Cooperative Communications With Unreliable Relays

The advantage of cooperative communications often depends on the relay that can reliably receive a message from a source node, process it, and forward it to its intended destination node. In this paper, we consider the situation in which the relays cannot reliably decode the message due to a low signal strength or high interference. We propose a new cooperative relaying strategy that utilizes the information provided by unreliable relays to improve the reliability of decoding at the destination. The basic idea is to allow the relays to forward their decoder outputs as long as the destination can decode the message using the information provided by the source and all relays (called global information). The motivation is that even if each relay may not be able to decode the message based on its local information, the destination may still decode it using the global information. Only when the destination cannot decode the message, the relays discard their incorrect decoder outputs. We analyze the end-to-end outage probability of the proposed scheme as a function of the transmission rate, signal-to-noise ratio, and number of relays. We show that the proposed scheme can provide a significant improvement over the conventional schemes, particularly when the relays cannot reliably decode the message. If the SNR is high enough or the transmission rate is below a threshold, the proposed scheme can provide the same outage probability that can be provided by the genie-aided relaying that assumes perfect knowledge on the message by all relays.