Non-Orthogonal Multiple Access (NOMA) is one of the promising techniques to ensure very high spectral efficiency in 5G mobile communications and beyond.In contrast to the orthogonal multiple access (OMA) technique, the NOMA shows outstanding performances in terms of throughput, user fairness, low latency and compatibility with the current and future communication systems. In this paper, we analyze the capacity region in NOMA system and the limited number of multiplexed users under a given power allocation vector (i.e. symmetric/asymmetric channel). Moreover, we have investigated the effect of large constellation order on the power allocation and bit error rate (BER). Comparisons between the coded and uncoded schemes are also presented.