Due to the increasing demand for higher capacity in mobile communication devices, non-orthogonal multiple access (NOMA) has become a promising technique. Compared to existing orthogonal multiple access methods, NOMA can serve multiple users with better spectral efficiency by allowing co-channel interference, thus its reliability performance over various channel models needs further research. In this paper, closed-form outage probability, average channel capacity and symbol error rate are derived for a downlink-NOMA system with an arbitrary number of users over generalized-K fading channels. Finally, theoretical findings are validated via Monte Carlo simulations for several channel parameters. (C) 2021 Elsevier B.V. All rights reserved.