Directional MAC Protocols
Various MAC protocols using smart antennas or directional antennas, typically referred to as directional MAC protocols, have been proposed for ad hoc networks.
Ko et al. (2000) propose DMAC (Directional MAC) in which all frames are transmitted directionally except for the CTS (Clear To Send). Choudhury et al. (2002) propose MMAC (Multi-hop RTS MAC), which involves the multi-hop RTS (Request To Send) to take advantage of the higher gain obtained by directional antennas. These protocols, however, need various additional mechanisms to provide the location information and to forward the RTS.
In (Fahmy, 2002; Nasipuri, 2002) and (Takai, 2002), RTS is transmitted omni-directionally in order to find the receiver in case location information is not available. Each node estimates the direction of neighboring nodes for pointing the beam with AOA (Angle of Arrival) when it hears any signal. Because these protocols employ at least one omni-directional transmission, it limits the coverage area provided by directional transmissions and do not exploit one of the main benefits of directional antennas, i.e., the increase of the transmission range, either.
Ramanathan (2001) proposes circular directional transmission of periodic hello packets to obtain node information that is located farther away than the omni-directional transmission range. Korakis et al. (2003) proposes circular RTS, which scans all the area around the transmitter to find the addressed receiver and to tackle the deafness and the hidden-terminal problem arisen from directional transmissions. Bandyopadhyay et al. (2001) develops additional frames in order to determine the neighbor topology by recording the angle and signal strength under consideration of propagation conditions. Although these schemes attempt communication range extension, circular transmission increases the delay and incurs large control overhead.
In mobile ad hoc evaluation, Ramanathan (2005) shows a directional MAC testbed and simulation results, but not including actual experimental results. Although Nishida (2005) evaluated AODV (Ad hoc On demand Distance Vector routing) protocol based on IEEE802.11b DCF with omni-directional antennas in the experiment, not used directional antennas.