Spatial reuse efficiency of medium access control in wireless ad hoc networks

In the past few years wireless ad hoc networks have witnessed an explosion of interest as they increasingly play a significant role in the deployment of multi-hop data and sensor networks. In this thesis we investigate the spatial reuse efficiency of decentralized random MAC (Medium Access Control) protocols in wireless ad hoc networks. Spatial reuse, with its ability to determine the number of simultaneous connections allowed in a given region, has a substantial impact on the network performance in terms of its throughput and delay characteristics. We begin our study by showing that the channel reservation mechanism of IEEE 802.11 virtual carrier sensing (VCS) results in sub-optimal spatial reuse and the area reserved by the RTS/CTS handshake depends on the distance between transmitter and receiver. We propose a novel VCS scheme that improves the spatial reuse by incorporating this distance information in the decision making process for the channel reservation. Both analytical and simulation results are used to quantify and demonstrate the substantial performance improvements obtained by the proposed VCS scheme. We then present an analytical framework which quantifies the spatial reuse for a bank of random access MAC protocols using their equivalent saturation throughput. Using appropriate stochastic assumptions, we obtain the upper bound for equivalent saturation throughput as a function of node density and show that our model further adds a constant bound to the result from the capacity analysis by Gupta and Kumar. A numerical evaluation is also used to visualize the results and show the potential of our model to facilitate system design. In the final part, our spatial reuse study is further extended to multi-hop environments in which MAC coordination among neighboring nodes is examined and shown to influence the spatial reuse for persistent flows. We propose adaptive pacing in 802.11 ad hoc networks to orchestrate the transmissions of adjacent nodes in a more balanced manner so as to tune the spatial reuse towards its optimum. Simulation results verify that our scheme significantly outperforms the original 802.11 MAC by boosting the end-to-end throughput without adversely affecting the latency.