Performance of 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 fourway collision avoidance handshake. We present an analytical model that considers both directional reception and the possible difference in gain between omnidirectional and directional transmissions. 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 as long as the directional transmission/reception beamwidth is narrow. 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; and show that side lobes affect little on throughput if they are reasonably low and the carrier sensing threshold is raised to make nodes less sensitive to channel activities. It is argued that the all-directional scheme in which both transmission and reception of packets is directional is the most attractive collision avoidance approach.