Energy-efficient spectrum access in Cognitive Radio

Cognitive radio (CR) and energy-efficient design have emerged as two promising technologies to achieve high spectrum efficiency (SE) and energy efficiency (EE), respectively. In this paper, we study energy-efficient opportunistic spectrum access strategies for multiple secondary users (SUs) in an orthogonal frequency division multiplexing (OFDM)-based CR network. The worst EE is considered and optimized to achieve the max-min fairness. Since the original optimization issue belongs to nonconvex integer combinatorial fractional program and are essentially NP-hard for optimal solution, we use continuous and convex relaxation to modify the problem for somewhat better mathematical tractability. For the modified problem, we first develop a framework to find the optimal solution based on efficient root finding and convex optimization. We also develop a low-complexity alternative for suboptimal solution to further reduce complexity. Simulation results show that the energy-efficient spectrum access strategies significantly boost EE compared with the conventional spectral-efficient spectrum access ones. And the low-complexity suboptimal approaches can well balance the performance and complexity.