multiple access (NOMA) systems with quadrature phase shift keying (QPSK) modulation in
fading channels. In particular, we exploit the joint maximum-likelihood (ML) detector at a
base station (BS) with multiple antennas since it results in the optimal BER performance of a
super-imposed QPSK modulated symbols from multiple users. In particular, we obtain a
closed-form integral of Q-function with Erlang distributed random variable to derive the BER …