| Directional antennas are known to reduce the interference and to increase the range and the capacity for wireless networks due to the increased gain and spatial spectrum reuse. However, in IEEE 802.11 based wireless LANs, directional antennas result in higher network capacity but with an increase in the occurrence of collisions. The original 802.11 protocols were designed assuming omni-directional antennas. Directional antennas cause two new problems: the enhanced hidden node problem and the deafness problem. These are caused by an inconsistent view of the channel status by neighboring nodes resulting from the directional antennas. Other proposed 802.11 protocols using directional antennas such as DMAC do not completely solve these problems and are not fully compatible with standard 802.11 stations with omni-directional antennas.
We have developed an architecture and protocol for 802.11 radios consisting of multiple directional antennas and multiple receivers called Sectorized-MAC (S-MAC). S-MAC consists of two parts: 1) a self-interference cancellation scheme at the PHY layer, and 2) a MAC protocol for proper coordination of channel access. S-MAC uses the multiple directional antennas and multiple receivers to provide 360 degree coverage and extended range around a node and allows a node to be simultaneously transmitting in some sectors while receiving in others using the self-interference cancellation method. Thus, S-MAC prevents the enhanced hidden node problem and the deafness problem by having a consistent view of channel status. In this paper, the S-MAC protocol and architecture are described and methods for handling user mobility are discussed. Despite differences in the internal S-MAC architecture, the standard 802.11 air-interface is not changed. Consequently the new scheme is compatible with the normal 802.11 protocol and can coexist in networks with a mix of normal and S-MAC-based nodes. The performance of S-MAC is studied via simulations and compared with the original 802.11 protocol using omni-directional antennas. Our studies show that S-MAC achieves significant capacity gains in both infrastructure and ad hoc modes, even if only used in parts of the network.
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| Tamer Nadeem is a research scientist at Siemens Corporate Research. He
received a Ph.D. degree from the department of Computer Science,
University of Maryland, College Park in 2006. He was a member of the Maryland
Information and Network Dynamics Laboratory (MIND) at the University of
Maryland. He has interned at Hughes Networks Systems, IBM T.J. Watson
Research Center, and Fujitsu Laboratories of America at College Park. His research
interests include wireless communications, mobile ad hoc networks,
peer-to-peer systems, pervasive computing, vehicular networks, cross layer
designs, management of wireless networks, and location determination
systems. His research resulted in several peer-reviewed publications in
several top journals and conferences in computer networks and systems. He
is a member of IEEE and ACM, an elected member for the honor societies Phi
Kappa Phi and Sigma Xi, and a member of Association of Egyptian American
Scholars. |