Cooperative Schemes for a Source and an Occasional Nearby Relay in Wireless Networks

A wireless network is considered where a source is communicating with a remote destination, and where a relay terminal is occasionally present in close proximity to the source but without the source's knowledge. The channel source-relay is assumed fixed due to the short distance, while the channels source-destination and relay-destination are affected by block flat Rayleigh fading, where channel state information is known only to respective receivers. Oblivious cooperative protocols are addressed which improve performance when the relay is present and do not degrade it when the relay is absent-and all this while the source is uninformed of the actual topology. Using the expected throughput as a performance measure, several such protocols based on decode-and-forward and quantize-and-forward are proposed for this network. It turns out that block Markov decode-and-forward (BMDF) with appropriate decoding is in fact an oblivious cooperative protocol. The optimal correlation between the transmissions of the source and the relay in decode-and-forward and the corresponding optimum performance are characterized. Finally, quantize-and-forward is considered for rates which preclude the usage of decode-and-forward, and several schemes are proposed for incorporating partial side information in the relay's quantization.