A Selective CSMA Protocol with Cooperative Nulling for Ad Hoc Networks with Smart Antennas

In this paper a media access control protocol is proposed for ad hoc network stations with adaptive antenna arrays. The protocol is based on the IEEE 802.11 distributed coordination function (DCF) and uses omnidirectional RTS/CTS exchanges to initiate data packet transmissions. Unlike previous directional antenna MAC protocols, the proposed protocol accommodates the active nulling of co-channel interferers that may arise during the course of ongoing transmissions. This is done using a three-way handshake where neighboring stations cooperate during link activations thus allowing the active receiver to dynamically null potential future interfering packet transmissions. The protocol uses omnidirectional transmission combined with beamformed reception, and is referred to as selective CSMA with cooperative nulling (SCSMA/CN). Packet type identification is used to enable/disable carrier sensing based upon whether or not existing transmissions are protected by antenna beamforming. Simulation results are presented which show that the proposed protocol can achieve higher capacity than previous comparable MAC protocols. The results also include the effects of transmit power control on system performance.