Non-orthogonal multiple access (NOMA) is a promising technique to improve spectral efficiency and support massive connectivity in 5G-enabled IoT networks. This paper evaluates the bit error rate (BER) performance of a downlink NOMA system employing an optimized power allocation strategy based on the Lagrange multiplier method and Karush–Kuhn–Tucker (KKT) conditions. The analysis incorporates both perfect and imperfect successive interference cancellation (SIC), with residual interference (ϵ) representing practical system impairments such as hardware limitations and channel estimation errors. Simulation results show that the proposed scheme outperforms conventional methods, including fixed power allocation NOMA (FPA-NOMA), dynamic power allocation NOMA (DPA-NOMA), and hybrid NOMA-OMA. At Eb/N0= 24 dBm, N=4, m = 3, and ϵ = 0.005, the corresponding BER values are 0.1009 (FPA-NOMA), 0.0648 (DPA-NOMA), 0.0058 (hybrid NOMA-OMA), and 0.0051 (optimized power allocation). These results confirm the effectiveness of the proposed method in improving BER performance for massive IoT deployments. © This article is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. License details: https://creativecommons.org/licenses/by-sa/4.0/