Carrier-sense protocols for packet-switched smart antenna basestations

Smart antennas can be used to improve the performance of various wireless communication systems. Recently researchers have considered their use in wireless packet-switched networks 1, 2, 3, 4, 5]. In 6], a single-beam system was described which employs a smart antenna basestation operating in carrier-sense (CSMA) mode. Performance improvements can be obtained since the antenna can dynamically point pattern nulls in the direction of interfering stations. This reduces the chance of channel collisions amongst competing and hidden stations. In this paper we consider the reverse link throughput and capacity achieved by a set of portable stations sharing a single indoor radio channel. The stations communicate to a basesta-tion which is equipped with a smart antenna operating in multibeam SDMA mode. To accommodate this, the basestation contains several beamforming modules, each of which is capable of directing a beam at a desired packet transmission. A basic multibeam CSMA protocol is rst described for this type of system. Stations access the channel using a form of CSMA which has been adapted to accommodate the smart antenna basestation. Wireless networks often have very poor inter-station connectivity due to channel fading and shadowing eeects. For this reason, successfully initiated packet transmissions may be corrupted by hidden station transmissions 6]. In this paper we also present a multibeam CSMA/SDMA protocol which incorporates basestation/portable signalling to reduce this eeect. The protocol is motivated by the RTS/CTS packet exchanges used in IEEE 802.11 for this purpose 7]. In the paper we present throughput and capacity models for the systems being considered.