Cooperative communication is considered one of efficient techniques to overcome fading in wireless networks since it can achieve spatial diversity. Employing all the relays in the network to transfer the data, consumes high power and cost. Selection of one or more relays reduces the complexity, decreases the power consumption and reduces the interference. In this paper, with these goals in mind, a relay selection algorithm is proposed in a slow flat fading channel. The algorithm is based on evaluating the log-likelihood ratio of all relays and the relay which has the largest magnitude of log-likelihood ratio (LLR) is selected to help the source to transmit the data to the destination.

The performance of the algorithm is evaluated by simulation and compared with other relay selection algorithms. The performance is measured in terms of bit error rate (BER) and outage probability. Both decode and forward and amplify and forward protocols are investigated. The performance is evaluated for known and estimated channel. The channel is estimated using least square algorithm. The performance of channel estimation algorithm is measured in terms of the mean square error. The results show that BEP provided by LLR-based selection combining algorithm is lower than BEP provided by the conventional SNR-based selection combining schemes.

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