An Optimal Finite Alphabet NOMA Scheme for Two-User Uplink Channels

In this paper, we investigate the design of an optimal non-orthogonal multiple access (NOMA) transmission scheme with finite alphabet inputs for a typical two-user uplink wireless communication system, in which channel state information are available at both transmitters and receivers, and each terminal equips a single antenna. For such a system, two users utilize four quadrature amplitude modulation constellations to transmit their data information to a common base station and the receiver employs an optimal maximum likelihood detector for jointly estimating the two users’ transmitted signals. We consider the design of a pair of scalar beamformers for these two users such that the minimum Euclidean distance of the received sum-constellation is maximized subject to each individual user power constraint. A closed-form optimal solution and the corresponding optimal sum constellation are attained. Numerical simulations verify that when signal to noise ratio is slightly high, our proposed NOMA scheme outperforms other existing NOMA designs for the same system.