MIMO Transmission through Reconfigurable Intelligent Surface: System Design, Analysis, and Implementation (1912.09955v2)

Abstract: Reconfigurable intelligent surface (RIS) is a new paradigm that has great potential to achieve cost-effective, energy-efficient information modulation for wireless transmission, by the ability to change the reflection coefficients of the unit cells of a programmable metasurface. Nevertheless, the electromagnetic responses of the RISs are usually only phase-adjustable, which considerably limits the achievable rate of RIS-based transmitters. In this paper, we propose an RIS architecture to achieve amplitude-and-phase-varying modulation, which facilitates the design of multiple-input multiple-output (MIMO) quadrature amplitude modulation (QAM) transmission. The hardware constraints of the RIS and their impacts on the system design are discussed and analyzed. Furthermore, the proposed approach is evaluated using our prototype which implements the RIS-based MIMO-QAM transmission over the air in real time.

• The paper introduces an analytical formulation for RIS-based MIMO systems that applies high-order QAM via programmable metasurfaces.
• A novel non-linear modulation technique is proposed to operate under a constant envelope constraint, enabling efficient QAM implementation.
• The practical viability is confirmed through a prototype demonstration achieving 20 Mbps, showing promise for cost-effective 6G wireless solutions.

An Overview of RIS-Based MIMO Transmission for Wireless Communications

The paper, "MIMO Transmission through Reconfigurable Intelligent Surface: System Design, Analysis, and Implementation," offers a comprehensive investigation into the potential of Reconfigurable Intelligent Surface (RIS) as a cost-effective and energy-efficient solution for enabling advanced wireless communication technologies such as ultra-massive MIMO (UM-MIMO) and holographic MIMO. The authors focus on designing and analyzing RIS-based MIMO transmission systems capable of supporting high-order modulation schemes, specifically Quadrature Amplitude Modulation (QAM).

Core Contributions and System Model

The authors present a detailed mathematical model that characterizes RIS-based MIMO transmission by considering the electromagnetic (EM) nature of RISs. This model demonstrates that RIS-based systems follow the same foundational principles as conventional MIMO systems, whereby modulation is directly applied to the EM carrier signals using programmable metasurfaces without the need for conventional radio-frequency (RF) chains. This approach opens up possibilities for realizing advanced wireless communication systems with reduced hardware costs and power consumption.

Key contributions of the work include:

1. Analytical Formulation: The paper provides an analytical formulation for RIS-based MIMO systems that considers hardware constraints, such as phase-dependent amplitude and discrete phase shifts.
2. Non-linear Modulation Technique: A novel non-linear modulation technique is introduced to facilitate high-order QAM under the constant envelope constraint, which typically challenges RIS deployments. This approach enables the implementation of QAM on harmonics by controlling the phase response of RIS elements over time.
3. Experimental Validation: The practical feasibility of the presented systems is validated through the world's first prototype demonstrating real-time RIS-based MIMO-QAM communication, achieving a data rate of 20 Mbps.

Numerical Results and Prototype System

The authors detail the prototype setup, which includes a varactor-diode-based programmable metasurface as the RIS component, capable of achieving continuous phase manipulation over a significant range. The experimental setup integrates an RF signal generator, digital baseband processing, and control circuitry to manage the RIS elements. Numerical results from the experimental validation confirm the robustness and efficiency of the system design, illustrating closely matched measured and theoretical bit error rates (BER).

Implications and Future Directions

This paper highlights the potential of RIS technology to revolutionize the design of wireless communication systems by offering a flexible and energy-efficient alternative to traditional MIMO architectures. The RIS-based approach provides an attractive solution for 6G technology development, especially in addressing the challenges of realizing large aperture systems such as UM-MIMO and holographic MIMO.

The implications of this research extend to theoretical perspectives, where future studies could explore optimization algorithms for enhanced RIS-based beamforming and modulation schemes. Practically, the findings invite further investigation into scalable RIS architectures and the integration of RIS technology into existing wireless networks.

In summary, the development of RIS-based MIMO systems holds promise for transformative impacts on future communications, balancing performance, cost, and energy usage more effectively than current methodologies.