Diversity Combining Technique for Soft Handoff in OFDMA Cellular Systems

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

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

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

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

7. A B C D MS 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 : 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. System Models • One subchannel occupies subcarriers • Total subchannels • Subcarrier assignment schemes • Subband based schemes • Interleaved schemes

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

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

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

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

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

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

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

18. Appropriate ΔN • 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

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

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

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

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

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. Received Signals • For interleaved schemes with l -th symbol (l+1) -th symbol Standard Alamouti decoding andMRC 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 definition : overall bit energy is noise enhancement bit energy from one BS

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

30. 0 10 -1 10 -2 10 -3 10 -4 10 0 5 10 15 System Performance Bit Error Probability

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

32. 0 10 -1 10 -2 10 Non-handoff state Analytical “ ” case Analytical “ ” case Analytical “ ” case Simulation “ ” case Simulation -3 10 -4 10 15 0 5 10 Subband Based Schemes BER Bit Error Probability

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

34. 0 10 -1 10 -2 10 Analytical Simulation : Case I : Case II : Case III : Case IV : Analytical non-handoff state -3 10 -4 10 0 5 10 15 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. 0 10 -1 10 -2 10 Non-handoff state Analytical Subband based “ ” Simulation Subband based “ ” Simulation Strategy I = Strategy II Analytical Strategy I Simulation Strategy II Simulation -3 10 -4 10 0 5 10 15 Strategy I, II, III Comparison 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.