La tecnología de acceso múltiple no ortogonal (NOMA) se ha convertido en la clave de las futuras redes inalámbricas 5G. Puede lograr la multiplexación de múltiples usuarios en el dominio de potencia de transmisión mediante la asignación de diferentes potencias, lo que puede mejorar efectivamente la capacidad del sistema y la eficiencia espectral. Aiming at the problem of high computational complexity and improving system capacity in non-orthogonal multiple access (NOMA) based on orthogonal frequency division multiple access (OFDMA) for 5G wireless cellular networks, this paper proposes an improved low complexity radio resource allocation algorithm for user grouping and power allocation optimization. The optimization model is established with the goal of maximizing system capacity. Through the step-by-step optimization idea, the complex non-convex optimization problem is decomposed into two sub-problems to be solved separately. Firstly, all users are grouped based on the greedy method, and then the power allocation is performed on the sub-carriers of the fixed group. Simulation results show that the proposed algorithm has better system capacity than the existing state-of-the-art algorithms and reduced complexity performance.