Error Analysis of NOMA-Based VLC Systems with Higher Order Modulation Schemes

Nonorthogonal multiple access (NOMA) is a technique with high spectral efficiency that is expected to be applied in future wireless networks. However, its high spectral efficiency comes at the expense of increased error in data detection. This paper presents theoretical error analysis of a downlink power-domain NOMA-based visible light communication (VLC) system with higher order modulation schemes in which the scenario of imperfect successive interference cancellation (SIC) is considered. Exact closed-form expressions for the symbol error rates (SERs) of the users are derived when the modulation is the square quadrature amplitude modulation (QAM). The derived expressions are applicable to any modulation order of each user. In addition, a necessary and sufficient power allocation (PA) constraint is provided for NOMA with higher order modulation to ensure that the decision regions of the symbols in the superimposed constellation do not intersect or overlap. The simulation results support the theoretical analysis and the accuracy of the derived expressions. The results demonstrate that the user SER in NOMA-based systems can be minimized with a suitable PA that depends on the modulation orders of the users.