1. Iterative Multi-user Detection for STBC DS-CDMA Systems in Rayleigh Fading Channels Derrick B. Mashwama And Emmanuel O. Bejide

2. 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.

3. 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

4. Generalized structure of a FEC coded DS-CDMA system in fading channel FEC Encoder Interleaver FEC Encoder Interleaver Multiuser Receiver Scattering Channel Figure 1: FEC Coded Direct Sequence CDMA System in Fading Channels

5. 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.

6. Performance of PIC detector in AWGN CDMA Channel Figure 2: Four Stage PIC BER performance AWGN, K=5, PG=15 chips

7. 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.

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

9. 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

10. 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).

11. MIMO Signal Processing Techniques Figure 5: Classification of MIMO techniques

12. Performance of STBC in Fading Channels Figure 6: BER Performance of BPSK system with no diversity and up to 2x2 transmit diversity for a Rayleigh fading channel K=1, Rayleigh Fading

13. Methodology Classification of concatenation schemes –Partitioned Approach (PA) –Iterative Approach (IA)

14. Turbo Space-Time PA Receiver Figure 7: Turbo Space-Time Partitioned Approach Receiver Structure

15. Turbo Space-Time IA Receiver Figure 8: Turbo Space-Time Approach Receiver Structure

16. Parameters and Assumptions ParameterValue Active Subscribers, KVariable or as stated User Frame Length, L1024 bits Encoder Generators Code Rate1/3 Processing Gain15 Transmission ModeSynchronous Propagation ChannelFlat Fading Total bits per SNR MIMO Signal Processing TechniqueSTBC, up to 2x2 Interference Cancellation Stages4 BPSK Modulation

17. 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)

18. 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.

19. 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

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

21. 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

22. ?…Q & A…? deemash@crg.ee.uct.ac.za