Rate Adaptation, Power Adaptation and Mutual Exclusion in Variable Rate Wireless Networks
We consider the joint design of rate adaptation, power adaptation and mutual exclusion for the MAC layer of a multi-hop, ad-hoc wireless network. We assume the physical layer supports a variable bit-rate. Most of the existing MACs analyze impacts of only one of these elements, and the jointly optimal strategy is not known. We assume that successive decoding is not implemented, i.e. one receiver decodes only one source at a time. Using a theoretical model that neglects protocol overhead, we numerically find the optimal combination of the three basic elements. Our results suggest that the optimal strategy has the following properties: (1) When a node transmits it should always transmit with the maximum power and no power adaptation is necessary. (2) There is an optimal exclusion region around a destination. While a destination is receiving, nodes inside the exclusion region should stay silent. Nodes outside of the exclusion region should transmit in parallel. The size of the exclusion region does not depend on link sizes, nor on position of nodes, but only on maximum transmitted power. (3) A sender should adapt the transmission rate to the amount of interference generated by nodes outside of the exclusion region of a receiver. We present the results in detail for the 802.11a/g physical layer (but our conclusions hold for other rate functions as well). We show by simulations that the optimal protocol outperforms the existing 802.11 rate adaptation protocols; the exclusion region of 802.11a/g is too large and the spatial reuse is too low, in other words, the efficiency of 802.11 could be improved by allowing more interference.