BER analytical expression for two-user PD-NOMA with square QAM by polynomial approximation

Power-Domain Non-Orthogonal Multiple Access (PD-NOMA) is a new multiple access technique to increase system performance. In turn, a bit error rate (BER) is one of the performance metrics. There are currently no analytical expressions for the exact BER calculation for PD-NOMA. The issue is related to the complexity of decoding, which makes it difficult to derive a BER expression for arbitrary multiplexing configurations. By configuration, we mean the number of multiplexed users, types of modulation, and power allocation coefficients they use. This paper considers a PD-NOMA scenario when two subscribers using square quadrature amplitude modulation (QAM) are multiplexed. In the paper, a BER analytical expression using a polynomial approximation for such a scenario is proposed. The work is divided into three conceptual parts. Firstly, we provide an overview and describe the system model of the typical scenario of PD-NOMA. Secondly, we show an analytical expression for the exact symbol error rate (SER). Based on it, we propose a BER expression using the polynomial approximation of SER. In the final section, we compare the BER obtained by the proposed method and the simulation. As a result, we have obtained the BER analytical expression for a two-user PD-NOMA system with square QAM by polynomial approximation.