# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 3 RIS Field Trials
### Field Test Reviews  
In November 2021, NTT and NTT Docomo announced the world's first use of 5G base stations and RIS metasurface reflectors in the 28 GHz band to improve indoor communications by dynamically changing the direction of radio waves as mobile devices move in an indoor environment [156]. NTT's reflection control technology dynamically controls the direction of reflection based on the movement of the receiving device, and is applied to the millimeter-wave RIS reflector device developed by AGC Inc. (Tokyo, Japan) to create an indoor mobile environment that enables the use of high-frequency bands for 5G and next-generation mobile communications. The test was conducted at NTT Yokosuka R&D Center in Japan from October 8 to 22 with the measurement setup shown in Figure 37. Docomo designed the indoor network and operated the base station. The tests confirmed that the RIS reflector improves the strength of the signal received by the moving receiver. Radio waves from a remote base station entered the test room through a window, and the RIS was used to control the direction of reflection. From the measurements in Figure 38, the strength of the received 28 GHz signal was improved by up to 20 dB compared to the signal received without the RIS. In China, China Mobile conducted the world's first field trial for RIS application in the urban area of Nanjing to investigate the practical performance of coverage improvement in 5G commercial mobile networks [157]. A 32 \(\times\) 16 (1.6 m \(\times\) 0.8 m) RIS with a phase resolution of 1 bit in each unit cell and independently programmable was introduced and utilized in the field trial, as shown in Figure 39. In addition, three common under-coverage scenarios, known as under tower shadow zone, indoor shadow zone, and outdoor building shadow zone, were selected as test sites to verify the improvement capabilities of the RIS, as shown in Figure 40. The measurement results confirm that the application of the RIS can significantly improve the communication quality for users in current 5G commercial mobile networks. In the shadow zone under the building, deploying the RIS increased the reference signal receiving power (RSRP) of the user at the edge of the cell by 4.03 dB and the average RSRP by 3.8 dB, and the average throughput of the user increased from 91.50 Mbps to 109 Mbps, which is an improvement of about 19%. In indoor shadow zone, most test locations, including meeting rooms, offices, and supermarkets, showed significant improvements, with average gains of 9.9 dB in RSRP, 2.87 dB in signal-to-interference-plus-noise ratio (SINR), and 78.61 Mbps in throughput. However, testing in the studio did not show any significant improvement with the deployment of the RIS, which may be due to the limited beamwidth of the RIS reflector beam not being able to reach the area, indicating that it may be necessary to deploy the RIS inside the building. With respect to the shaded area at the outdoor of the building, gains of 3.3 dB, 1.45 dB, and 79 Mbps in the RSRP, SINR, and throughput were observed, respectively, when the RIS was placed in the area where the RSRP was lower than \(-90\) dBm [157]. Figure 41 shows the comparative performance of the RSRP, SINR, and throughput with and without the RIS in the shadowed area under the building and the shadowed area behind the building. The significance of these experimental results is that the aRIS was tested using a working 5G base station. Since the RIS with a 1-bit phase resolution used in the experiment can generate non-negligible sidelobes when performing beamforming, causing electromagnetic pollution, signal interference, and energy consumption, higher performance improvements can be achieved through joint optimization of the BS, RIS, and UE. Figure 38: Received signal strength of the received 28 GHz signal with and without RIS [156]. Figure 37: Testbeds for RIS system evaluation [156].