RIS-Aided Cell-Free Massive MIMO Systems for 6G: Fundamentals, System Design, and Applications

TL;DR

This paper tackles the challenge of meeting 6G requirements by surveying RIS-aided CF mMIMO systems, detailing system models, channel modeling, and protocols that enable distributed, environment-aware communication. It analyzes system operation and resource allocation, highlighting channel estimation, joint beamforming, and multi-layer signal processing, while addressing practical impairments such as hardware nonidealities, EMI, and fronthaul limits. The survey further explores integration with NGMA, SWIPT, mmWave/THz, and UAVs, and discusses future directions including semantic communications, ISAC, SAGIN, XL-MIMO, and security. Together, these insights provide a comprehensive guide for designing, deploying, and evolving RIS-aided CF mMIMO networks toward robust, high-capacity 6G systems.

Abstract

An introduction of intelligent interconnectivity for people and things has posed higher demands and more challenges for sixth-generation (6G) networks, such as high spectral efficiency and energy efficiency, ultra-low latency, and ultra-high reliability. Cell-free (CF) massive multiple-input multiple-output (mMIMO) and reconfigurable intelligent surface (RIS), also called intelligent reflecting surface (IRS), are two promising technologies for coping with these unprecedented demands. Given their distinct capabilities, integrating the two technologies to further enhance wireless network performances has received great research and development attention. In this paper, we provide a comprehensive survey of research on RIS-aided CF mMIMO wireless communication systems. We first introduce system models focusing on system architecture and application scenarios, channel models, and communication protocols. Subsequently, we summarize the relevant studies on system operation and resource allocation, providing in-depth analyses and discussions. Following this, we present practical challenges faced by RIS-aided CF mMIMO systems, particularly those introduced by RIS, such as hardware impairments and electromagnetic interference. We summarize corresponding analyses and solutions to further facilitate the implementation of RIS-aided CF mMIMO systems. Furthermore, we explore an interplay between RIS-aided CF mMIMO and other emerging 6G technologies, such as next-generation multiple-access (NGMA), simultaneous wireless information and power transfer (SWIPT), and millimeter wave (mmWave). Finally, we outline several research directions for future RIS-aided CF mMIMO systems.

Figures (15)

• Figure 1: Network model evolution. The wireless communication network has evolved from a traditional BS-centric cellular network to a user-centric CF mMIMO network and is further progressing into a more intelligent RIS-aided CF mMIMO network.
• Figure 2: The organization structure of the survey.
• Figure 3: Application scenarios of RIS-aided CF mMIMO systems. The scenarios mainly include data demand scenarios such as mMTC, high-mobility, XL-MIMO, mmWave, and Metaverse, as well as energy demand scenarios such as wireless energy transfer in the physical layer.
• Figure 4: Illustration of the considered system frame structure under TDD communication protocol.
• Figure 5: Multi-stage transmission procedure and multi-layer signal processing of RIS-aided CF mMIMO systems. (a) Illustration of the system architecture. (b) Illustration of the four-stage transmission procedure, including the wireless energy transfer, UL pilot transmission, UL data transmission, and DL data transmission.