Outage Performance Analysis and Optimization of Two-User Downlink NOMA Systems With Imperfect SIC and Improper Signaling

Non-orthogonal multiple access (NOMA) is a promising technology to increase system throughput and accommodate massive connectivity via non-orthogonal resource allocation. This paper studies the benefits of the improper Gaussian signaling (IGS) technique on the two-user downlink NOMA system with imperfect successive interference cancellation (SIC) over Rayleigh fading channels by analyzing the outage probabilities of both the strong user (SU) and the weak user (WU). The SU is assumed to use proper Gaussian signaling (PGS), while the WU employs IGS. Only statistical channel state information (CSI) is available at the base station, which is more practical than perfect knowledge of the instantaneous CSI. Both users' outage probabilities are formulated in terms of the impropriety degree of the WU's IGS, taking into account the residual interference introduced by the imperfect SIC. The optimization problem is formulated to minimize the outage probability of one user by adjusting the transmitted power and the impropriety degree of the WU while maintaining a certain quality-of-service (QoS) of the other user. Numerical results on the NOMA system verify that IGS provides an opportunity to further improve the outage performance over the conventional PGS.