Designing MAC Protocols for Spread Spectrum Ad Hoc Networks: Thinning versus Spatial State-Dependent Packing

Achieving high spatial reuse in ad hoc networks exploiting a spread spectrum physical layer can be quite challenging. Designing a MAC protocol for such systems must take into account the physical layer characteristics. Most existing MAC designs for spread spectrum ad hoc networks perform contention resolution to thin the intended transmissions. This is fundamentally similar to the designs for a narrow-band system. In this paper, we show that fundamentally a thinning approach is not efficient toward achieving a high level of spatial reuse and significantly compromises system capacity. We then propose a novel design concept: state dependent spatial packing of transmissions. We design a multistage contention protocol to realize this spatial packing concept and show it is very efficient in terms of spatial reuse and handling power control, robust to network load and imperfect configuration, and easily implemented in a distributed fashion with small overhead. The performance of this scheme is impressive, with only two stages capable of offering 100%-500% gain over simple thinning protocols such as ALOHA-like random channel access.