Low-complexity approaches to spectrum opportunity tracking

We consider opportunistic spectrum access under design constraints imposed at both node and link levels. First, hardware and energy limitations at node level may prevent a secondary user from sensing all the channels in the spectrum simultaneously. A channel selection strategy is thus necessary to track the time-varying spectrum opportunities. Second, sensing errors are inevitable. A secondary user needs to decide, based on imperfect sensing outcomes, whether to access the sensed channel and how to update its statistical knowledge of spectrum dynamics for better tracking in the future. Third, a secondary transmitter and its intended receiver need to hop synchronously in the spectrum in order to communicate. When a dynamic opportunity tracking strategy is used where the channel selection depends on the sensing history, achieving this synchrony is nontrivial in the absence of a dedicated control channel and in the presence of sensing errors. These practical constraints significantly complicate the design of opportunistic spectrum access, and the optimal performance requires the joint design of the spectrum sensor, opportunity tracking strategy, and spectrum access decisions. The focus of this paper is on developing low-complexity approaches for opportunistic spectrum access. We show that under certain conditions on the spectrum dynamics, simple myopic strategies can provide optimal performance for the joint design of spectrum sensor, opportunity tracking, and opportunity exploitation. We also propose an alternate low-complexity indexing strategy for other conditions that takes into account the expected time to channel availability.