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Iterative Multi-user Detection for STBC DS-CDMA Systems in Rayleigh Fading Channels. Derrick B. Mashwama And Emmanuel O. Bejide. Summary. Investigate performance of Turbo Space-Time Multiuser Receivers Classify such receivers as either Partitioned Approach (PA) or Iterative Approach (IA)

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iterative multi user detection for stbc ds cdma systems in rayleigh fading channels

Iterative Multi-user Detection for STBC DS-CDMA Systems in Rayleigh Fading Channels

Derrick B. Mashwama

And

Emmanuel O. Bejide

summary
Summary
  • Investigate performance of Turbo Space-Time Multiuser Receivers
  • Classify such receivers as either Partitioned Approach (PA) or Iterative Approach (IA)
  • Employ Turbo codes for FEC and MIMO techniques to mitigate fading effects
  • Results:
          • IA outperforms PA at low SNR
          • Both schemes are dependent on diversity level, system loading, channel conditions and the detector parameters.
introduction
Introduction
  • Mitigating the effects of Multiple Access Interference (MAI) inherent in CDMA systems
  • Forward Error Correction in DS-CDMA systems
  • Multiple-Input Multiple-Output (MIMO) Techniques
generalized structure of a fec coded ds cdma system in fading channel
Generalized structure of a FEC coded DS-CDMA system in fading channel

FEC

Encoder

Interleaver

Multiuser

Receiver

Scattering

Channel

FEC

Encoder

Interleaver

Figure 1: FEC Coded Direct Sequence CDMA System in Fading Channels

mitigating mai effects
Mitigating MAI Effects
  • Optimum MUD
  • Suboptimal MUDs

1. Linear Detectors

- Decorrelator Detector

- Minimum Mean Square Error (MMSE) Detector

2. Interference Cancellation Schemes

- Parallel Interference Cancellation (PIC) Detector

- Successive Interference Cancellation (SIC)

3. Combined Detection Schemes: Zero-Forcing Decision Feedback, Decorrelator/PIC, MMSE/PIC, etc.

performance of pic detector in awgn cdma channel
Performance of PIC detector in AWGN CDMA Channel

AWGN, K=5, PG=15 chips

Figure 2: Four Stage PIC BER performance

forward error correction
Forward Error Correction
  • enables DCS to reduce the amount of erroneous data at the receiver
  • BER - used as measure of the system performance for given SNR.
  • Generally FEC codes are classified into: block codes, convolutional codes and Turbo Codes (TC).
  • A TC is the parallel concatenation of two RSC codes separated by an interleaver.
slide8

Turbo Codes

Figure 4: Rate r=1/3 Turbo Encoder Structure

mimo techniques
MIMO Techniques
  • Signals propagating through the wireless channel experience path loss and distortion due to multipath fading and additive noise.
  • Diversity helps improve the receiver performance in the presence of fading.
  • Diversity Schemes:
        • Space Diversity
        • Frequency Diversity
        • Time Diversity
        • Polarization Diversity
mimo techniques1
MIMO Techniques
  • MIMO Communication systems have been introduced as a viable approach to providing significant performance improvements
  • Promises high bit rates and improved channel capacity
  • MIMO signal processing techniques can be divided into two main categories:
      • Spatial multiplexing (SM)
      • Space-time codes (STC).
mimo signal processing techniques
MIMO Signal Processing Techniques

Figure 5: Classification of MIMO techniques

performance of stbc in fading channels
Performance of STBC in Fading Channels

K=1, Rayleigh Fading

Figure 6: BER Performance of BPSK system with no diversity and up to 2x2 transmit diversity for a Rayleigh fading channel

methodology
Methodology
  • Classification of concatenation schemes
    • Partitioned Approach (PA)
    • Iterative Approach (IA)
turbo space time pa receiver

Space-time MUX

Space-time MUX

Space-time MUX

TDp

TD1

TD1

TDp

TD1

TDp

DD

1st

PIC Stage

pth

PIC Stage

DD

MIMO Matched Filtering

DD

Turbo Space-Time PA Receiver

Figure 7: Turbo Space-Time Partitioned Approach Receiver Structure

turbo space time ia receiver

TD

TD

TD

Space-time DeMux

DD

Space-time Mux

HDD

Space-time DeMux

DD

Space-time Mux

MIMO Matched Filtering

MAI Reconstruction

HDD

DD

Space-time DeMux

Space-time Mux

HDD

Turbo Space-Time IA Receiver

Figure 8: Turbo Space-Time Approach Receiver Structure

parameters and assumptions1
Parameters and Assumptions
  • Assume:
    • Equal power users with perfect power control
    • Perfect channel state information (CSI) at receiver
    • Quasi-static channel
    • No inter-symbol interference (ISI)
results
Results

FIGURE 9: BER Performance vs. SNR for both PA and

IA as a functions of increasing iterations. Here, users=4, 2x1 antennas

FIGURE 10: BER performance vs. SNR for both PA and IA as a function of diversity. Here, users=4, Iterations=4.

results1
Results

FIGURE 11: Performance comparison between PA and IA

for different BER vs. SNR system loads.

Here Iterations=4,2x2 antennas

FIGURE 12: BER performance as a function of system load. Here, SNR =4 and 2x1 antennas

results2
Results

FIGURE 13: BER performance as a function of the number of PA and IA iteration. Here, SNR =4, 2x1 antennas

conclusion
Conclusion
  • Both PA and IA MUDs achieve considerable capacity gains
  • At low SNR the IA scheme outperforms PA scheme
  • As the fidelity of the signal improves PA gradually gains more performance improvements over IA
slide22
?…Q & A…?

deemash@crg.ee.uct.ac.za