Opportunistic Spectrum Access for Energy-Constrained Cognitive Radios

We consider a scenario in which a secondary user makes opportunistic use of a channel allocated to a primary network. The primary network operates in a time-slotted manner and switches between idle and active states according to a stationary Markovian process. At the beginning of each time slot, the secondary user can choose to stay idle or to carry out spectrum sensing to check if the primary network is idle or active. If the primary network is detected as idle, the secondary user can carry out data transmission. Spectrum sensing consumes energy and introduces false alarms and mis-detections. Given the delay cost associated with staying idle, the energy cost associated with spectrum sensing and data transmission, and the throughput gain associated with successful transmissions, the objective is to find an optimal sequence of idle/sensing actions, together with the optimal spectrum sensing durations, to maximize the expected net reward for the secondary user. We formulate this problem as a partially observable Markov decision process (POMDP) and obtain optimal control policies. Heuristic control policies that can be obtained at low complexity are also proposed.