  In uplink non-orthogonal multiple access (NOMA) networks, the order of
decoding at the base station (BS) depends on the received instantaneous signal
powers (ISPs). Therefore, the signal-to-interference-plus-noise ratio (SINR)
coverage probability for an uplink NOMA user should be calculated using the
ISP-based ranking of the users. In the existing literature, however, mean
signal power (MSP)-based ranking is used to determine the decoding order and
calculate the SINR coverage probability. Although this approximation provides
tractable results, it is not accurate. In this letter, we derive the coverage
probability for ISP-based ranking, and we show that MSP-based ranking
underestimates the SINR coverage probability.
