This letter studies the multiple-input single-output (MISO) non-orthogonal multiple-access (NOMA) downlink using regularized zero-forcing (RZF) precoding with imperfect channel state information (CSI). We first propose a new user scheduling scheme based on imperfect CSI and a model to characterize the channel correlation between the weak and strong users. Then we derive an approximate expression of the ergodic sum-rate using large-system random matrix theory. This approximation permits us to derive the optimal power allocation scheme that satisfies the rate requirement of the weak users. Simulation results are presented to confirm the accuracy of the approximation and reveal the relationships between the ergodic sum-rate, the channel correlation, and other system parameters. Index Terms-NOMA, imperfect CSI, regularized zero-forcing precoding, user clustering. I. INTRODUCTION Non-orthogonal multiple-access (NOMA) is regarded as the radio access technique to address the massive connectivity problem for Internet of Things (IoT) [1, 2]. With successive interference cancellation (SIC) at the receiver and superposition coding at the transmitter [3], NOMA has the capability to deliver greater performance, such as higher user fairness and improved spectrum efficiency, when compared to orthogonal multiple access (OMA) methods. User clustering, scheduling, power allocation, and multiuser beamforming are among the methods that can further improve the sum-rate and energy efficiency of the multiple-input singleoutput (MISO)-NOMA downlink [4, 5]. To reduce users' intercluster interferences, [4] applied zero forcing (ZF) precoding and clustering to MISO-NOMA. In [5], user clustering,