6. Diversity Relaying with Spatially Random Mobile Relays

In many studies on wireless cooperative diversity, it is often assumed that the number of helpers and their locations are deterministic or known a priori. In this chapter, we relax such assumptions and investigate a wireless diversity system with distributed cooperation and spatially random helpers subject to random direction (RD) mobility. To enable opportunistic relaying with multiple helpers, we consider an ALOHA-like medium access control (MAC) scheme and a timer-based random backoff scheme for multi-helper coordination. Particularly, we analyze the upper bound of combined signal-to-noise ratio (SNR) and unconditional success probability with multi-helper cooperation. We also provide numerical approximations for the delay of the two MAC schemes. To characterize the tradeoff between the success probability and delay, we further define a success/delay ratio, which can be maximized by adapting the intensity of selected helpers. The numerical and simulation results validated the analysis accuracy and demonstrated insightful observations.