I. Introduction
The ever-increasing demand of high system capacity, reliable quality of service (QoS), and low processing latency has been calling for new techniques in wireless communications, especially in the fifth-generation and beyond, to support massive connectivity with stringent QoS requirements [1]. Among existing promising candidates for performance enhancement, multiple-input multiple-output (MIMO) with precoding techniques has been widely used to provide diversity and/or multiplexing gains via spatial degrees of freedom [2]. In addition, the achievable rate region can be improved by power-domain non-orthogonal multiple access (NOMA), which allows multiple users to transmit data simultaneously in the same frequency band at the same time. With the use of superposition coding (SC) and successive interference cancellation (SIC), NOMA has superior spectral efficiency over the conventional orthogonal multiple access [3].