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 ) Wireless communication system LAB. NTHU

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

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

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

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

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)

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)

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

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

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

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

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

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

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

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

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

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

Similar results can be obtained for interleaved schemes Appropriate ΔN

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

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

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

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

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

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

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

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

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

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

Bit Error Probability System Performance

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

Bit Error Probability Subband Based Schemes BER

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

Interleaved Schemes BER Bit Error Probability

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

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

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.

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

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.