdd 4 “0” bits

488 coded bits

456 bits

20 msPuncturing of 32 coded bitsPuncturing of 32 coded bits

Data and Channel Coding


Channel Coding of Signaling Channels• Signaling information contains a maximum of 184 bits. It does NOT make a difference whether the type ofsignaling information to be transmitted is mapped onto a BCCH, PCH, SDCCH or SACCH. The formatalways stays the same.

• Special format are reserved for the SCH & RACH• FCCH requires no coding at all

Signaling InformationSignaling Information

Block Encoder (Fire Code)Block Encoder (Fire Code)

184 bits

Fire coded adds 40 parity bits to the 184 bit = 224 fire-coded bits, then adds 4 “0” bits

456 bits

1/2 Encoder 1/2 Convolutional Encoder

Signaling and Channel Coding


Ciphering

■ XOR operation of data and a specific key

■ Double-XOR recovers the original data• A/5 algorithm generates the pseudo random key

sequence

• phone No., SIM info, time/date

Data

Key

Ciphered


GMSK Modulation

■ Information becomes NRZ signals.• k=1, if di=di-1

• k=-1, if di di-1≠φ π

µ σπ

πσσ

πσ σ

© ª © © ª © ªª

íï

¯¯

© ª

ùÕ È ù È ù

Õ ô

È ù ù æ æ

õù õ

= + − −

= = =

= +−

−∞

−

∫

³

²

³

²

³

µ¹

²´

³

³ ± ´± µµ²·¹µ

²

³ ³

³

³

³

³³ ³

Æ õ â õ õ

õ ì õ êÕê

õ

© ª äðô© © ªª

© ª © ª

= × +

= + −∑ω ϕ

ϕ ϕ φ±

±

is equal to a ramp convolved with Gaussian functionφ


GMSK Modulation

■ ROM table lookup for Gaussian, I and Q

■ Digital multiplier for IF modulation


Gaussian Filter

■ BT=0.3; the low-pass filter is equal to a 5-tagFIR.

1 2 3 4 5 6 7 8 9 10 110

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0 200 400 600 800 1000 12000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

-3


GMSK Graph

0 200 400 600 800 1000 1200 1400 1600 1800 2000-1.5

-1

-0.5

0

0.5

1

1.5

0 500 1000 1500 2000 25001

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

0 500 1000 1500 2000 25000

1

2

3

4

5

6

0 500 1000 1500 2000 2500-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

NRZ:

phase:

Gaussian:

Carrier:


GMSK Properties

■ 1 bit difference:


GMSK Demodulation

■ Maximum Likelihood Estimation• Viterbi Algorithm

■ Training sequence pattern is utilized fordemodulation decision

■ Combined demodulation and channelequalization

■ Demodulation and Error decoding


GMSK Demodulation

■ 5-bits state: Gaussian filter

■ 20 us separated multipath


Frequency Assignment

■ Frequency allocation (GSM-900):• Ful( n ) = 890.0 MHz + (0.2 MHz) n

• Fdl( n ) = ful( n )+ 45 MHz

■ Frequency allocation (DCS-1800):• Ful( n ) = 1710.0 MHz + (0.2 MHz)( n - 511 )

• Fdl( n ) = ful( n )+ 95 MHz


GSM Key Words

■ GSM: combined FDM and TDMA (200 KHz)

■ 1 TDMA frame = 8 time slots (577 s)

6 7 20 1 3 4 5 6 7 0 1

4.615 ms

2 320 1 3 4 5 6 7 0 1

6 7 20 1 3 4 5 6 7 05

BTS

MS

Time-Division Duplex

µ


GSM Key Words

■ Pulsed Transmission• power vs. time template: burst

■ Timing Advance (Synchronization for TDMA)• delay time due to distance

• avoidance of collision

■ Power Control• signal attenuation due to distance


GSM Keywords

■ Radio Channel Properties• Shanon’s Eq.:

• C=B log2 (1+S/N)

■ R/B=1/BTbit (bps/Hz)• B=81.3 KHz, R=270833 bps

• for GSM with GMSK, BT=0.3 or 3.33 bps/Hz


GSM Key Words

■ Physical Channel• Different bursts for different situations

• 147bits = 542.8 s (3.69 s/bit)

■ Logical Channel• Messages for the communication between BTS and

MS.

