Spectral efficiency (SE) enhancement of NOMA system through iterative power assignment

Abstract

This paper studies enhancing non-orthogonal multiple access (NOMA) spectral efficiency (SE) through optimizing the assigned power to each NOMA user depending on their channel states. This would improves the overall NOMA system performance in an adaptive manner in accordance with the channel state variations. An optimization problem for SE maximization is presented to optimize the performance of NOMA system. To solve the formulated problem, three power assignment schemes are proposed; firstly, a constant weight based water- filling approach is proposed, where it assigns the power to all subcarriers per each NOMA user iteratively. Secondly, two power assignment schemes are proposed, namely; high weight and low weight based power assignment (PA) schemes. Where they depend on constant weights to assign the power. In particular, the high weight based PA scheme depends on a weighting factor that uses the user index within the decoding order to fractions a high portion of the available total transmission power and allocate it to the respective user. On the other hand, the low weight based PA works based on a weighting factor that fractions low amount of the total transmission power. Both of these techniques take into account each user rank index within the decoding order with respect to its counterpart, and they also take into consideration the total number of active users. The results obtained from the simulations reveal that the proposed PA techniques maintains very good SE performances that are close to the numerical solution. It is worth mentioning that the numerical solution is obtained based on genetic algorithm. Nevertheless, the simulations results also confirm that the proposed approaches surpass the fractional transmission power approach simulated from literature works.