The increasing demands of high throughput and capacity in 5G are posing different challenges on the design of multiple access schemes. To meet these challenges, Non-orthogonal multiple access (NOMA) is considered as one of the potential candidates. Broadly, NOMA is classified as power-domain and code-domain NOMA. Power-domain NOMA serves multiple users with the same time, frequency and code, but using different power levels. This feature make NOMA capable of increasing spectral efficiency gain over the conventional multiple access schemes. In this paper, an in-depth analysis of downlink power domain NOMA is provided under various fading channels, modulation schemes and channel coding rates. Power domain multiplexing with superposition coding is done at the transmitter side and practical successive interference cancellation (SIC) is used at the receiver side. Performance is analyzed by simulations on the basis of Bit error rate (BER) using AWGN, Rician and Rayleigh channel models for various modulation schemes and code rates. This analysis provides a good foundation for the development of Adaptive Modulation & Coding (AMC) schemes for future NOMA-based 5G systems.