One of the most promising technologies for future mobile networks is non orthogonal multiple access (NOMA). Hence, most of academic and industry people are working on this technique to evaluate the error performance and to know the capacity of NOMA scheme. In this paper, we have considered a NOMA system in which two users are separated with an equal distance from base station (BS). The bit error rate (BER) performance is evaluated over non-fading (AWGN) and fading (Rayleigh, Rician, Nakagami-m and Weibull) environments for an arbitrary NOMA user. For two users, different power levels are provided. During the transmission of data, BPSK modulation scheme is considered and superposition coding is used at base station (BS). Successive interference cancellation (SIC) decoding mechanism in applied at receiver. Finally, with the help of MATLAB simulations, BER performances are evaluated. The comparative BER analysis for different environments is also provided.