Coexistence of Cyclic Waveforms in Nonorthogonal Multiple Access Systems

ABSTRACT Multiple access schemes that employ nonorthogonal resource elements require advanced interference mitigation techniques to ensure reliable communication. Cyclic spread nonorthogonal multiple access (NOMA), an interference resilient scheme, has been introduced to enhance diversity, particularly in frequency‐selective environments. Previous studies have demonstrated that pairing maximum‐length (ML) sequences with Walsh–Hadamard (WH) sequences can improve NOMA performance. However, the correlation properties of ML sequences are not well‐suited for optimal detection in multipath fading channels. To overcome this limitation, we propose a novel sequence pairing strategy within the NOMA framework, utilizing a minimum mean square error (MMSE) receiver. Specifically, we investigate the use of Zadoff–Chu (ZC) sequences paired with either WH or ML sequences. To assess performance and fairness, we compare the traditional ML–WH pair with the proposed ZC–WH and ZC–ML pairs. While ZC‐based combinations generally offer improved performance, our findings indicate that the noncyclical nature of the WH sequence in the ZC–WH pair introduces performance degradation due to imperfect channel estimation in NOMA scenarios. In contrast, the proposed ZC–ML pairing outperforms both the ZC–WH and ML–WH pairs in terms of bit error rate (BER) and user fairness, demonstrating its effectiveness in multipath fading environments.