Energy-Efficient Joint User Association and Power Allocation in Relay-Aided Massive MIMO Systems

Energy efficiency is an important metric for downlink transmission in an amplify-and-forward relay-aided massive multiple-input multiple-output system,but has not been well investigated.In this work,considering the characteristics of such a system and quality-of-service requirements of users,the energy-efficient joint user asso-ciation and power allocation problem is studied.First,the closed-form expression of system energy efficiency under the proportional fairness criterion is derived.Then,the proportionally fair utility of system energy efficiency is maximized under constraints of minimum signal-to-noise ratio requirements of users and maximum transmit powers of the base station(BS)and relay stations.As it is difficult to solve this optimization problem directly due to its mixed-integer and non-convex features,the original problem is decomposed into a user association sub-problem and a power allocation sub-problem.For the former,optimum user association is determined by solving a Lagrangian dual problem with a sub-gradient algorithm;for the latter,optimum transmit powers of the BS and each relay station are determined by using Newton’s method.Finally,a sub-optimal solution of the original problem is obtained by a low-complexity iterative algorithm.Simulation results show that the proposed joint user association and power allocation algorithm can offload the traffic of the BS effectively,keep the BS and relay stations operate at low power levels,and improve the system energy efficiency significantly,compared with user association-only schemes.