Realization of Base Station Antennas Using RIS Technology and the Characteristics of RIS Antennas

1. Overview of Reconfigurable Intelligent Surface (RIS) Technology

With the introduction of high-frequency bands like the millimeter-wave spectrum in 5G communication, new challenges have emerged. One of the main issues is the weakening of signals received by devices due to the distance from base stations and the effects of surrounding buildings.
Generally, to solve coverage issues, technologies that increase power density by narrowing radio waves into thin beams and electronically controlling the beam direction (a.k.a beamforming technology / phased array technology) are introduced to expand the communication area.
In addition to beamforming technology, research and development is progressing on another approach that uses a special reflective panel known as RIS (Reconfigurable Intelligent Surface). This technology reflects radio waves into areas shadowed by buildings, thereby expanding the communication area.
Figure 1 illustrates the concept of a wireless communication environment using RIS. The area construction method using RIS involves reflecting radio waves transmitted from the base station in the direction of the terminal using RIS to expand the communicable area.
In addition to the purpose of expanding the area, there are also attempts to improve the efficiency of MIMO (Multiple Input Multiple Output) by adaptively controlling the wireless environment existing between the base station and the terminal.
Due to these developments, RIS—an advanced functional device—is gaining attention, and various research institutions are actively exploring its potential applications.

Figure 1. An Example of a Wireless Communication Environment Using RIS Technology

2. Mechanisms of RIS and Challenges for Practical Implementation

Reflective RIS consists of multiple passive elements, as shown in Figure 2. By changing the size and shape of these passive elements, we can modify the surface characteristics (surface phase). This surface phase adjustment enables us to direct the beam in any desired direction.

Figure 2. Mechanisms of RIS

However, there are many problems associated with attempts to dynamically control the wireless environment with RIS. Technical challenges include implementing control technologies to accurately direct radio beams to RIS positioned at a significant distance from the base station and developing RIS that operates only within specific frequency bands for individual telecom operators. Additionally, there are numerous considerations such as the difficulty in installing RIS in ideal locations, meaning practical implementation is expected to take more time.

At SoftBank, we are focusing on the ability of RIS to change the surface phase to strengthen beams in any desired direction. By leveraging this characteristic, we are working to develop simpler base station designs by significantly reducing the number of wireless components, such as phase shifters and amplifiers, within the base station. This has led to research and development of "RIS antennas," which integrate RIS technology into parts of the base station.

3. Simplification of Base Stations with RIS Antennas

In base stations supporting the millimeter wave band, the short wavelength allows for a large number of antenna elements to be arranged. The more elements an antenna has, the more effective the beamforming becomes. An antenna that achieves beamforming by arranging many elements and controlling their phases is called a phased array antenna.

As shown in Figure 3, a phased array antenna is composed of numerous phase shifters, amplifiers, and antenna elements, resulting in a complex structure and relatively high power consumption. On the other hand, with RIS antennas, installing the RIS near the antenna and controlling the reflection direction of the radio waves allows even base stations with fewer amplifiers and antenna elements to achieve the same functionality as a highly-element phased array antenna.
Thus, RIS antennas simplify the structure of millimeter wave base stations by reducing complex circuitry involving phase shifters and amplifiers. They are anticipated to be a technology that can reduce equipment costs and power consumption.

Figure 3. Phased Array Antenna and RIS Antenna

4. Design of RIS Antennas

At SoftBank, we are advancing research and development from the design stage of RIS antennas. Figure 4 shows the simulation results of the radiation pattern of a millimeter wave base station with and without RIS. The results indicate that placing the RIS near the antenna successfully enhances the antenna gain. Additionally, in Figure 5, by altering the surface phase distribution of the RIS, we have succeeded in changing the beam direction of the RIS antenna. The graph demonstrates that by setting the surface phase of the RIS to direct the beam at a 30-degree angle, we can control the beam direction.

Figure 4. Simulation Results of Antenna Gain with and without RIS

Figure 5. Beam Direction Control by Adjusting Surface Phase (30-degree direction)

These results demonstrate that RIS antennas can achieve both high antenna gain and beam direction control comparable to phased array antennas, without requiring the numerous phase shifters and amplifiers that make up traditional phased array antennas.

5. Research and Development of Polarization-Separated RIS

Furthermore, at SoftBank, we are conducting research and development on RIS antennas capable of separating beam directions by polarization to improve frequency utilization efficiency. As shown in Figure 6, radio propagation simulations have confirmed that directing beams formed by the RIS antenna in different directions for vertical and horizontal polarizations respectively can improve frequency utilization efficiency.

When attempting to realize such RIS antennas with these characteristics, using passive elements like those shown in Figure 2 posed a challenge, as the passive elements respond to both polarizations, making it difficult to direct beams in different directions for each polarization. To overcome this challenge and achieve RIS antennas that independently control each polarization, we have been conducting joint research with Kumamoto University. Specifically, as shown in Figure 7, we are considering an antenna structure that forms an RIS using passive elements that respond to vertical polarization and passive elements that respond to horizontal polarization, thereby providing different surface phases for each polarization.
Using this structure, it becomes possible to direct beams in different directions for each polarization. Looking ahead, we expect that by combining RIS antennas capable of directing beams in various directions for each polarization with AI technologies such as deep learning, we can perform beamforming even more efficiently.

Figure 6. MIMO Communication Using Polarization-Independent Control RIS Antenna

Figure 7. Structure of Polarization-Independent Control RIS Antenna

In this article, we have introduced RIS antenna technology. At SoftBank, we will continue to explore various technological elements in our pursuit of providing highly efficient wireless communications.