Controlling Error Propagation in Network-Coded Cooperative Wireless Systems

In cooperative communications, error propagation at relays degrades the diversity order of the system. To combat that effect, it has been suggested to implement a reliability threshold at the relay to control error propagation. The relay calculates log-likelihood ratio (LLR) values for the bits sent from the source. These values are subjected to a threshold to selectively forward bits that are most reliable and discard bits that are less so, resulting in less errors propagating to the destination. We investigate the application of this technique to a network-coded two-way relay channel, where the relay is assisting two sources simultaneously. We investigate two modes of thresholding at the relay: at the individual-bit level and at the combined-bit level. 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 separate thresholds yields better results than utilizing a combined threshold scheme.