Impact of Cooperation in Ad-hoc Networks with Heterogeneous Smart Antennas

The use of smart antennas in wireless ad-hoc networks has garnered significant attention over the last few years. Given the unique capabilities of smart antennas, and how they can improve performance in a typically constrained ad-hoc network environment, the attention is with merit. However, not much light has been shed on wireless ad-hoc networks that have nodes with varying antenna capabilities. While homogeneous ad-hoc networks with all nodes having the same antenna capabilities will have certain applications, we argue that ad-hoc networks with nodes having heterogeneous antenna capabilities are more likely to be the norm due to a variety of motivating factors. In the context of such heterogeneous smart antenna networks (HSANs), we investigate and motivate the need for a simple form of node cooperation called retransmit diversity. We show that while such a simple form of node cooperation cannot bring significant benefits to homogeneous omni-directional antenna networks, they can bring great gains (several folds improvement) to heterogeneous smart antenna networks. We then present several key properties pertaining to node cooperation in HSANs along with analytical reasoning. In the process, we identify a fundamental trade-off between exploiting smart antenna gain and cooperation gain, that undermines the ability of HSANs to leverage node cooperation to their maximum potential. To address this tradeoff, we then present an adaptive cooperation mechanism and incorporate this mechanism through the design of a simple but efficient MAC protocol. The performance of the MAC protocol is evaluated through ns2 simulations along with a realistic physical layer channel model for smart antenna environments.