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Cooperative STBC-OFDM Transmissions with Imperfect Synchronization in Time and Frequency

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 {fngnone1,xli}@binghamton.edu h ttp://ucesp.ws.binghamton.edu/~xli.

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Cooperative STBC-OFDM Transmissions with Imperfect Synchronization in Time and Frequency

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  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 {fngnone1,xli}@binghamton.edu http://ucesp.ws.binghamton.edu/~xli

  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 dj and frequency offset fj OFDM demodulation. CP removed prior to this point. where channels are distorted with delay and carrier offset

  8. Analysis of inter-carrier interference (ICI) Desired Signal Part ICI Part OFDM symbol illustration

  9. Joint OFDM detection and STBC decoding Construct vector channel modelJNxJN channel matrix due to STBC (J) andOFDM (N)Estimate symbols by solving the equations Ci: coefficients of desired signal part. Ci’: 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.

  12. 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

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