Cooperative Protocols for a Source and an Occasional Nearby Relay

We consider a wireless network where a source is communicating with a remote destination, and where a relay terminal is occasionally present in the source's vicinity without the source's knowledge. Cooperative protocols based on decode-and-forward (DF) and quantize-and-forward (QF) are proposed which improve upon single-user performance when the relay is present, and do not degrade single-user performance in the relay's absence. For DF, optimal correlation between the source's and relay's transmissions is investigated, and for QF, a scheme is suggested for incorporating partial side information without the relay terminal knowing the destination's channel state.