It is shown that the throughput capacity of wireless ad hoc networks using omni-directional antennas (OMN networks) is significantly decreased with the increased number of nodes. One major reason lies in the interference caused by using omni-directional antennas, which just broadcast radio signal in all directions. Thus, a communication with multipleshort-rangedhops is suggested in such networks to avoid interference and improve the throughput. However, the multi-hop transmission can also significantly increase the end-to-end delay.

In this paper, we investigate the throughput improvement and the delay reduction by using directional antennas in wireless ad hoc networks. We call such wireless ad hoc networks using directional antennas as DIR networks. In particular, we investigate the effective transmission range of directional antennas with consideration of various channel conditions, such as the large scale path loss and the shadow fading effect. We have found that directional antennas can significantly increase the transmission range compared with omni-directional antennas even under the same channel condition. Besides, we derive the throughput and the delay of a DIRnetwork by constructing a routing scheme and a time-division multi-access (TDMA) scheme. We have found that using directional antennas not only can increase the throughput capacity but also can decrease the delay by reducing the number of hops.