1 Cooperative STBC-OFDM Transmissions with Imperfect Synchronization in Time and Frequency Fan Ng and Xiaohua(Edward) Li Department of Electrical and Computer Engineering State University of New York at Binghamton

2 Problems:  In cooperative transmissions, synchronization of distributed transmitters is difficult  Imperfect time synchronization Channel dispersion, ISI, STBC structure destroyed  Imperfect frequency synchronization Residue carrier, channel time-variation

3 OFDM  Good candidate for broadband transmissions  Inherently tolerate time-asynchronism Transmission delays resolved as phase of subchannels  Frequency asynchronism is challenging Create inter-carrier interference (ICI) Our contributions  Study time/frequency asynchronism among cooperative STBC-OFDM transmitters  Propose a receive scheme to resolve both asynchronism

4 Cooperative STBC-OFDM transmissions Objective: redesign STBC-OFDM receiver to tolerate asynchronous transmitters  Reduce synchronization cost by increasing computational complexity of receiver

5 A special J=2 STBC-OFDM transmission scheme

6 Conventional: OFDM without frequency offset Transmitted baseband signal Received baseband signal Signal after OFDM demodulation

7 Cooperative OFDM with delay and frequency offset Received signal distorted by delay d j and frequency offset f j OFDM demodulation. CP removed prior to this point. where channels are distorted with delay and carrier offset

8 Analysis of inter-carrier interference (ICI) OFDM symbol illustration

9 Joint OFDM detection and STBC decoding Construct vector channel model JNxJN channel matrix due to STBC (J) and OFDM (N) Estimate symbols by solving the equations C i : coefficients of desired signal part. C i’ : coefficients of ICI part

10 Simulations

11  ICI & delay not compensated  Proposed joint detection method  Conventional STBC-OFDM with perfect synch. STBC: J=2. OFDM: N=32, CP=, Delay=2+fractional Carrier offset: 100 Hz, 80 Hz.

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13 Summary:  OFDM resolve delay asynchronism for cooperative transmission, but suffer from carrier-offset-induced ICI  Analyzed ICI structure, proposed a joint detection method  Simulations show the method successfully resolve delay & carrier asynchronism among transmitters  Future work is to develop more efficient detection algorithm