In this work, we investigate resource allocation and user pairing to improve the system’s throughput for the down-link non-orthogonal multiple access (NOMA) based 5G networks. The proposed resource allocation involves user pairing, subchannel power allocation, and proportional power allocation among the multiplexed users. The resource allocation is a nondeterministic polynomial (NP) NP-hard problem where it is difficult to tackle throughput maximisation. The user pairing and power allocation are coupled to address the substantial requirements of the NOMA system. The NOMA system requires an efficient deployment of resource allocation techniques to enhance the system’s throughput performance. In this work, we propose simulated annealing (SA) to optimise the power allocation and perform user pairing to maximise throughput for the NOMA system. Also, we provide mathematical proof on the near-optimal solution for subchannel power and mathematical analysis on the optimal value of the power ratio for the multiplexed users in NOMA system. The SA provides a significant throughput performance that increases by 7% compared to the existing numerical optimisation methods. Results obtained show that SA performs with sufficient reliability and low time complexity in terms of throughput improvement.