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

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technique for mobile wireless communications because of its good performance under multipath propagation environments. The concept of cooperative communication is currently under intensive research. It uses antennas of other terminals in the network to create virtual Multiple Input Multiple Output (MIMO) systems, providing capacity gains similar to those of MIMO systems. However, OFDM based communication systems suffer from Carrier Frequency Offset (CFO) due to Doppler shift, and frequency synchronization error at the transmitter-receiver oscillators. And this CFO disturbs the subcarriers by making them lose their orthogonality, which in turn causes a severe problem in wireless communications called Subcarrier Leakage or Inter-Carrier Interference (ICI). Due to complex relaying at the channel and the combining techniques used at the receiver the issue of ICI will become worse in the case of cooperative communication. In this thesis, OFDM based cooperative communication system is studied and developed and also a model to mitigate the effects of Doppler shift and frequency synchronization error in an OFDM communication system is developed. The system uses orthogonal frequency division multiplexing as a transmission technique. In the design of OFDM, maximum likelihood estimation is used to compensate for the effects of carrier frequency offset. Performance comparison is also done for maximum likelihood estimation with the most common carrier frequency estimation method called the self-cancellation estimation method and proved that it overweighs the self-cancellation estimation series. xii In designing the cooperation system, the channels between the source node, the cooperating node and the destination (base station) are modeled containing thermal noise, Rayleigh fading, Rician fading and path loss. Amplify-and-Forward (AAF) cooperation protocol is used at the cooperating node when the system is in cooperation mode. Depending on the channel state between the source network access node and the cooperating network access node, the cooperating node turns its cooperation switch to either ON or OFF state. For a relatively short distance between the cooperating nodes, when compared to the distance between them and the base station, AAF has a better performance than Decode-and Forward (DAF) protocol, unless an error correcting code is simulated. The wireless communication links, the link between the source node and the base station and the link between the cooperating node and the base station, are assumed to have six different propagation paths. Next, the system performance is investigated (bit-error-rate against signal-to-noise-ratio) for different types of signal combination techniques. The performance of different combination protocols at the receiver is simulated and Maximum Ratio Combiner (MRC) 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. Matlab application software package was used to perform the simulations.