1. Diversity Combining Technique for Soft Handoff in OFDMA Cellular Systems Xiu-Sheng Li and Yuh-Ren Tsai Presented by Xiu-Sheng Li ( 李修聖 ) 2007 05 18 Wireless communication system LAB. NTHU

2. Outline Introduction System Model Diversity Combining Techniques System Performance and Simulation Results Conclusion

3. Introduction OFDMA cellular system – Ex : WiMAX Mobility Handoff problem Soft handoff against hard handoff Reduces the “ ping-pong ” effect Communication link is always active Enhanced performance by diversity combining Goal : to propose a novel diversity combining technique so that soft handoff can be implemented in OFDMA cellular systems

4. Introduction Reuse factor : To reduce co-channel interference D A B C D B B C A B A BC D Cellular system with Each with bandwidth subcarrier number Total bandwidth subcarrier number

5. Conventional Strategies Strategy I  conventional Strategy II  conventional Strategy III  to be proposed in this work

6. When soft handoff is activated, all bandwidth is collected, DFT with size is operated. Diversity combining is performed. Sampling rate : DFT size : Strategy I D C A B MS ABC D Diversity combining (ex:MRC)

7. Strategy II When soft handoff is activated, two receiving front ends are necessary. The signal from each BS is processed independently, and finally diversity combining is performed. Two receivers Sampling rate : DFT size : D C A B MS A C Diversity combining (ex:MRC)

8. Strategy III Strategy III key points changing sampling rate and DFT size Alamouti space time code (STC) is applied Compared to Strategy I smaller sampling rate and smaller DFT size Compared to Strategy II one DFT block despite of a little higher sampling rate and DFT size

9. One subchannel occupies subcarriers Total subchannels Subcarrier assignment schemes Subband based schemes Interleaved schemes System Models

10. Down-converted by carrier System Models bandwidth BS 1 BS 2 Backbone Network MS Synchronous cellular system ABC D

11. Diversity Combining Technique By sampling rate - point DFT is applied With the assumption Filtering

12. Diversity Combining Technique By sampling rate Repeat in every - point DFT to extract values By

13. Diversity Combining Technique By changing appropriate sampling rate, desired signals can be gathered and interference can be avoided Alamouti space time code (STC) is required for desired signals After sampling by : subchannel index for BS1 : subchannel index for BS2

14. After sampling by Diversity Combining Technique Another example for subband based schemes

15. Appropriate Δ N Appropriate is determined by subcarrier assignment schemes : subchannel index for BS1 : subchannel index for BS2 : subchannel number : subcarrier number of a subchannel : carrier spacing parameter

16. For subband based schemes with Appropriate ΔN C=1 C=2 (a) C=2 (b)

17. For subband based schemes with Criterion Overlapped desired signals Interference avoidance DFT size restriction Final result Appropriate ΔN

18. For subband based schemes with Criterion To guarantee that desired signals from BS1 do not interfered by signals from BS2 To guarantee that desired signals from BS2 do not interfered by signals from BS1 Final result Appropriate ΔN

19. Similar results can be obtained for interleaved schemes Appropriate ΔN

20. Alamouti STC Alamouti STC with 2TX&1RX Information source

21. BS 1 BS 2 Information Source Modulator MS Backbone Network At l-th symbol time At (l+1)-th symbol time Frequency Domain on -th subchannel At l-th symbol time At (l+1)-th symbol time Frequency Domain on -th subchannel Alamouti STC

22. Received Signals For subband based schemes with Standard Alamouti decoding is applied & diversity order is 2 l -th symbol (l+1) -th symbol

23. Received Signals For subband based schemes with l -th symbol (l+1) -th symbol Maximal ratio combining (MRC) is applied & diversity order is 2

24. Received Signals For interleaved schemes with l -th symbol (l+1) -th symbol Standard Alamouti decoding is applied & diversity order is 2

25. For interleaved schemes with Received Signals l -th symbol (l+1) -th symbol Standard Alamouti decoding and MRC are applied & diversity order is 2

26. System Performance Uncoded BPSK BER over Rayleigh fading channel BPSK Alamouti BER with 2TX&1RX over Rayleigh fading channel BPSK BER with 2-branch MRC overall bit energy bit energy from one branch

27. System Performance Strategy III BER at non handoff state Strategy III BER at soft handoff state : the proportion that Alamouti decoding is applied : the proportion that MRC is adopted bit energy from one BS

28. System Performance Strategy III BER at handoff state with Alamouti decoding Strategy III BER at handoff state with MRC bit energy from one BS noise enhancement definition : overall bit energy is

29. Noise Enhancement Noise enhancement for Alamouti decoding Twice noise power No noise enhancement for MRC

30. Bit Error Probability System Performance

31. Subband Based Schemes BER Simulation parameters OFDM symbol length (without CP) : CP length : ITU-5 channel model  

32. Bit Error Probability Subband Based Schemes BER

33. Interleaved Schemes BER Simulation parameters OFDM symbol length :, CP length : ITU-5 channel model, Case I : Case II : Case III : Case IV :  

34. Interleaved Schemes BER Bit Error Probability

35. Strategy I, II, III Comparison BPSK BER for Strategy I Non handoff state Soft handoff state BPSK BER for Strategy II Non handoff state Soft handoff state Simulation parameters

36. Strategy I, II, III Comparison Non-handoff state Analytical Subband based “ ” Simulation Strategy I = Strategy II Analytical Strategy I Simulation Strategy II Simulation Bit Error Probability

37. Strategy I, II, III Comparison Strategy I Sampling rate : DFT size : Strategy II Two receivers Sampling rate : DFT size : Strategy III Sampling rate : can be restricted by DFT size : can be restricted by Channel re-allocation is sometimes required.

38. Conclusion The proposed diversity combining consists of Changing sampling rate Appropriate DFT size Alamouti STC By the proposed technique, diversity combining can be realized for OFDMA cellular systems The proposed scheme may have some implementation advantages against conventional ones

39. Conclusion The proposed scheme can be considered as a generalized scheme of conventional ones. The proposed diversity combining technique may have applications other than soft handoff.