BER Performance of Downlink MIMO-NOMA with Combination of SFBC and Joint Maximum-Likelihood Detector

Reducing the influence of multipath channels and simplifying the equalizer at receivers are the main benefits of orthogonal frequency division multiplexing (OFDM). In this paper, we take advantage of OFDM to implement downlink multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) systems under the view of space-frequency block code (SFBC). In particular, we implement a combination of SFBC at a base station and a joint maximum-likelihood (JML) detector at two users to improve the bit-error-rate (BER) performance. For evaluating the system's performance, we mathematically obtain closed-form approximate expressions of the BER performance and the maximum diversity gain for two users on Rayleigh fading channels under the assumption of a small number of channel taps. Through computer simulations, we validate that analytical results match quite well with simulation results and asymptotic results in the low and high signal-to-noise ratio (SNR) regimes, respectively. Based on expressions of BER, the fairness problem of the system is considered, and it shows that the power allocation for achieving justice becomes inconsequential when both the transmit power and the number of receive antennas are sufficiently large.