Exploiting Distributed Spatial Diversity in Wireless Networks

We develop energy-ef£cient transmission protocols for wireless networks that exploit spatial diversity available at distributed antennas to combat multipath fading through coordinated transmission and/or processing by several radios. We discuss the problem in a general network setting and focus on a multiple-access case with suf£cient symmetry to make the presentation concise. In particular, we examine several possibilities for the strategy employed by the assisting radios, or relays, including decoding and forwarding and amplifying and forwarding. To characterize performance, we develop outage regions and associated outage probabilities that indicate robustness of the transmissions to varying signal-to-noise ratios (SNRs). The outage regions are treated as set-functions of the observed SNR γ on the channel between the two source radios. The outage regions are thus (conditional) events de£ned in terms of the other SNR parameters of the channel, and can be readily interpreted in coded and uncoded settings. Our results suggest that the relays employ a threshold rule, namely, for SNRs satisfying γ > γ∗, for some value γ∗, the relays decide to cooperate and pass along each other’s transmissions, preferably by decoding and re-encoding with a separate codebook for suf£ciently high γ; for SNRs satisfying γ < γ∗, the relays can do better by simply retransmitting their own signals.