  In a cognitive radio scenario we consider a single secondary user (SU)
accessing a multi-channel system. The SU senses the channels sequentially to
detect if a primary user (PU) is occupying the channels, and stops its search
to access a channel if it offers a significantly high throughput. The optimal
stopping rule and power control problem is considered. The problem is
formulated as a SU's throughput-maximization problem under a power,
interference and packet delay constraints. We first show the effect of the
optimal stopping rule on the packet delay, then solve this optimization problem
for both the overlay system where the SU transmits only at the spectrum holes
as well as the underlay system where tolerable interference (or tolerable
collision probability) is allowed. We provide closed-form expressions for the
optimal stopping rule, and show that the optimal power control strategy for
this multi-channel problem is a modified water-filling approach. We extend the
work to multiple SU scenario and show that when the number of SUs is large the
complexity of the solution becomes smaller than that of the single SU case. We
discuss the application of this problem in typical networks where packets
arrive simultaneously and have the same departure deadline. We further propose
an online adaptation policy to the optimal stopping rule that meets the
packets' hard-deadline constraint and, at the same time, gives higher
throughput than the offline policy.
