1. Pipelining and number theory for multiuser detection Sridhar Rajagopal and Joseph R. Cavallaro Rice University This work is supported by Nokia, TI, TATP and NSF

2. Motivation Several multiuser detection schemes Hardware implementation infeasible Optimize algorithm + hardware Design a reduced complexity multiuser detection algorithm and its implementation for 3GPP W-CDMA

3. Approaches Algorithm –parallel interference cancellation –reduced complexity, parallel structure Pipelining –bit-streaming, avoid block detection Number theory –Most Significant Digit First (MSDF) computation –sign detection

4. Contributions decrease detection latency and storage requirements by window length (12X) On-line arithmetic based on MSDF computation –further latency reduction by 1.9X –increase in throughput by 3X –possible savings in area

5. Outline Parallel interference cancellation Pipelining On-line arithmetic Conclusions Current research directions

6. Multiuser detection r i-2 r i-1 riri r i+1 Interference from previous bits of other users Interference from future bits of other users Desired user User 1 User j riri bibi b i+1 time

7. Multiuser detection Optimal - MLSE Decorrelating MMSE Serial/Parallel interference cancellation Top 3 require inversion of matrices Block based detection

8. Parallel interference cancellation Block based detection

9. Block detection

10. Outline Parallel interference cancellation Pipelining On-line arithmetic Conclusions Current research directions

11. Parallel interference cancellation Block Toeplitz structure - suitable for pipelining

12. Pipelined detection

13. Being designed as a class project in Elec 422/423 VLSI class

14. Outline Parallel interference cancellation Pipelining On-line arithmetic Conclusions Current research directions

15. Redundant number systems Conventional systems ( 0.34578, r=10) –radix r has r possible digits Redundant (0.34578,0.35578,…. r=10) –>r possible digits. Limit carry propagation Totally parallel addition/subtraction ONLY.

16. On-line arithmetic Uses a redundant number system Pipelined bit-serial arithmetic Most Significant Digit First computation Successive computations as soon as  inputs available (  = 1-4, typically) Can do operations such as addition, multiplication, division, square-root etc.

17. On-line detection and decoding Entire chain can be done on-line Work with hard decisions (sign of MSD) simple way to use softer decisions (2 or more digits)

18. On-line arithmetic for detection

19. Conventional arithmetic - matched filter

20. On-line matched filter

21. Conventional multiuser detection

22. On-line multiuser detection

23. Comparisons N = K =32, d = 8, S = 3, r = 4, t ol = 2, t conv = 1, t stop = 2

24. Outline Parallel interference cancellation Pipelining On-line arithmetic Conclusions Current research directions

25. Conclusions Techniques such as pipelining and on-line arithmetic can be used to implement multi- user detection for W-CDMA. Lower latency Higher throughput Smaller area Simple hardware - adders and multipliers

26. Current research directions Reconfigurable computing -RENE Chameleon - hardware mNIC card