Virtual Channel Optimization Downlink NOMA with High-Order Modulations without CSIT

This paper presents a virtual channel optimization (VCO) method for downlink non-orthogonal multiple access (NOMA) systems on high-order modulations up to 16-QAM. With this VCO scheme, we present a new optimization criterion based on maximizing the minimum Euclidean distance among superposition constellation's points without channel state information at the transmitter (CSIT). Prior works on VCO scheme only focus on quadrature phase-shift keying (QPSK) modulations and the analytical solutions are obtained. However, the analytical solution is challenging to obtain for high-order modulations. We exploit the particle swarm optimization (PSO) algorithm to search the optimal solution. Simulation results for high-order modulations demonstrate the improvement of the overall throughput based on the merit of constellation constrained capacity (CCC).