MIMO and TCP: A Case for Cross Layer Design

TCP has not fared well in wireless ad hoc networks. Even in a perfectly static network, there is the problem of throughput degradation due to the interference between flows, which becomes more severe as the nodes come close to each other. Moreover, when two or more TCP flows compete in the same collision domain, often one of the flows "captures" the channel and blocks other flows, giving flows an unfair access to the channel. These problems have been traced in the 802.11 MAC protocol and the interaction between TCP and MAC protocol retransmission timeouts. Proposed solutions range from modifications in the 802.11 MAC protocols to the use of network layer selective drop strategies. However, none of these schemes offers full protection. Moreover, they require change in the existing protocols. The advent of Multiple-Input Multiple-Output (MIMO) system offers a way of increasing total throughput by reducing the interference. The same interference blocking feature leads to unfairness reduction. Recently proposed MIMO MAC protocols such as SPACE-MAC reduce interference by separating flows. In this paper, we test the efficacy of SPACE-MAC in eliminating the TCP capture problem and reducing throughput degradation due to interference. We evaluate the fairness and throughput performance of SPACE-MAC in various scenarios, and then compare with conventional MAC results via Qualnet simulation.