Directional collision avoidance in ad hoc networks

This paper analyzes the performance of directional collision avoidance schemes, in which antenna systems are used to direct the transmission and reception of control and data packets in channel access protocols based on four-way handshakes to try to avoid collisions. The first analytical model to consider directional reception and the possible difference in gain between omni-directional and directional transmissions is presented. Analytical results show that, when the directional collision avoidance scheme in which all transmissions are directional is augmented with directional receiving, one-hop throughput does not decrease due to the increased spatial reuse, even when the number of competing nodes within a region increases. It is also shown that, as expected, the performance of directional collision avoidance schemes degrades when directional transmissions have much higher gain than omni-directional transmissions. However, this degradation is relatively small. Simulations of the IEEE 802.11 protocol and its directional variants validate the results predicted in the analysis. The simulation results also show that the presence of broadcast traffic does not degrade the performance of the all-directional collision avoidance scheme significantly, even for relatively large percentages of broadcast traffic. The performance results of this study indicate that the most attractive collision avoidance approach consists of using directional transmissions of control and data packets, together with the directional reception of packets whenever a node is expecting a particular packet. Given the high tolerance to broadcast traffic of directional collision avoidance schemes, it is argued that the periodic transmission of beacons omni-directionally suffices to provide such schemes with the relative location of neighboring nodes.