  Unlicensed secondary users (SUs) in cognitive radio networks are subject to
an inherent tradeoff between spectrum sensing and spectrum access. Although
each SU has an incentive to sense the primary user (PU) channels for locating
spectrum holes, this exploration of the spectrum can come at the expense of a
shorter transmission time, and, hence, a possibly smaller capacity for data
transmission. This paper investigates the impact of this tradeoff on the
cooperative strategies of a network of SUs that seek to cooperate in order to
improve their view of the spectrum (sensing), reduce the possibility of
interference among each other, and improve their transmission capacity
(access). The problem is modeled as a coalitional game in partition form and an
algorithm for coalition formation is proposed. Using the proposed algorithm,
the SUs can make individual distributed decisions to join or leave a coalition
while maximizing their utilities which capture the average time spent for
sensing as well as the capacity achieved while accessing the spectrum. It is
shown that, by using the proposed algorithm, the SUs can self-organize into a
network partition composed of disjoint coalitions, with the members of each
coalition cooperating to jointly optimize their sensing and access performance.
Simulation results show the performance improvement that the proposed algorithm
yields with respect to the non-cooperative case. The results also show how the
algorithm allows the SUs to self-adapt to changes in the environment such as
the change in the traffic of the PUs, or slow mobility.
