Relay selection based on the log likelihood ratio for cooperative communication networks

This paper proposes a relay selection scheme for wireless cooperative communication systems (CCS) utilizing a decode and forward protocol. The proposed scheme is suitable for systems employing M-ary PSK modulation schemes. A single relay node is chosen to cooperate with the direct link in delivering the message signal. This choice is based on the absolute value of the log-likelihood ratio (LLR) of the received signal at each relay node. The proposed scheme is derived for BPSK and QPSK as illustrative examples. Numerical simulations are carried out and presented to evaluate the system performance. The bit-error-rate (BER) curves of the proposed scheme are evaluated for known and estimated channels and are compared to other relay selection methods. Channel estimation is performed by the least square algorithm. Convergence curves of known and estimated channels are employed to evaluate the performance of the estimation algorithm.