Modeling Smart Antennas in Synchronous Ad Hoc Networks Using OPNET's Pipeline Stages

Smart antennas have been proposed as a physical layer device that can increase the capacity of ad hoc networks. The effectiveness of smart antennas depends on whether access mechanisms create the conditions that enable receivers to adapt to both desired signals and interfering signals and enable transmitters to discern where they must avoid causing interference. The ease of implementing solutions and modeling the antennas are both affected by whether the access schemes are asynchronous or synchronous. Asynchronous access mechanisms are more difficult since they allow new transmitters to begin transmissions during ongoing exchanges. Thus, past adaptation becomes irrelevant and current adaptation is done with insufficient information. Arbitrating the effects in simulation requires detailed models of antenna adaptation and the resulting power patterns. Synchronous access mechanisms, however, overcome these shortcomings because they force ongoing exchanges to conclude before new exchanges start and because they cause all new exchanges to occur simultaneously. Receivers can sample both the desired signals and the interfering signals to arrive at a weighting solution. Since conditions do not change after adaptation, the adaptation is more effective and simulation models can be more abstract. In this paper we describe how we built models of adaptive antennas in OPNET using a radio process model and the radio pipeline stages. We use this model in conjunction with our Synchronous Collision Resolution (SCR) medium access control protocol and evaluate the relative merits of different antenna technologies and capabilities. We found that those technologies that improve capture soonest in an exchange most improve the capacity.