In this paper we present a routing protocol (Modified HWMP, MHWMP) for IEEE 802.11s WLAN mesh networks with smart antennas that incorporates the optimum selection of the PHY-layer transmission/reception mode (multiplexing, beamforming and diversity). MHWMP adaptively selects between spatial multiplexing and beamforming for data transmission according to the wireless channel conditions. We also modified the traditional RTS/CTS mechanism to take into account the different behaviors of both transmission modes at MAC layer: the advantages of diversity are harvested by sending RTS/CTS frames with space time block coding in case of beamforming while standard RTS/CTS frames are used in case of spatial multiplexing. Simulation results illustrate that MHWMP leads to significant better throughput and delay performance in certain situations. Moreover, it enables a high degree of robustness wrt. wireless link failures for stationary mesh networks.

Abstract. The performance of multi-hop wireless networks can be substantially improved by using smart antennas. However, the utilization of smart antennas is quite challenging as the benefit of the different transmission modes strongly depends on specific channel conditions. In this paper we present a novel path selection protocol PSPSA (path selection protocol for smart antennas) and a modified MAC protocol that exploits the benefits of smart antennas. PSPSA selects an optimum path considering the specific ranges and transmission rates implied by the different transmission schemes (spatial multiplexing and beamforming). The MAC protocol is a modification of the traditional DCF scheme to cope with the different PHY layer transmission schemes. Our solution enables mesh nodes to dynamically select their transmission scheme on a per packet basis according to the current channel conditions. Simulation results show that the overall packet success rate (throughput) of multi-hop wireless networks are substantially improved. IntroductionIn the last decade multi-hop wireless networks have captured a great importance due to their diverse applications. However the multi-hop structure and the uncertainty of the wireless channel are critical performance limiting factors. The amendments at PHY layer proposed in IEEE 802.11n [1] could increase the efficiency of multi-hop networks but the incorporation of smart antenna features is quite challenging due to the mutual influence of the transmission scheme and the MAC mechanism. Furthermore the choice of the transmission scheme strongly depends on the current network topology and channel conditions, see [4]. For example, beamforming enables directional transmission with extended range thus being suitable for sparse network topologies whereas spatial multiplexing enables high bit rate omni-directional transmission with lower range thus exploiting advantages in dense networks. Thus, a transmitting node might also influence a different number of neighboring nodes depending on whether beamforming or spatial multiplexing is used. Hence it is advisable that path selection protocol, MAC mechanisms and PHY layer transmission techniques are operating in a well aligned manner.

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