A quantized reliability-relay selection method for wireless cooperative communications

In this paper, we propose a relay selection method for cooperative communication systems (CCS) employing the QPSK modulation scheme. The relay nodes forward their reliability to the destination node, utilizing a quantized reliability-relaying (QRR) scheme. The reliability is divided into five different levels by the relay nodes, according to the log-likelihood ratio (LLR) value of the received signal from the source node. Then, a suitable symbol is forwarded to the destination node if the relay node is set as active. A relay node is set as active if the reliability of the received signal is beyond a certain threshold. We provide analysis of the LLR for QPSK and carry out numerical simulations comparing the bit error rate (BER) of the QRR method against regular decode-and-forward (DF) and the best relay selection algorithms. A channel estimation algorithm is utilized to evaluate the performance of the system, as well as calculations of the normalized mean squared error (NMSE) of the channels. The special case of a system employing BPSK is also investigated.