Error recovery with soft value combining for wireless cooperative systems

Due to the multi-path, multi-hop transmission structure in wireless cooperative systems, when packet errors occur, retransmissions may take longer time and more energy, and thus affect the system performance and user-perceived quality of services. How to reduce the error rate for cooperative systems is a critical issue. In this paper, a novel and simple error recovery scheme for wireless cooperative systems has been proposed by using the soft values of each bit, which are related to the confidence levels when the physical layer demodulates and decodes the bit. The receiver has a better chance to obtain the correct bit by combining the soft values associated with the relay and the direct transmission paths, respectively. We then derive the bit error rate (BER) performance for the amplify and forward (AF) cooperative systems with soft value combining. Next, we extend the soft value combining for the demodulation and forward (DMF) cooperative systems, where the relay demodulates the received signal and forwards it to the destination in a separate modulation type, in order to further improve spectrum efficiency. Numerical results have validated the analysis, and demonstrated that the proposed soft value combining scheme is simple to implement and can achieve near optimal performance.