Modeling Smart Antennas in Synchronous Ad Hoc Networks
Using OPNET's Pipeline Stages
October 2005
John A. Stine, The MITRE Corporation
ABSTRACT
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.
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