Medium Access Control in Ad Hoc Networks with Omni-directional and Directional Antennas

An ad hoc network is a dynamic network formed on demand by a group of nodes without the aid of any pre-existing network infrastructure. An efficient and effective medium access control (MAC) protocol which regulates nodes access to the shared channel(s) is essential in an ad hoc network. Our work is focused on the throughput and fairness properties of existing omni-directional MAC protocols as well as enhancement of their performance with directional antennas via both analytical and simulation approaches. In the first part, we present the first analytical modeling of collision avoidance MAC protocols including the popular IEEE 802.11 MAC protocol in multi-hop ad hoc networks. We show that in ad hoc networks with a lot of hidden terminals, collision avoidance even if done correctly, can still limit achievable throughput significantly because of the much reduced spatial reuse. Then we advance the analytical modeling to evaluate those MAC protocols which use directional antennas and can achieve much higher throughput through directing transmissions and receptions to desired directions only. We show that the gain in spatial reuse outweighs that of collision avoidance and hence an aggressive all-directional scheme is more advantageous than other hybrid schemes that take conservative (or unnecessary) tradeoff between collision avoidance and spatial reuse. All the analytical work has provided very useful insight on the interaction between spatial reuse, interference reduction and collision avoidance that previous work lacked.