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Energy efficiency is crucial in wireless communication systems, especially in cognitive radio (CR) systems in which the exclusive functionality of spectrum sensing inevitably incurs additional energy consumption. In this paper, we study energy-efficient link adaptation for the secondary users (SUs) with heterogeneous quality of service (QoS) requirements in an interference-limited CR system. Two classes of SUs with different QoS are considered: delay-sensitive SUs (DS-SUs) and delay-tolerant SUs (DT-SUs). We focus on energy efficiency (EE) maximization taking into account the SUs' heterogeneous QoS and PU interference constraint. The problem of EE maximization is formulated as a nonlinear fractional programming problem, which is transformed into an equivalent parametric programming problem. Moreover, optimal solution to joint subcarrier assignment and power allocation is derived with the bisection method and dual decomposition method (DDM) in convex optimization theory. Simulation results illustrate the significant performance improvement of our scheme over an existing one which aims at maximizing system throughput rather than EE.

Energy-Efficient Link Adaptation for Cognitive Radios with Heterogeneous QoS requirements