Power control for distributed MAC protocols in wireless ad hoc networks

In wireless networks that use centralized transmission scheduling, reducing the transmission power normally leads to higher network transport throughput. In this paper, we investigate powercontrol and spatial utilization in a different scenario, where the network adopts distributed MAC-layer coordination mechanisms. We first consider the widely adopted RTS/CTS-based MAC protocols. We show that an optimal power control protocol should use higher transmission power than the "just-enough" power in order to improve spatial utilization. The optimal protocol has a minimal transmission floor area of Theta(d(i,j)d(max)), where d(max) is the maximal transmission range, and d(ij) is the link length. This surprisingly implies that if a long link is broken into several short links, then the sum of the transmission floors reserved by the short links is still comparable to the floor reserved by the long link. Thus, using short links does not necessarily lead to higher transport throughput. Another consequence of this is that, with the optimal RTS/CTS based MAC, rate control can at best provide a factor of two improvement in transport throughput. We then extend our results to other distributed MAC protocols that use physical carrier sensing or busy tone as the control signal. Our simulation results show that the optimal power control scheme outperforms other popular MAC-layer power control protocols. We also validate our analysis regarding the impact of routing choices via extensive simulations.