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GSM BTS Development
&
GSM/EDGE Receiver based on FDE
Dinakar. P
[email protected]. of Electrical Engineering
Indian Institute of Technology - Madras
ComNet - 2007
mailto:[email protected]:[email protected]
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Introduction: GSM
Based on FDD TDMA
Raw bit rate: 270.83 kbps
Each physical channel is shared (TDMA) by 8 users
Data service: HSCSD (Circuit-Switched) & GPRS
(Packet-Switched)
Global System for Mobile Communication (GSM)
Air Interface
Currently ~ 670 networks in 213 countries and territories
Frequency bands (MHz) 450, 850, 900, 1800, 1900
Very rapid growth in subscribers (1275M-2/05, 1600M-12/05,
2000M-12/06)
Aggressive evolution to 3G EDGE, Wideband CDMA
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GSM Cellular Network
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GSM BTS Specification
Single TRX (Single carrier)
Low power
Supports multiple bands GSM 900, 1800 and 1900Standard
compliant
Remote monitoring capability
Low cost
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Base Station Unit
RF Section
Baseband
Section
Processor
Section
POE
BSC Antenna
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GSM Receiver - Equalizer
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GSM Receiver EqualizerCont
MLSE based receiver
Generates soft-bit (SOVA)
Implemented on DSP
Processor loading: ~60 % (MIPS)
Standards compliant
Transparent to GPRS reception
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1 2 3 4 5 6 7 8
GSM TDMA frame
4.615 ms
GSM time-slot (normal burst)
577 s
guard
spacetail user data TrainingS S user data tail
3 bits 57 bits 26 bits 57 bits1 1 3
GSM: Time Division Multiple Access
Raw Bit Rate per Slot: 270.8*(114/156)*(12/13)/8 = 22.8 kbps
General Packet Radio Service (GPRS) = 14.4 kbps / Slot
High Speed Circuit-Switched Data (HSCSD) = 64 kbps / 4 Slot
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Need higher date rate ? Use higher order modulation !!!
EDGE Enhanced Data rate for Global Evolution
Enhancement of GPRS is EGPRS and HSCSD is ECSD
Retains GSM characteristics:
Symbol durationFrame structure
Spectral characteristics
Constellation: GMSK (1bit/sym) and 8PSK (3bit/sym)
Symbol duration: 3.69 S. Delay Spread as high as 17 S
Memory Length (L):5 No. of State in MLSE: GSM2L (32) EDGE8L
(32768)
** Number of Channel Taps: L + 1
Traditional GSM Equalizer: MLSE
Evolution of GSM: EDGE
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Is Frequency Domain Equalization Techniques are feasible for
EDGE ?
Delay Decision Feedback Sequence Estimation (DDFSE)
Reduced-State Sequence Estimation (RSSE)
DFE combined with nearest-neighbor symbol perturbation
Turbo Equalization with a core low complexity equalizer
Equalizers like MAP-DFE, Zero-forcing filter . . .
Traditional Equalization Techniques:
Equalization Techniques for EDGE
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EDGE: Simulation Model
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A quick glance at Frequency Domain Equalization
v samples
CP CP
copy copy
Nsamples
B l o c k i B l o c k (i+1)
Time-Domain Convolution Frequency-Domain Multiplication
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TailSymbols
(3)
TailSymbols
(3)
DataPre-Amble
(58)
DataPost-Amble
(58)
TrainingSequence
(26)
UniqueWord-1
(3)
UniqueWord - 2
(3)
Data & Training Sequence (58+26+58)
Block (Length N= 145)
(i)
Guard
IntervalGuard
Interval
Interfering Block
(i-1)
IBI Removal and CP Construction
A different look on Burst Structure
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BER Performance: Proposed FDE vs MMSE Equalizer
** Ideal Channel Estimation
Static Channel
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TailSymbols
(3)
TailSymbols
(3)
DataPre-Amble
(58)
DataPost-Amble
(58)
Training Sequence
(26)
Efficient Frequency-Domain Equalizer
64-Pt Block 64-Pt Block
Training Sequence is used as known pattern and the cyclic prefix
is
constructed accordingly
Separate FDE for Post-Amble and Pre-Amble
Same channel values are used for both the FDE
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BER Performance: Full Block FDE and Modified 64 Pt FDE
** Ideal Channel Estimation
Static Channel
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Tail
Symbols
(3)
Post-Amble (58)Training Sequence
(26)
64-Pt Block
FDE for longer channels
Equalization of Post-Amble
Cyclic Prefix Reconstruction
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FDE for longer channels
Equalization of Pre-Amble
Tail
Symbols
(3)
Tail
Symbols
(3)
Data
Pre-Amble
(58)
Data
Post-Amble
(58)
Training Sequence
(26)
Guard
Interval
TailSymbols
(3)
Pre-Amble (58)Training Sequence(26)
64-Pt Block
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BER Performance with proposed CP reconstruction procedure
** Ideal Channel Estimation
Hilly Terrain: Slow fading channel
CP Reconstruction: 2 Iterations
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EDGE: System Model
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Summary
MIPS Memory
Cycles/Burst
Mega Cycles/Second
BF 537
600 MIPSkBytes
BF 537
64 kBytes
Best Case(ShorterChannel)
13622 24 4% 7.4 11%
Worst Case(Longer Channel)
25114 45 8% 7.6 11%
Compared with Time-Domain MMSE Much better
Implemented on DSP
Standards compliant
Possible enhancement: Over-sampling & receive diversity
