Adaptive Rate Splitting for Uplink Non-Orthogonal Multiple Access Systems

In this paper, a fixed rate splitting (FRS) scheme and an adaptive rate splitting (ARS) scheme are proposed for an uplink non-orthogonal multiple access (NOMA) system, in which a near user and a far user communicate with a basestation (BS) using power-domain NOMA. Considering back-off power control, information signal of near user is split into two data streams for uplink transmissions, whereas information signal of far user is transmitted without rate splitting. In the FRS scheme, a fixed power allocation factor is applied at near user for allocating transmit power between data streams. At the BS receiver, successive interference cancellation is applied for recovering all data streams. Motivated by cognitive radio principles, a dynamic power allocation factor is designed for the ARS scheme based on instantaneous channel state information. Closed-form expressions are derived for outage probabilities of the near and far users, respectively. In addition, analytical expressions are derived for asymptotic outage probabilities of the FRS and ARS schemes, respectively. The superior outage performance achieved by the FRS and ARS schemes are corroborated by system-level simulation results.