  The Internet of Things (IoT) framework requires a massive number of
connection thus demanding spectral efficient solutions such as Non-Orthogonal
Multiple Access (NOMA). However, the main drawback of NOMA with successive
interference canceler (SIC)-based detectors is the error floor in the uplink.
In this paper, a reliable multi-user detection in uplink IoT NOMA is guaranteed
by a Joint Maximum-Likelihood (JML) detector (i.e., optimum detection
algorithm). We derive a closed-form upper bound of bit error rate (BER) of JML
over Rayleigh fading channels for arbitrary number of IoT devices and an
adaptive M-ary phase shift keying (M-PSK). Based on the extensive simulations,
the derived expressions are validated and it is revealed that the JML improves
the error performance in uplink NOMA and removes the error floor. Furthermore,
regardless of the number of the IoT devices and modulation order, a full
diversity order (i.e., number of receiving antennas) is guaranteed for each
device.
