Load Adaptive MAC: A Hybrid MAC Protocol for MIMO SDR MANETs

The performance of prevalent wireless MAC protocols is a function of network contention level and the capabilities of the underlying network nodes. While contention-based MAC protocols such as CSMA experience significant performance degradation under high contention levels, slot-based MAC protocols such as TDMA perform in the opposite way. In this paper, we propose a hybrid MAC protocol to which we refer as Load-Adaptive MAC (LA-MAC) protocol for MANETs formed by a collection of Multiple-Input Multiple-Output (MIMO) equipped nodes. By adaptively switching its mode of operation between CSMA and TDMA, LA-MAC achieves a high channel utilization and a reasonable delay profile under both high and low contention levels identified by the measured collision rate of the MAC frames. As a cross-layer protocol, LA-MAC relies on the information received from the physical layer in order to distinguish between corrupted frames due to actual collisions as opposed to those corrupted by wireless link effects such as fading. We describe the design of LA-MAC protocol and report on its implementation in a MANET testbed formed by a collection of MIMO Universal Software Radio Peripheral (USRP) Software Defined Radio (SDR) nodes. Through analytical and experimental studies, we compare the performance of LA-MAC with that of CSMA and TDMA under different traffic conditions. The results show the improvements achieved by LA-MAC in comparison with other alternatives.