Somprakash Bandyopadhyay & Siuli Roy 
Enhancing the Performance of Ad Hoc Wireless Networks with Smart Antennas [PDF ebook] 

A large portion of the network capacity of an ad hoc network can be wasted by the medium access mechanisms of omni-directional antennas. To overcome this problem, researchers propose the use of directional or adaptive antennas that largely reduce radio interference, improving the utilization of wireless medium and the resulting network throughput.