Cooperative diversity has been recently proposed as a way to form virtualantenna arrays that provide dramatic gains in slow fading wirelessenvironments. However most of the proposed solutions require distributedspace-time coding algorithms, the careful design of which is left for futureinvestigation if there is more than one cooperative relay. We propose a novelscheme, that alleviates these problems and provides diversity gains on theorder of the number of relays in the network. Our scheme first selects the bestrelay from a set of M available relays and then uses this best relay forcooperation between the source and the destination. We develop and analyze adistributed method to select the best relay that requires no topologyinformation and is based on local measurements of the instantaneous channelconditions. This method also requires no explicit communication among therelays. The success (or failure) to select the best available path depends onthe statistics of the wireless channel, and a methodology to evaluateperformance for any kind of wireless channel statistics, is provided.Information theoretic analysis of outage probability shows that our schemeachieves the same diversity-multiplexing tradeoff as achieved by more complexprotocols, where coordination and distributed space-time coding for M nodes isrequired, such as those proposed in [7]. The simplicity of the technique,allows for immediate implementation in existing radio hardware and its adoptioncould provide for improved flexibility, reliability and efficiency in future 4Gwireless systems.
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