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 D³is 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.