Energy-Efficient Delay-Constrained Transmission and Sensing for Cognitive Radio Systems

In this paper, we study energy-efficient transmission for Cognitive Radio (CR) that opportunistically operates on the primary user's channel through spectrum sensing. Spectrum sensing and compulsory idling (for incumbent protection) introduce energy overheads for Secondary User (SU) operations, and thus, an appropriate balance between energy consumption in data transmission and energy overheads is required. We formulate this problem as a discrete-time Markov decision process in which the SU aims at minimizing its average cost (including both energy consumption and delay cost) to finish a target traffic payload through an appropriate rate allocation. Based on certainty equivalent control, we propose a low-complexity rate-adaptation policy that achieves comparable performance with the optimal policy. With the low-complexity policy, we quantify the impact of energy overheads (including the power consumption for spectrum sensing and compulsory idling) on the SU transmission strategy. Specifically, the SU rate increases with the increase of energy overheads, whose marginal impact, however, diminishes. Moreover, the marginal impact of energy overheads is more significant for delay-insensitive traffic compared with that for delay-sensitive traffic. To mitigate the loss due to imperfect spectrum sensing, we quantify that the SU decreases (increases) its rate with a larger misdetection probability (false alarm probability).