Mitigating the Effects of Mobility and Synchronization Error in OFDM based Cooperative Communication Systems

An Orthogonal Frequency Division Multiplexing based mobile wireless network with a sender, a destination and a third station acting as a cooperating node is modelled and analyzed. The length of cyclic prefix in the orthogonal frequency division multiplexed symbols is made to vary depending on the channel conditions and maximum likelihood estimator is used at the receiver in order to compensate for the carrier frequency offset that occurs during transmission. Simulation results show that maximum likelihood estimator has better performance than self-cancellation estimations. The channels between the source, the cooperating node and the destination are modelled containing thermal noise, Rayleigh fading, Rician fading and path loss. Amplify-and-Forward cooperation protocol is used at the cooperating node when the system is in cooperation mode. For a relatively short distance between the cooperating nodes, when compared to the distance between them and the base station, amplify and forward cooperation protocol has a better performance than decode-and forward protocol, unless an error correcting code is simulated. The cooperating node turns its cooperation mode switch ON or OFF depending on the channel state between the source and the cooperating nodes.  The performance of different combination protocols at the receiver is simulated and maximum ratio combining is found to have better performance. However, for immobile wireless sensor networks Extended SNR (ESNR) combiner has also better performance. The system has also showed that with any kind of combination protocol at the receiver it is possible to achieve second order diversity when there is only one cooperating node in the system.