Twin-Layer RIS-Aided Differential Index Modulation Dispensing With Channel Estimation

In this correspondence, we propose a twin-layer hierarchical differential index modulation scheme for a reconfigurable intelligent surface (RIS)-aided transmitter architecture, termed as HD-RIS-IM. More specifically, the RIS array is partitioned into perfectly tiling sub-arrays. Each sub-array is mapped across multiple time slots, where each RIS element is activated only once at a particular time slot. These sub-arrays represented by sub-matrices are then used for constructing a block-based permutation matrix for data transmission. With the aid of this hierarchical structure, additional information can be conveyed by the specific order of the sub-arrays that are activated. Furthermore, extra bits are also embedded in the particular order of the RIS elements that are activated within each sub-array. By exploiting all the distinct permutations of the activated matrices, a differential modulation scheme is proposed for the RIS-aided transmitter, which maintains an
$M$
-ary phase shift keying (MPSK) and facilitates CSI-free demodulation. Furthermore, at the receiver, we propose a low-complexity distributed maximum likelihood (ML) detector, which significantly reduces the detection complexity. Our simulation results demonstrate the performance benefits of the proposed HD-RIS-IM scheme.