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GSM Physical Layer
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Advanced Technology Center Chung-Wei Ku
Advanced Technology Center Chung-Wei Ku
Outlines
■ Overview of PHY
■ Source Coding
■ GMSK Demod and Equalization
■ Baseband Process
■ Burst Structure
Advanced Technology Center Chung-Wei Ku
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.
Advanced Technology Center Chung-Wei Ku
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
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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
Advanced Technology Center Chung-Wei Ku
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
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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
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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
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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
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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
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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.
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Channel Coding
■ Convolutional Coding• depends on the channels
■ Fire Code (Cyclic coding)• (X23+1)(X17+X3+1)
■ Parity Coding
■ Interleaving
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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
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Summary of Convolutional Codes
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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.
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Interleaving
■ To avoid burst errors which are fetal forconvolutional coding, interleaving is necessary.
■ Implementation: transpose memory
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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.
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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
Advanced Technology Center Chung-Wei Ku
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
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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
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• Structure of Interleaver– interleaving speech frames onto TDMA frame
Interleaver
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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
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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
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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
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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
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GMSK Modulation
■ ROM table lookup for Gaussian, I and Q
■ Digital multiplier for IF modulation
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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
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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:
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GMSK Properties
■ 1 bit difference:
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GMSK Demodulation
■ Maximum Likelihood Estimation• Viterbi Algorithm
■ Training sequence pattern is utilized fordemodulation decision
■ Combined demodulation and channelequalization
■ Demodulation and Error decoding
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GMSK Demodulation
■ 5-bits state: Gaussian filter
■ 20 us separated multipath
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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
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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
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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
Advanced Technology Center Chung-Wei Ku
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
Advanced Technology Center Chung-Wei Ku
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
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Burst Structure
147 (87) bits
-70dB(-36dBm)
-30dB
-6dB
+4dB 2dB
10 8 10 10 8 10 (us)
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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
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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
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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
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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
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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
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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
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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
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Frame Structure
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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
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Channel to Burst Mapping
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Mapping Example
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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
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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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