Energy efficiency maximization by joint transmission scheduling and resource allocation in downlink NOMA cellular networks

In this paper, we study joint transmission scheduling, rate selection, sub-channel allocation and power assignment problem to maximize energy efficiency in downlink power-domain non-orthogonal multiple access (NOMA) cellular networks. For the study, a fractional optimization problem is formulated to determine the energy efficient optimal configurations of the parameters for transmission scheduling and transmit power, data rate and sub-channel allocation under a constraint of maximum to minimum throughput ratio of the users. To make the fractional optimization problem tractable, the optimization problem is reformulated as a linear programming (LP) problem. The LP problem is solved numerically for many instances of wireless cellular networks by using the optimization tool CPLEX. The numerical results show that the jointly optimal transmission scheduling and resource allocation in NOMA system provides significantly better performance than the existing resource allocation schemes in NOMA and orthogonal multiple access (OMA) systems. We find that the maximum number of users for energy efficient NOMA downlink transmission is limited by 2 and the majority of the downlink transmissions use OMA technique instead of NOMA technique. Again, we apply the duality theory in the LP problem, develop a sub-gradient algorithm and propose an iterative algorithm for solving the problem optimally. The key feature of the iterative algorithm is that it can be implemented in real time operation of a NOMA system. Via extensive simulation, it is demonstrated that the solution provided by the iterative algorithm is very close to the optimal solution. (C) 2019 The Author. Published by Elsevier B.V.