Exploiting Time and User Diversity in Distributed Medium Access Control for Mobile Ad-Hoc Networks

In this paper, we consider a distributed medium access control (MAC) for mobile ad-hoc networks (MANET). We propose an adaptive threshold-based medium contention scheme to exploit diversity gain in multiple dimensions, in particular, the time and user diversity. In our scheme, each node performs a carrier sensing multiple access with collision avoidance (CSMA/CA) MAC protocol and access the channel only when the channel quality is above a threshold. We develop a practical solution to address node mobility and time varying fading channels in a general ad-hoc network model. Our solution consists of channel estimation and prediction, transmission rate selection, and threshold adaptation. We show that an efficient threshold-based solution will need to employ a dynamic load-based threshold adaptation. Moreover, we show that a distributed MAC algorithm requires additional overhead for time and user diversity control and thus need appropriate tradeoff design to maximize the system capacity. We conduct network simulations to evaluate the performance of our proposed protocol and demonstrate promising throughput increase via time and user diversity. We conclude that the achievable diversity gain in MANET highly depends on the network topology and heterogeneity of link quality and thus remains as a challenging task.