In energy-limited communication systems

energy efficiency is a key factor that affects the performance of the systems. In this paper

we consider a wireless powered hybrid non-orthogonal multiple access network consisting of a base station and multiple clustered users. In this network

the base station transfers energy to the users in a wireless manner

and the users utilize the harvested energy to transmit information to the base station. To simplify the complexity of the base station’s information receiver

a hybrid multiple access scheme is adopted by the users

which are partitioned into multiple clusters. The users in the same cluster transmit in the non-orthogonal multiple access manner

while the users from different clusters transmit in the time division multiple access manner. To maximize the energy efficiency of the network

we jointly optimize the time length of wireless energy transfer from the base station to the users and the time length of information transmission from the users to the base station

as well as the transmit powers of the users. The formulated problem is non-convex and thus is difficult to solve. To tackle this problem

we first find the structure of its optimal solution

and then propose an efficient iterative algorithm to solve it based on the fractional programming technique. Simulation results show that the energy efficiency performance of the proposed algorithm significantly outperforms that of two benchmark schemes

namely the throughput maximization scheme and the fixed time allocation scheme.