On optimal outage in relay channels with general fading distributions

This correspondence deals with the outage capacity of relay networks in the low signal-to-noise ratio (SNR) regime. This work is motivated by the fact that in relay channels, unlike multi-antenna point-to-point links, the transmitters, i.e., source and relays, may not be co-located and therefore their channel statistics can be quite different. It has been recently shown that bursty amplify and forward (BAF) is outage optimal when all the links have a Rayleigh distribution. In this correspondence, it is shown that BAF is in fact outage optimal for a wide class of independent channels with smooth distribution functions. Optimality of BAF is further generalized to a special class of dependent channels, namely, the case where we opportunistically use only the best relay (out of N relays). It turns out that relative to the strategy where all the N relays are used, this opportunistic best relay strategy uses significantly smaller average power (independent of N) while suffering negligible increase in outage. This holds out potential for significant gains in an ad hoc network scenario where minimizing interference to possibly other users as well as conserving power are important considerations. Index Terms--Cooperation, diversity, fading channels, outage capacity, relay channel.