Power and Discrete Rate Adaptation in Wideband NOMA in Frequency-Selective Channels: A Systematic Approach

Non-orthogonal multiple access (NOMA) superimposes signals of multiple users and transmits them simultaneously. To be implementable in 5G and beyond cellular systems, it must adhere to the constraint imposed by the standard that the same modulation and coding scheme (MCS) and power must be used across all physical resource blocks (PRBs) assigned to the users. However, the channel gains of different PRBs are different in wideband channels and the MCSs must belong to a discrete, pre-specified set. We propose a method that uses the exponential effective signal-to-noise ratio mapping (EESM) to systematically determine whether a feasible power allocation exists for a given choice of MCSs, and to find the MCSs that maximize the weighted sum rate. We then propose a novel, lower complexity method called power-normalized EESM, which leads to explicit analytical criteria for the existence of a feasible power allocation. We prove that this method is a relaxation of the original problem under various conditions and is exact for narrowband channels. Wideband NOMA achieves a higher average weighted sum rate than orthogonal multiple access, which is employed by 5G.