  In this paper, we examine a cognitive spectrum access scheme in which
secondary users exploit the primary feedback information. We consider an
overlay secondary network employing a random access scheme in which secondary
users access the channel by certain access probabilities that are function of
the spectrum sensing metric. In setting our problem, we assume that secondary
users can eavesdrop on the primary link's feedback. We study the cognitive
radio network from a queuing theory point of view. Access probabilities are
determined by solving a secondary throughput maximization problem subject to a
constraint on the primary queues' stability. First, we formulate our problem
which is found to be non-convex. Yet, we solve it efficiently by exploiting the
structure of the secondary throughput equation. Our scheme yields improved
results in, both, the secondary user throughput and the primary user packet
delay. In addition, it comes very close to the optimal genie-aided scheme in
which secondary users act upon the presumed perfect knowledge of the primary
user's activity.
