Twin-shift co-existence non-orthogonal multiple access

Non-orthogonal multiple access (NOMA) is a communication technique that allows multiple users to share the same resource block for simultaneous data transmission. However, NOMA faces challenges in the presence of frequency-selective fading, leading to intersymbol interference (ISI) caused by overlapping data from multiple propagation paths and users. To address this issue, hybrid NOMA has been introduced, employing spreading codes in the power domain and simple cyclic interleaving to mitigate the impact of ISI effectively. Furthermore, a simple cyclic interleaved co-existence method for NOMA has been developed, aiming to reduce ISI and multiple access interference (MAI) by co-existing different spreading codes within the NOMA cluster. In the current scheme, cyclic shifted codes support bit error rate (BER) improvement for users with poor reception. However, introducing cyclic shifts to the code and data can enhance performance further, a concept not proposed for the co-existence NOMA. Therefore, this paper introduces a new twin-shift cyclic interleaved co-existence method for NOMA, involving a process that cyclically shifts and interleaves the data and code. This method significantly improves multipath diversity compared to the existing simple cyclic interleaved co-existence method, demonstrating a performance gap of \(5\) dB in \(E_{b} /N_{0}\) at a BER of \(10^{ - 5}\), while also providing a maximum diversity order of \(11\) for a \(14 -\) length channel.