Non-orthogonal multiple access (NOMA) systems use superposition coding (SC) at transmitter and successive interference cancellation (SIC) at the receiver in order to simultaneously transmit the messages of multiple users. In this paper, closed from expressions for the symbol error rate (SER) of superposition codes are derived. Moreover, an approximate expression is presented and it is shown that it closely follows the exact symbol error rate. The derived formulas are applicable to general scenarios with $N$ users when each individual user uses pulse amplitude modulation (PAM) or quadrature amplitude modulation (QAM) signalling with arbitrary constellation sizes. While the error expressions are for additive white Gaussian noise (AWGN) channels, it will be shown that they can be extended to fading channels. Numerical simulations verify the validity of both expressions.

非正交多址（NOMA）系统在发射机处使用叠加编码（SC），在接收机处使用连续干扰消除（SIC），以便同时发送多个用户的消息。在本文中，从表达式的封闭式中得出了叠加码的符号错误率（SER）。此外，给出了一个近似表达式，并表明它近似遵循精确的符号错误率。导出的公式适用于具有以下条件的一般方案：$ñ$当每个用户都使用具有任意星座图大小的脉冲幅度调制（PAM）或正交幅度调制（QAM）信令时，就会出现“用户”状态。虽然误差表达式是针对加性高斯白噪声（AWGN）信道的，但可以证明它们可以扩展到衰落信道。数值模拟验证了两个表达式的有效性。