Reconfigurable Intelligent Surfaces May Become a Key Technology for Future Wireless Networks

5G has been deployed globally, with the key enabling technology being massive Multiple Input/Multiple Output (MIMO). Although it is offering a capacity boost compared to previous cellular systems, it still faces several challenges, including high energy consumption and high cost. Moreover, 5G deployments still focus on traditional deployment process, meaning that new equipment needs to be deployed on existing cell sites or new sites are necessary for wide coverage. However, there are now new types of technologies being developed to reduce energy consumption, cost, and enhance network capability. Reconfigurable Intelligent Surfaces (RIS) has emerged as a low-power, cost-effective, and sustainable technology to address the issues and improve network performance.

RIS could be a real value-added technology, but certain technical challenges must be adequately addressed before it gets integrated into future communication standards and move towards commercialization. On October 4, 2021, the European Telecommunications Standards Institute (ETSI) launched an industry specification group on RIS (ISG RIS) for a duration of two years to address some of the challenges and establish global standardization of RIS.

RIS is a software-controlled system utilizing smart radio surfaces with hundreds or thousands of metamaterial elements to shape and control the radio signal dynamically. In this deployment scenario, RIS panels would be deployed throughout an existing 5G cell, reflecting signals accordingly while increasing coverage and capacity through passive elements. This technology has a number of competitive advantages. First, RIS only needs mostly passive elements, so they have lower hardware cost and energy consumption compared to 5G technologies based on active antenna arrays. Second, they are almost passive, so they can be made with low weight and constructed into any shape to be deployed in a wide range of scenarios (i.e., ceilings, walls, and lampposts). RIS is also free of antenna noise amplification and self-interference. Furthermore, RIS can be configured to operate in any frequency level, from sub-6GHz to THz.  

The Japanese operator NTT DOCOMO completed a demonstration of 5G mobile communication system in the 28GHz band using a metasurface reflect-array in November 2018. The metasurface reflector was developed using small structures that were arranged to have different shapes according to their positions in the array. A downlink speed of 560Mbps was achieved, compared to 60Mbps without a reflector. In January 2020, NTT DOCOMO conducted another trial, which was a transparent dynamic metasurface with 5G radio signals using 28GHz. It showed that the designed metasurface can reflect the incident signals with both full and partial power and enable the signal to penetrate with nearly zero power loss. They re-conducted the trial in January 2021 to prove that a transparent metasurface could improve the power level of received signals at indoor focal points. ZTE also completed a demonstration of RIS during Mobile World Congress (MWC) 2021, showing that it can improve network quality in blind spots.

However, several challenges need to be addressed before RIS can be widely deployed in practice. Channel modeling is a fundamental problem to be studied to evaluate the performance of RIS, as the electromagnetic characteristics of each RIS element needs to be properly captured and modeled. Moreover, channel estimation is another challenge as RIS is mostly passive and thus cannot send pilot signals, and a low complexity but efficient algorithm needs to be developed for RIS-enabled networks.

The deployment of RIS must take into account a number of factors, including location availability, hardware cost, and user distributions. A fundamental question must be answered: given the number of reflection elements, should it be a centralized deployment or a distributed deployment? RIS would be deployed outdoors, and exposed to very harsh weather conditions (i.e., sunlight, temperature fluctuations, rain, snow, and wind). The materials that constitute the RIS elements must be resilient to those hostile weather conditions for years, during which their electromagnetic properties should not deteriorate dramatically. With substantial investments from research institutes and industry, it is set to see fast progress.  

Currently, 3GPP is focused to complete Release 17 for 5G systems, and there is no formal plan for 6G yet. Each generation of wireless communication standards last about ten years. Since 3GPP started work group meeting on 5G systems in 2016, discussions on 6G systems may begin around 2026.

RIS has been a popular research area for academic researchers for the past a few years and several major research projects on RIS with emphasize on wireless technologies are to be completed in 2022-2024. Standardization of RIS has only kicked off at regional levels. China Communications Standards Association has initiated a study item to look at channel modeling, channel estimation, beamforming with RIS, RIS and Artificial Intelligence (AI), etc., and a technical report is expected in June 2022. The second Chinese organization FuTURE was formed in December 2020 with the focus to study potential use cases and key technologies to support RIS standardization and commercialization. In March 2021, a proposal was submitted to 3GPP by companies in China to call for support of RIS for 5G advanced. Within the European Telecommunication Standards Institute, the newly formed group ISG RIS is set to review and establish global standardization of this technology within the next two years.

Addressing the challenging issues on RIS-enabled networks will be a lengthy process. Academic researchers have contributed vastly to building and testing proof-of-concepts as well as prototypes of RIS. The industry progress mainly originates from NTT DOCOMO. To accelerate the maturity of RIS, the industry should engage themselves by running more implementations to learn the actual performance in practical scenarios. The future of RIS at the moment is not clear, though more updates should be available by 2023. Given the timeline of 5G systems, RIS seems to be a candidate technology for 6G systems.