Energy-Efficient Relay Selection and Resource Allocation in Nonregenerative Relay OFDMA Systems

Discrete modulation in multiuser nonregenerative relay systems is less investigated in current resource allocation but is more commonly used in practical systems. In this paper, based on a set of discrete modulation levels, we formulate a joint optimization of relay selection and resource allocation under rate requirements to minimize the system transmitting power consumption and to improve energy efficiency. Since the problem cannot be easily solved due to its NP-hardness, we decompose it into two subproblems, namely, a simple relay selection method and a resource allocation method, and we develop a heuristic algorithm to find suboptimal solutions. The two-step solutions are based on power saving and virtual direct transmission, and they can reduce the dimensionality and can simplify the resource allocation of the relay networks. In addition, the power constraints at the relay stations (RSs) are considered in a way that the subcarriers on overloaded RSs are adjusted according to the joint optimization. Our results show that a satisfactory suboptimal solution close to the optimal one could be achieved, and the power constraints at RSs may have a significant impact on the total power consumption.