Joint Space Time Coding on RIS for Simultaneous Direct Modulation and Beamforming

TL;DR

This work introduces a novel joint space–time coding framework for RIS-based direct modulation, combining time coding for harmonic-sideband modulation with space coding for beamforming via an XOR operation: $\phi_{m,n}(t)=\Gamma_{m,n}\oplus G(t)$. The approach decouples modulation and steering, enabling simultaneous data transmission and directional gain using a 1-bit phase RIS. Numerical simulations and FPGA-implemented experiments demonstrate substantial performance improvements, including ~22 dB SNR gains and a 55% rmsEVM reduction in hardware tests, along with up to three orders of magnitude BER improvement in directional communication. The results indicate strong potential for IoT and future 6G networks where low-complexity RIS-based modulation and adaptive beamforming are desirable.

Abstract

Reconfigurable Intelligent Surface (RIS)-based direct modulation communication systems have garnered significant attention due to their low cost, low power consumption, and baseband-less characteristics. However, these systems face challenges such as the random time-varying coding state of the RIS and the difficulty in implementing beamforming in direct modulation. In this paper, we propose a simple and effective joint space-time coding approach for RIS that enables simultaneous realization of both direct modulation communication and beamforming. By modeling the transmitted signals of the RIS using space-time coding, we show that the time coding determines the direct modulation functionality, while the space coding governs the beamforming. Consequently, we introduce a joint time-space coding technique by performing exclusive-or (XOR) operations on the time and space coding sequences, enabling both functionalities to be achieved concurrently. Numerical simulations demonstrate the effectiveness of the proposed method. Furthermore, we design and fabricate a transmissive 1-bit phase reconfigurable RIS operating in the 3.4~3.79 GHz frequency band for the implementation of a direct modulation communication system. Experimental results reveal that the bit error rate (BER) is significantly reduced when joint space-time coding is used, compared to using time coding alone. Additionally, the root-mean-square error vector magnitude (rmsEVM) of the constellation diagram is reduced by 55%. This technique is promising for applications in the Internet of Things (IoT), contributing to the development of intelligent networks for electronic devices.

Figures (16)

• Figure 1: Schematic diagram of enhanced direct modulation communication through joint space-time modulation.
• Figure 2: Time sequence $g_l(t)$ of the $l$th modulation period.
• Figure 3: Simulated received signal before and after applying joint space--time coding: (a) spectrum and (b) constellation with time-only coding; (c) spectrum and (d) constellation with joint space--time coding.
• Figure 4: BER versus SNR for different modulation orders with and without beamforming using joint space--time coding.
• Figure 5: Power pattern of the transmitter before and after beamforming using joint space-time coding.