Joint OMA-NOMA cell-free massive MIMO with limited fronthaul

We consider a joint orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) cell-free massive multiple-input multiple-output (CFmMIMO) system with limited fronthaul capacity. In this system, some users (UEs) are grouped to be served by access points (APs) in NOMA mode, while other UEs are served in OMA mode. We formulate a mixed-integer nonconvex problem of optimizing the power control and AP-group association to maximize the sum spectral efficiency (SE) in the considered system. This problem is subject to minimum SE requirements of each UE, per-AP transmit power, and limited fronthaul capacity. We propose an algorithm based on the successive convex approximation (SCA) optimization technique to obtain a stationary-point solution for the formulated problem. Numerical results demonstrate that the proposed joint optimization approach increases significantly sum SE compared to other heuristic baseline schemes, especially under a tight fronthaul capacity limitation. Also, the joint OMA-NOMA CFmMIMO system provides remarkably higher 95%-likelihood sum SE compared to a CFmMIMO system using only the OMA scheme, especially up to 42% when the coherence interval is short.