Due to the problem of spectrum underutilization and energy inefficiency in wireless communications,the research on energy efficient Cognitive Radio Networks (CRNs) has received significant attention in both industry and academia.In this paper,we consider the problem of optimal spectrum selection and transmission parameters design with the objective of minimizing energy consumption in CRNs.Since the system state cannot be directly observed due to miss detections and estimation errors,we formulate the optimal spectrum access problem as a Partially Observable Markov Decision Process (POMDP).In particular,the proposed scheme selects the optimal spectrum,modulation and coding scheme,transmission power,and link layer frame size in each time slot according to the belief state,which captures all the history information of past actions and observations.The optimal policy can be acquired by solving POMDP problem with linear programming based algorithm.Simulation results show that significant energy savings can be achieved by the proposed scheme.