RIS Helps Sustainable and Efficient Evolution of 5G-Advanced and Beyond

5G supports a wide range of high-speed and large-bandwidth applications, which not only enrich people's daily lives, but also stimulate the demand for network capacity. It is expected that in the next decade, the capacity of communication networks will increase by a thousandfold, and ubiquitous wireless connectivity will become a reality. However, highly complex networks, high-cost hardware, and increasing energy consumption have become the key issues. It is urgent to explore new technologies and new materials to provide low-cost, low-energy, high-performance wireless network evolution solutions. Among the candidate new technologies, reconfigurable intelligent surface (RIS) stands out for its unique configurability, low cost, low energy consumption, and easy deployment features.

The static metasurface in the early stage after being designed and manufactured had solidified electromagnetic wave responses and functions, which cannot be adjusted. We call it the RIS 1.0 phase. Because it does not support on-demand dynamic adjustment, its application scenarios are very limited, and it is only suitable for technical verification, and unsuitable for commercial deployments. ZTE is the first vendor to enter the stage of RIS 2.0. ZTE not only developed the dynamic RIS hardware but also launched the industry's first 5G base station and dynamic RIS collaborative beamforming algorithm to realize millimeter-wave dynamic beam steering. This greatly improves the coverage of RIS and ensures seamless connectivity of users even in mobile scenarios.

Multi-Band and Multi-Form Portfolio Adapts to Different Scenarios

In order to realize the dynamic adjustment of RIS, it is necessary to integrate active devices (such as PIN diodes, varactors, etc.) or phase-shift materials (such as liquid crystals) on the metasurfaces. By changing the external excitation, the metasurfaces with fixed physical structures can present dynamically tunable or reconfigurable electromagnetic properties. Based on the characteristics of different materials, ZTE has developed various forms of RIS to adapt to different application scenarios (Fig. 1).

For the millimeter-wave dynamic RIS, the control system adopts PIN diodes, and the state (size) of the electromagnetic unit is controlled by applying different bias voltages to the PIN diodes, thereby realizing the phase shift of the incident electromagnetic wave. Accurate beamforming is achieved through the independent control of different electromagnetic units combined with the effects of phase shift of all units. At present, ZTE's millimeter-wave dynamic RIS contains more than 16,000 electromagnetic units (antenna elements), which can form ultra-narrow reflected beams, and the single-point RSRP gain exceeds 30 dB. In addition, in terms of control accuracy, ZTE has achieved an industry-leading 2-bit dual-polarization control. Compared with the 1-bit solution commonly used in the industry, the coverage gain can be increased by 3 dB. The PIN diode has the advantages of high device maturity, low insertion loss, and fast switching speed (ns level), but it also has the drawbacks of high cost and power consumption. To this end, ZTE is also actively exploring other control materials, such as liquid crystals. Liquid crystal materials have the advantages of continuous phase shift (higher control accuracy), lower cost, and lower power consumption, but due to the high environmental requirements, they are currently considered suitable for indoor use. In addition, in the penetrated scenario, ZTE has also developed a transparent penetrated RIS. By superimposing a copper mesh on a transparent substrate such as glass or PC, it can realize the focused transmission of electromagnetic waves, and at the same time, it can achieve a high light transmittance, which is very suitable for being attached to building glass surfaces and high-speed rail window surfaces.

Collaborative Algorithm Realizes Flexible Coverage Reconfiguration

On the basis of hardware, the true realization of intelligent control of electromagnetic waves by RIS also depends on well-designed beamforming algorithms. A necessary condition for RIS to achieve energy convergence is the high gains produced by the narrow beam. But the problem with the narrow beam is that the coverage is very small, and when the UE moves, it is easy to leave the coverage of the RIS. This requires RIS to support beam scanning to achieve dynamic tracking of the UE and coverage of a larger area. However, due to the fact that RIS is a passive device, it does not have the ability of channel estimation and measurement, which requires the base station to help RIS realize joint channel estimation. ZTE has innovatively proposed the base station and RIS collaborative beamforming algorithm (Fig. 2). The algorithm uses the base station to instruct the RIS to realize time division-based beam scanning/switching, which expands the coverage of the RIS and achieves the effect of UE beam tracking. In addition, the algorithm is based on the 5G air interface protocol framework and supports 5G commercial terminals, laying a solid technical foundation for the early commercial use of RIS, a 6G technology in the 5G stage.

Typical RIS Application Scenarios

Due to its flexible control of electromagnetic waves, RIS will have a wide range of application scenarios in the 5G-A and 6G stages. In the 5G-A stage, RIS can be applied to enhance outdoor blind or weak coverage, enhance edge coverage, increase hotspot streams and throughput, outdoor-to-indoor coverage, and enhance coverage in the carriages of vehicles. In addition, RIS helps millimeter waves achieve continuous coverage in dense urban areas, making millimeter wave services move from FWA only to mobile services, unleashing the mmWave spectrum's untapped potential. In the 6G stage, with the introduction of higher frequency bands such as terahertz, RIS can help to improve high-frequency coverage. In addition, relying on RIS's ability to establish virtual line-of-sight (LOS) links in non-LOS environments, and ultra-large-scale antenna arrays, higher positioning accuracy can be achieved, facilitating the introduction of integrated sensing and communication.

RIS is a wireless technology with huge potential. ZTE dynamic RIS innovates on RIS by empowering operators to use beam steering in real-time. ZTE believes that the real-time capabilities of dynamic RIS are necessary for the successful commercial use of RIS, as 5G networks are increasingly and inherently dynamic.

Comprehensive Collaboration and Trials to Drive Commercial Progress

Since 2021, ZTE and operator partners have conducted comprehensive prototype trials. By exploring the feasibility of RIS to improve coverage and user experiences in multiple frequency bands and multiple scenarios, the commercial process of RIS is actively promoted.

In August 2022, ZTE and China Mobile Research Institute completed the industry's first prototype verification

of dynamic RIS technology. The trial results show that the 5G base station and dynamic RIS collaborative beamforming technology can not only greatly improve the coverage, but also support the seamless connectivity of users in mobile scenarios, laying an important technical foundation for the future commercial use of RIS.

In addition, in 2021, ZTE completed the industry's first field verification of RIS in the 5G high-band with China Telecom, cooperated with China Unicom to complete the industry's first field verification of RIS in the 5G mid-band, jointly released the industry's first RIS cascade prototype verification results based on the 2.6 GHz commercial network with China Mobile Beijing Branch.

RIS Technical Challenges and Trends

Although RIS has made great progress in technical research, engineering application, and prototype verification, as a cutting-edge technology, RIS still faces many technical challenges such as research on new materials with lower cost and power consumption, on simple deployment solutions, on interference between RIS of different operators, and on RIS management, operation, and maintenance. ZTE will continue to carry out research in the above-mentioned aspects and jointly promote the technological development and commercialization of RIS in cooperation with industry-university-research institutes.