# Differential Reflecting Frequency Modulation with QAM for RIS-Based Communications

## TL;DR

This paper introduces a new communication system using reconfigurable surfaces that reduces the need for channel information and improves performance with a new modulation technique.

## Contribution

The paper introduces DRFM with QAM, a novel modulation scheme that enhances throughput and performance without requiring channel state information.

## Key findings

- DRFM operates without channel state information at the transmitter, RIS, or receiver.
- The proposed QAM-aided DRFM outperforms traditional PSK, APSK, and star-QAM modulations.
- Simulation results show DRFM has acceptable SNR penalty compared to coherent detection methods.

## Abstract

Reconfigurable intelligent surface (RIS)-aided index modulation (IM) shows great potential for next-generation wireless communications. Nevertheless, obtaining channel state information (CSI) for RIS-based IM incurs high pilot overhead, particularly for multi-domain IM. In this paper, we integrate orthogonal frequency division multiplexing into RIS-aided differential reflecting modulation (DRM) communications, introducing the differential reflecting frequency modulation (DRFM) system. In DRFM, information bits are jointly conveyed through the activation permutations of reflecting patterns, grouped carriers, and constellation symbols. The transmitter combines the differentially coded reflecting-time block and the time–frequency block using the Kronecker product. This allows DRFM to operate without relying on CSI at the transmitter, RIS, or receiver. Moreover, we design a novel high-rate quadrature amplitude modulation (QAM) scheme for DRFM. Compared to PSK-based DRFM, this QAM scheme can boost either the throughput or the performance of DRFM. Simulation results illustrate the superiority of the DRFM system, along with an acceptable SNR penalty, compared to non-differential modulation with coherent detection. At the same spectral efficiency, the proposed QAM-aided DRFM outperforms schemes using traditional PSK, amplitude phase shift keying (APSK), and star-QAM constellation modulations.