Comparison of PD-NOMA Capacity with Conventional and Joint Gray-Mapping

The paper compares two bit-mapping approaches for non-orthogonal multiple access with channel separation in power domain in a system with two users. In the first approach, bit mapping to modulation symbols is implemented independently for each user, and in the second approach, joint Gray-mapping is implemented. This allows for increasing the bit error rate performance of the user channel with a “strong” propagation channel and extending the range of acceptable values for power allocation. For this paper, we have constructed capacity maps that show the total spectral efficiency of channels depending on their channel states. The system model uses signal-code constructions combining quadrature amplitude modulation and polar codes. It is shown that the joint bit mapping allows for greater spectral efficiency under identical conditions. We have performed a computer simulation to evaluate the average spectral efficiency that was calculated in a system with one base station and 10 users, observing the planning strategy of max-min fair. The results demonstrate that the joint Gray-mapping makes it possible to increase the average spectral efficiency of the system by 3.3% compared with the independent mapping.