MAC layer issues and challenges of using smart antennas with 802.11

Traditionally IEEE 802.11 systems have used omni-directional antennas. Omni-directional antennas are most suitable for the CSMA/CA based 802.11 MAC protocol. Recently increased market penetration of 802.11 is demanding higher range and higher capacity access points (AP). The use of smart antennas to enhance the coverage and capacity of 802.11 systems has become a compelling application. In an AP that uses smart antennas, each transmission is in the form of a beam, with the main lobe directed to a particular station (STA). Thus the coverage area is no longer omni-directional, which causes some undesirable side effects to the 802.11 MAC. The directional nature of transmissions and receptions and the interaction between them introduce certain characteristic problems. The problems such as the hidden terminal problem, the hidden beam problem and the 'ACK suicide' problem along with some other problems involved are described in this paper. The impact of the problems is quantified using simulations. Some possible solutions to the above problems are discussed.