This paper investigates the energy-efficient transmission under hybrid spectrum sharing scenario, where secondary users (SUs) can select a proper spectrum sharing method based on the state of primary users (PUs). Firstly, an optimization model to evaluate the energy-efficiency, as measured by the "throughput per Joule" metric, is proposed. Then, we show that there exists a unique globally optimal transmission power scheme for SU to achieve the maximum energy-efficiency by decomposing the optimization problem into two sub-problems. It's usually difficult to directly solve the optimal problem which is equivalent to solve joint nonlinear equations. So we propose a one-dimension low-complexity search algorithm, considering the unimodal characteristic of energy-efficiency function. Our simulation results show that the proposed transmission scheme can greatly improve energy savings with energy-efficiency maximization objective.
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