Cross-Layer Design of Wireless Ad-Hoc Networks

We consider a cross-layer design of wireless ad-hoc networks.
Traditional networking approaches optimize separately each of the three layers: physical layer (rate and power control), medium access and routing. This may lead to largely suboptimal network designs. In this work, we propose a jointly optimal design of the three layers, where the goal of the optimization is to achieve the proportional fair allocation of end-to-end flow rates. We show a significant performance improvement of the cross-layer design over the conventional approach.

We first analyze networks built on physical layers that have a rate which is a linear function of SNR at the receiver (such as UWB or low-gain CDMA systems). We find that the optimal solution is characterized by the following principles: (1) Whenever a node transmits, it has to transmit with the maximum power; otherwise it has to remain silent (0-P^MAX power control). (2) Whenever data is being sent over a link, it is optimal to have an exclusion region around the destination, in which all nodes remain silent during transmission, whereas nodes outside of this region can transmit in parallel, regardless of the interference they produce at the destination. (3) When a source transmits, it adapts its transmission rate according to the level of interference at the destination due to sources transmitting in parallel. (4) The optimal size of this exclusion region depends only on the transmission power of the source of the link, and not on the length of the link nor on positions of nodes in its vicinity. As for the routing, we restrict ourselves to a subset of routes where on each successive hop we decrease the distance toward the destination. We also show that (5) relaying along a minimum energy route is better than using longer hops or sending directly, which is not obvious since we optimize rate and not power consumption. Finally (6), the design of the optimal MAC protocol is independent of the choice of the routing protocol. We present a theoretical proof of optimality of 0-P^MAX power control, and the remaining findings we show numerically on a large number of random network topologies.

Next, we consider narrow-band networks, where rate function is a strictly concave function of SNR. There, previous findings do not always hold. We show that in some cases, the size of the exclusion region and the optimal routing depend on transmission powers, and that the optimal MAC design depends on the choice of routing. Nevertheless, as we show with the example of 802.11 networks, a significant improvement over the existing 802.11 MAC can be achieved even with simpler, suboptimal strategies. Although this result is shown by simulations on a simplified model, it still gives further directions on how to improve the performance of RTS/CTS based protocols.