Design of Non-Coherent RIS-Empowered DCSK With Two-Level Nested Index Modulation

Non-coherent chaotic communication has gained increasing attention as it provides an efficient solution for reliable communications without requiring channel state information. In this paper, we propose a non-coherent reconfigurable intelligent surface (RIS)-empowered differential chaos shift keying scheme with two-level nested index modulation (RIS-DCSK-TLNIM). In the proposed RIS-DCSK-TLNIM scheme, the reference index and information index are nested to form two-level nested index modulation. This design enhances both the spectral efficiency and bit error rate (BER) performance of RIS-DCSK-TLNIM, albeit at the expense of slightly increased complexity. Furthermore, we propose a joint detection algorithm to recover the information bits transmitted via the reference index, information index, and two distinct-mode signals. We then extend RIS-DCSK-TLNIM into an enhanced system to achieve higher spectral efficiency. Subsequently, we analyze the BER performance, spectral efficiency, and system complexity of RIS-DCSK-TLNIM, and compare these metrics with those of benchmark systems. Comparison results demonstrate that the proposed RIS-DCSK-TLNIM, when configured with a small number of time slots, can achieve more than twice the spectral efficiency and at least a 6 dB gain in BER performance compared to benchmark systems.