A New Protocol to Mitigate the Unheard RTS/CTS Problem in Networks with Switched Beam Antennas

Wireless networks with the capability for directional transmissions using switched beam antennas have increasingly been used to increase the coverage area of nodes as well as to improve spatial reuse. This paper is concerned with the unheard RTS/CTS problem that arises due to the use of directional transmissions. The problem occurs because a node, while beam-formed in one direction, cannot hear the RTS/CTS messages that arrive on another direction with information pertaining to channel reservation. A node, therefore, transmits when it should defer, leading to unnecessary collisions and degraded performance. In this paper, we propose a new MAC protocol that uses a combination of three features to combat the problem: fragmentation of packets, the use of a tone signal to alert potential collision-causing nodes during ongoing transmission, and the use of a pause period when transmission is likely to lead to a collision. As opposed to other work on this problem, our protocol does not assume separate data and control channels. We present simulation results showing that our protocol can reduce the number of retransmissions of data packets due to the unheard RTS/CTS problem by as much as 86%, thus improving the delay and throughput characteristics of the network.