Joint Rate Adaptation and Channel-Adaptive Relaying in 802.11 Ad Hoc Networks

Channel-adaptive relaying has recently been proposed as a means to exploit spatial and temporal diversity in multihop ad hoc networks with fading. In conjunction with appropriate routing protocols, adaptive relaying enables each forwarding node in a multihop path to dynamically select the next-hop relay as a function of the measured (time-varying) channel state, providing a form of selection diversity at each hop. Based on the notion that links to diversity-selected relays have higher information capacity and therefore can support higher data rates than links obtained with traditional routing, this paper proposes marrying channel-adaptive relaying with rate adaptation (or adaptive modulation-coding). In particular, we specify a protocol for performing joint rate and relay adaptation in 802.11 ad hoc networks with geographic routing. Using both analytical and simulation tools, synergistic gains are observed in throughput, capacity and delay. Performance results are given for individual links as well as for multihop networks, in time-varying, correlated Rayleigh and Ricean fading channels over a range of channel speeds. Of particular interest in this study is the robustness of the adaptation to increasing channel Doppler. As a by-product of this work, we propose a new, SNR-based rate adaptation scheme for use in 802.11 systems that requires no modification to the standard 802.11 frame structure