Energy Efficient Joint User Association and Power Allocation Design in Massive MIMO Empowered Dense HetNets

When massive MIMO technology is combined with dense heterogeneous networks (HetNets), the user association and power allocation problems are fundamentally different although the energy- efficiency benefits can be intensified. This paper aims to investigate the energy efficient joint user association and power allocation problem in downlink massive MIMO empowered dense HetNets under proportional fairness criterion. The joint optimization problem is a non-convex mixed-integer nonlinear program (MINLP) which is NP-hard, and hence it is difficult to efficiently obtain exact solution. In order to obtain the highquality suboptimal solution, the joint optimization problem is first decomposed into two subproblems with alternating iterative method. Then a two-layer iterative suboptimal algorithm is proposed to solve the joint optimization problem with guaranteed convergence. The involved association subproblem adopts dual decomposition to achieve the optimal association index, whilst the power allocation subproblem allocates the transmit power of each BS with Newton's method. Numerical results verify the effectiveness of our proposed algorithm and show that our proposed algorithm outperforms conventional association schemes in the enhancement of energy efficiency performance. Furthermore, it can be seen that the energy efficiency performance is enhanced by increasing the number of antennas at macro base station (MBS).