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Cooperative Diversity Orthogonal Frequency Division Modulation (CD-OFDM) systems are very
sensitive to synchronization errors. In CD-OFDM, synchronization is more complex because all
cooperative nodes (CNs) have their own frequency oscillator and different channel path which results
in different timing and carrier frequency offset (CFO) for each node. Consequently, each node has to
be synchronized separately without affecting the synchronization process of other nodes. All CNs
transmit simultaneously during cooperation phase (C-phase) and their aggregate signal is received at
the destination node. A unique frequency domain (FD) preamble is proposed for each CN during Cphase that will allow simple separation of cooperative nodes. These FD multiplexed preambles make
the synchronization problem identical to OFDMA uplink. OFDMA system typically uses highly
complex iterative CFO estimators for uplink synchronization. However, a simple one-shot CFO
estimator is proposed that uses repeated preamble of two OFDM symbol duration. The proposed
method is computationally efficient because it relies on FFT operation for user separation and
interference mitigation. Subsequently, time domain (TD) multiplication is used for CFO correction of
each CN. Furthermore, a CD-OFDM protocol for data transmission is presented that suites the
proposed estimator and harnesses spatial diversity. The proposed estimator shows good statistical
results during simulations in AWGN and Rayleigh environments. During evaluation, estimator
variance, mean square error and symbol error rate are used as performance measure.
Mudassar I. Cheema, Shoab A. Khan, Member, IEEE. (2014) A Simple Carrier Frequency Offset Synchronization Strategy for Multiple Relay Cooperative Diversity OFDM System, Pakistan Journal of Engineering and Applied Sciences, Volume 14, Issue 1.
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