On the Number of Admissible Users in NOMA

Non-orthogonal multiple access (NOMA) attracts much attention from industry and academia as it promises more connections. One interesting yet important question is, "What is the number of maximally admissible users in NOMA?" In this letter, a lower bound of admissible user equipment (UE) number is derived, which relates to the spreading factor, the required signal to interference and noise ratio, and the noise power only. Also, the overloading factor, i.e., the normalized admissible UE number, can be presented as 1 + Delta, where Delta denotes the capability of interference tolerance in NOMA. Further, with considering Rayleigh-distributed fading channels, stochastic analysis is used to get a clean expression of the probability of accessing the network successfully in NOMA. Numerical results show the current demodulation reference signal pattern in 5G needs to be enhanced and a lower modulation and coding scheme is more suitable to achieve a higher overloading.