Carrier-Sense Protocols for Packet-Switched Smart Antenna Basestations

Abstract

Studies have considered the possible performance improvements when smart antennas are used in packet-switched data networks [1, 2, 3]. This work has included systems which operate using various ALOHA, polling, and reservation-based protocols. Recently however, a single-beam system was described which uses a smart antenna basestation to communicate with a set of stations using a carrier sense multiple access (CSMA) protocol [4]. In this system, performance improvements are obtained by having the antenna dynamically point pattern nulls in the direction of interfering stations, thus reducing the effects of channel collisions.

In this paper, we consider the performance of CSMA systems where stations access a smart antenna basestation using multibeam SDMA. As in other SDMA networks, the objective is for the basestation to transmit or receive multiple packets simultaneously. A basic CSMA/SDMA protocol is proposed for this purpose. Note that unlike conventional systems, the CSMA objective of isolating a single successful transmission is not desirable. Instead, our protocol uses carrier-sensing to synchronize various smart antenna operations. In this paper we also present a more sophisticated CSMA/SDMA protocol which incorporates novel basestation/portable signalling which mitigates the effects of hidden stations. The proposed mechanism takes into account the transient connectivity of such systems using the coherence time of the channel as an operating parameter. The performance of these systems is characterized and compared using analytical throughput/capacity models and mean delay simulations. It is shown that when hidden station effects are present, the capacity performance of the more sophisticated protocol may be much higher than that of the basic version.