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Exact bit error rate of cooperative NOMA without diversity combining

This paper investigates a downlink cooperative non-orthogonal multiple access without diversity combining (C-NOMA-WDC) when the near user works as a helper user in decode-and-forward (DF) mode to help the far user. The Binary Phase Shift Keying (BPSK) is used to encode the users' messages. Under the Rayleigh fading, we derive the analytical closed-form bit error rate (BER) expression for the near user. Then, we express the end-to-end BER of the far user. The results are compared with traditional cooperative orthogonal multiple access WDC (C-OMA-WDC). The optimal value of the power allocation and the best position for the assistance user are also discussed. The numerical results are verified by simulation. The results show that the BER of C-OMA-WDC outperforms C-NOMA-WDC. The power allocation coefficient does not always influence BER optimization but rather affects the performance of the successive interference cancellation (SIC). Moreover, to achieve fairness in BER performance between the far user and the near user, the best position for the assistant user is in the middle between the base station and the far user.