  Dynamic spectrum access under channel uncertainties is considered. With the
goal of maximizing the secondary user (SU) throughput subject to constraints on
the primary user (PU) outage probability we formulate a joint problem of
spectrum sensing and channel state estimation. The problem is cast into a
sequential framework since sensing time minimization is crucial for throughput
maximization. In the optimum solution, the sensing decision rule is coupled
with the channel estimator, making the separate treatment of the sensing and
channel estimation strictly suboptimal. Using such a joint structure for
spectrum sensing and channel estimation we propose a distributed (cooperative)
dynamic spectrum access scheme under statistical channel state information
(CSI). In the proposed scheme, the SUs report their sufficient statistics to a
fusion center (FC) via level-triggered sampling, a nonuniform sampling
technique that is known to be bandwidth-and-energy efficient. Then, the FC
makes a sequential spectrum sensing decision using local statistics and channel
estimates, and selects the SU with the best transmission opportunity. The
selected SU, using the sensing decision and its channel estimates, computes the
transmit power and starts data transmission. Simulation results demonstrate
that the proposed scheme significantly outperforms its conventional
counterparts, under the same PU outage constraints, in terms of the achievable
SU throughput.
