This paper investigates an energy-efficient transmission scheme in cognitive radio networks, where primary users (PUs) may reoccupy the spectrum when secondary users (SUs) is transmitting data. We aim to maximize the energy efficiency by jointly optimizing the transmission power, the fusion rule threshold and the sensing/transmission durations. Firstly, it is derived that for a given fusion rule threshold, the objective function is unimodal while only one optimization parameter varies. Furthermore, we provide the corresponding closed-form expressions of the optimal data transmission duration and transmission power. Finally, the globally unimodal property is proven, and hence, the globally optimal point can be easily found with the proposed algorithm based on the alternating direction method (ADM). Numerical simulation results show that our proposed scheme is much better than the existing ones. similar to [6], the optimization of sensing duration, with cooperative spectrum sensing based on k-out-of-N fusion rule, is considered to maximize energy efficiency by using golden section search, while transmission power is given a fixed valueHowever, all above research has not considered the optimization of the data transmission duration. If the transmission duration is too short, the throughput will be hard to improve, otherwise, it will result in more energy consumption for data transmission. Besides, the primary user's (PU's) activity is not investigated that transforms from OFF state to ON state during secondary user's (SU's) data transmission. Traditionally, the throughput of secondary system is simply measured without regard to the effective transmission duration, in this case where PUs randomly reoccupy the spectrum when SU transmits data. In addition, the longer transmission time may increase the risk of coming back of PU. It is noteworthy that the Cognitive radio (CR) is regarded as a promising technology significance of optimizing transmission time is unassailable to effectively resolve the contradiction between the low utiliza-in view of its contributions to the balance between throughput tion of the rare spectrum resource and increasing demanding and energy efciency in CRNs. Both work [9] and [10] take of wireless communication applications [1]. Recently, much PUs' state transformation into account. In [9], the maximum attention has been paid on energy efficiency in cognitive radio channel efficiency of SUs are designed by utilizing statisnetworks (CRNs) [2]-[7] and [9]-[11] which is vital to trade off tical information of the licensed channel occupancy. Based the capacity and energy consumption for the battery-powered on the PU's traffic pattern, collision-throughput problem is wireless devices.formulated to maximize the throughput in [10]. Work [9] and In our previous work [4] and [5], energy-efficiency mea-[10] have not concerned the energy and capacity tradeoff. sured with the "throughput per Joule" metric in CRNs are energy efficient scheme is investigated by jointly optimizing studied. Work [4] proposes ...