Wireless Network Design with Directional Antennas

The capacity of wireless networks is limited by mutual interference between transmissions by the different nodes. Deploying antennas that have a directional rather than omni-directional radiation pattern at nodes in the network permits the interference patterns in the network to be modified, and thus offers the potential for increasing capacity. However, to determine the effectiveness of directional antennas in achieving increased capacity, one has to account properly for all the possible effects of directional transmission and reception. For example, for the same average power (averaged over all directions) as in an omni-directional antenna, a directional antenna delivers a larger power within its beamwidth, and is thus capable of a larger transmission-range than an omni-directional antenna. The larger transmission range can lead to higher connectivity and thus to routes with fewer hops, but could also amplify the interference `footprint' of the transmission, leaving the net effect on capacity in doubt. If the same transmission range is maintained by reducing the transmitting power, a narrow beamwidth of directional transmission leads to a smaller `interference footprint' in comparison with omni-directional transmission, allowing a larger number of simultaneous transmissions to occur. However, a narrow beamwidth can also reduce network connectivity and force the use of longer routes.