Performance analysis of directional CSMA/CA in the presence of deafness

Although directional antennas can increase the spatial reuse in wireless ad hoc networks, the directional carrier sense multiple access/collision avoidance (CSMA/CA) protocols encounter unprecedented challenges that can offset this potential benefit. One critical problem is known as deafness that occurs when a transmitter repeatedly fails to communicate with its intended receiver because the receiver is beamformed towards another direction. The deafness problem has not yet been analytically studied since existing analytical models for directional CSMA/CA ignore the effect of deafness. In this study, the authors develop an analytical framework for directional CSMA/CA, which is the first analytical model to consider the problem of deafness as a source of transmission failures in multi-hop wireless networks with directional antennas. They also propose a deafness index to quantify the negative impact of deafness. Using their framework, the authors study the tradeoff between spatial reuse and deafness when a directional CSMA/CA protocol is employed. Their results demonstrate that decreasing the antenna beamwidth increases the saturation throughput up to a certain limit corresponding to an optimum beamwidth. However, by further lowering the beamwidth, the negative impact of deafness offsets the benefits of spatial reuse and results in a steep decrease in the saturation throughput. These results prove analytically that deafness is a critical problem if left unaddressed.