Abstract

In cooperative communications, error propagation at the relay nodes degrades the diversity order of the system. To combat that effect, we present a novel technique to control error propagation at the relays, which is implemented in the context of a distributed turbo code. In the presented technique, the relay calculates the log- likelihood ratio (LLR) values for the bits sent from the source. These values are subjected to a threshold to distinguish reliable decoded bits. The relay then forwards bits that are deemed reliable and discards bits that are not, resulting in less errors propagating to the destination. We develop upper bounds on the end-to-end bit error rate, enabling us to optimize the threshold in terms of the minimum end-to-end bit error rate. We compare our technique with existing techniques to control error propagation, including using only a cyclic redundancy code (CRC) check at the relay, forwarding analog LLR values, and with employing no error control at the relay at all. We demonstrate, via several numerical examples, that the performance of our proposed scheme is superior to all existing techniques. We investigate the application of this technique to a network-coded two-way relay channel where the relay is assisting two sources simultaneously. We propose two modes of thresholding: at individual bits and at combined bits. We analyze the bit-error rates of both thresholding modes and optimize the threshold for both. We show significant gains using thresholding over an unthresholded network-coded system. Based on system simulations, we conclude that utilizing