• Logical channels are mapped into physical channelburst structure.

µµ


Burst Structure

147 (87) bits

-70dB(-36dBm)

-30dB

-6dB

+4dB 2dB

10 8 10 10 8 10 (us)


Burst Structure

■ Normal Burst (MS, BTS)• The most common burst in GSM

• 8 kinds of training sequences (good correlation)

• The mid 16 in TS are used for equalization

• Stealing flag indicates signaling or user data

T3

Coded Data57

S1

Training Sequence26

S1

Coded Data57

T3

GP8.25

148 bits = 546.12 sµ


Burst Structure

■ Random Access Burst (MS)• MS randomly transmits it to gain initial access

• 68.25 s x 3 x 108 m/s = 75.5 km

T8

Synchronization Sequence 41

Training Sequence36

T3

GP68.25

88 bits = 324.72 sµ

µ


Burst Structure

■ Frequency Correction Burst (BTS)• BTS transmit it for MS with correct reference

• Due to the properties of GMSK, stuffing data canrepresent sinusoidal waveforms.

T3

fixed bit sequence142

T3

GP8.25

148 bits = 546.12 sµ


Burst Structure

■ Synchronization Burst (BTS)• some valuable system parameters

• BSIC=NCC+ BCC (3 bits) in coded data

• longer synchronization sequence

T3

Coded Data39

SynchronizationSequence 64

Coded Data39

T3

GP8.25

148 bits = 546.12 sµ


Burst Structure

■ Dummy Burst (BTS)• Rate-matching purpose

• The same as normal burst but with a fixed pattern

T3

Coded Data57

S1

Training Sequence26

S1

Coded Data57

T3

GP8.25

148 bits = 546.12 sµ


Cell Size

■ Recall 68.25 s x 3 x 108 m/s = 75.5 km

• cell size is approximate 37.75 km

■ Timing advance is from 0 to 63• 63 x 3.69 s/bit x 3 x 108 = 70 km

• cell size is around 35 km

■ Larger cell will cause signaling troubles andpower issues.

µ

µ


N v m u j . g s b n f t T u s v d u v s f

■ 1 t i m e sl o t = 57 7 s• 1 4 8 b i t s f o r a b o u t 54 7

s

■ 1 T DM A f r a m e = 4 . 61 6m s

■ 1 su p e r f r a m e = 1 3 2 6T DM A f r a m e s• 51 2 6- m u l t i f r a m e s o r 2 6

51 - m u l t i f r a m e s

• sm a l l e st c y c l e f o r

µ

µ


Frame Structure


Logical Channels

■ S e v e n k i n d s o fc o m b i n a t i o n s• T CH/ F S + F ACCH/ F S + S ACCH/ F S

• T CH/ HS ( 0 , 1 ) + F ACCH/ HS ( 0 , 1 ) +S ACCH/ HS ( 0 , 1 )

• T CH/ HS ( 0 ) + F ACCH/ HS ( 0 ) + S ACCH/ HS ( 0 ) +T CH/ HS ( 1 ) + F ACCH/ HS ( 1 ) + S ACCH/HS ( 1 )

• F CCH+ S CH+ CCCH+ B CCH

• F CCH+ S CH+ CCCH+ B CCH+ S DCCH/ 4 + S ACCH/ 4


Channel to Burst Mapping


Mapping Example


Synchronization■ Q u a r t e r b i t n u m b e r

( Q N) ( 0 - 62 4 )

■ B i t n u m b e r ( B N) ( 0 -1 56)

■ T i m e sl o t n u m b e r ( T N)( 0 - 7 )

■ T DM A f r a m e n u m b e r( F N) ( 0 - 2 7 1 564 7 )• Q N i s se t b y t r a i n i n g

se q u e n c e

T N i


Transmission Process

■ Up l i n k :• R a n d o m a c c e ss b u r st

f r o m M S t o B S

• No r m a l b u r st f o rt r a f f i c d a t a

■ Do wn l i n k• F r e q u e n c y c o r r e c t i o n

b u r st f r o m B S t o M S

• S y n c h r o n i z a t i o n b u r stf r o m B S t o M S

l b f

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