Non orthogonal multiple access (NOMA) is expected to be the most significant and viable solution for fifth generation (5G) communication system due to its incredible advantage of spectral efficiency. This paper proposes joint channel allocation and power optimization for downlink NOMA system, so as to maximize the system throughput while preserving the minimum data rate of every user equipment necessary to maintain quality of service and successive interference cancellation constraint. The optimization problem so formulated is a joint mixed integer nonlinear programming problem. Hence, two stage solution is proposed that uses mosek solver for channel assignment followed by power allocation across assigned channels. We use the difference between two convex functions (DC) programming technique for modifying the nonconvex optimization problem into a convex subproblem. Thus, we achieve efficient and fair power allocation coefficients across channels by solving the convex subproblem iteratively. Simulation results demonstrate better efficiency of our proposed scheme for resource allocation compared to some of the existing baseline schemes.