  This work provides a generalized performance analysis for the multi-user
uplink-NOMA system with adaptive square quadrature amplitude modulation (QAM)
over Rayleigh fading channels. Motivated by the massive IoT connections and
unavailability of orthogonal resources for each node, we consider a
multi-access scheme where multi-users with single-antenna transmit data to a
multiple-antenna base station through the same resource block. By taking
advantage of combining diversity paths with the proposed joint
maximum-likelihood detector (JMLD), a closed form expression for the upper
bound of bit error rate (BER) is obtained. Despite the number of users or the
order of modulation, the analytical results endorsed via computer simulations
reveal the ability of the MRC-JMLD detector to discard the error floor
completely. Moreover, the simulation results show that the MRC-JMLD surpasses
its counterparts significantly and ensures a full diversity order.
