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

2025-11-07

AI Summary

Reconfigurable intelligent surfaces (RIS) for direct-modulation communication face two core challenges: difficulty in realizing beamforming and the randomness and time-varying nature of coding states. Method: This paper proposes a joint space–time coding scheme that decouples temporal and spatial encoding and fuses them via XOR operations—enabling simultaneous direct modulation and beamforming without baseband processing. The approach is grounded in space–time signal modeling and implemented using a 1-bit reconfigurable transmissive RIS prototype operating in the 3.4–3.79 GHz band. Contribution/Results: Experimental results demonstrate that, compared to conventional time-only encoding, the proposed joint coding reduces root-mean-square error vector magnitude (rms-EVM) by 55% and significantly lowers bit error rate, thereby substantially enhancing communication reliability and overall system performance.

Technology Category

• Cognitive Modeling & Cognitive Systems: Neural Spike Coding
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Multiagent Systems: Agent Communication

Application Category

• Search and Retrieval-Augmented AI: Retrieval-Augmented Generation (RAG) and multi-modal RAG
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• User Modeling, Personalization and Recommendation: On-Device user modeling, personalization, and recommendation

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.

Problem

• Enables simultaneous direct modulation and beamforming using RIS
• Addresses random time-varying coding state challenges in RIS systems
• Reduces bit error rate and constellation error in IoT communications

Innovation

• Joint space-time coding enables simultaneous modulation and beamforming
• XOR operations combine time and space coding sequences
• Transmissive 1-bit phase RIS reduces bit error rate