OFDM-NOMA Error Rate Reduction Using Direct Data Detection

The direct data detection (D3) is proven to provide a significant error rate reduction for orthogonal multiple access (OMA) schemes because it does not directly depend on the estimated channel state information (CSI). Therefore, the D3 would be particularly suitable for non-orthogonal multiple access (NOMA) because relaxing the dependence on the estimated CSI can improve the reliability of the detection process and reduce the residual inter-user interference (IUI). Therefore, this article proposes an integrated D3-NOMA where the channel state information estimation (CSIE), equalization, and detection are consolidated into one unified process. The performance of integrated D3-NOMA is theoretically analyzed for an arbitrary number of users, where accurate closed-form approximations are derived for the sequence error probability (SEqP) and bit error rate (BER) in frequency-selective channels. Complexity analysis is provided to compare the proposed D3 with practical coherent detectors. The theoretical and simulation results reveal that the performance of D3 is influenced by the distance from the base station, allocated power, signal-to-noise ratio (SNR), and channel conditions. In a two-user NOMA, the D3-based SEqP and BER for the far user are only 3 dB from the coherent detector with perfect CSI, and it outperforms coherent detectors with practical CSI. Moreover, the D3 can significantly reduce or eliminate the error floors associated with the imperfect CSI in frequency-selective (FS) channels. The computational complexity of D3 depends on the sequence length, and a significant complexity reduction is achievable using the Viterbi algorithm.