NTT Corp., NTT DOCOMO and NEC Demonstrate Distributed MIMO Technology for High-Frequency 6G Communications in Automobiles and Trains
—Stable communications even when vehicle is traveling at high-speed or shielded by other vehicles—

Highlights:

TOKYO, JAPAN, March 25, 2025 - NTT Corporation, NTT DOCOMO, INC. and NEC Corporation announced today their successful demonstration of a distributed MIMO¹ technology system that suppresses the degradation of communication quality in the 40 GHz band when a large number of antennas are distributed among multiple locations. The technology selects the most appropriate antennas and beams² at high speed and uses frequency- and timing-compensation technology to prevent significant changes in reception frequency and timing on the mobile-terminal side, even if the terminals are moving at high speed or are shielded by other vehicles moving at high speed. The demonstration verified the potential of millimeter-wave distributed MIMO to achieve stable, high-capacity wireless communications in high-speed moving environments, such as automobiles and trains, for purposes such as providing large-volume content to many passengers and collecting massive sensing data for automated driving.
　In the future, the three companies will proceed with verifications of actual services on roads and railways, aiming to eventually implement high-frequency distributed MIMO in high-speed mobile environments, and will also consider the application of distributed MIMO to a wider range of frequency bands, such as the mid-band spectrum,³ to expand distributed MIMO use.
　The results of this demonstration will be exhibited at Tsukuba Forum 2025 on May 15 and 16 and Wireless Japan 2025 × Wireless Technology Park (WTP) 2025 (Japanese) from May 28-30.

1. Background

5G Evolution & 6G will enable the distribution of video and sensing information in a world where cyberspace and physical space are fused; Vehicle to X (V2X) for autonomous driving by linking vehicles and everything else; and multi-sensory communications including the five senses, atmosphere and sense of security. Such technologies will require increasingly faster and larger-capacity wireless communications. To this end, efforts are under way to make effective use of the millimeter wave band, which became available with 5G, and even higher frequency bands for mobile communications.

Since radio waves in high-frequency bands have high linearity, shielding measures are important. One promising solution⁴ is a high-frequency distributed MIMO system in which a single base station uses a large number of antennas distributed among various locations for transmission to each mobile terminal. In June 2022, NTT, DOCOMO and NEC announced a joint demonstration of high-frequency distributed MIMO technology. As part of this initiative, a demonstration of antenna-selection technology based on terminal-movement prediction, stable high-capacity radio transmission was achieved in a shielded indoor environment.⁵ In addition, in a separate demonstration of multi-user transmission technology, which fully utilizes the interference-suppression effect of analog beamforming at each distributed antenna when multiple mobile terminals simultaneously transmit on the same frequency channel, achieving the same radio transmission capacity when moving at walking speed as when stationary.⁶
　Promising use cases for high-frequency wireless communication with increased capacity include high-speed vehicles such as cars, trains, etc. In high-speed mobile environments, rapid changes in communication conditions occur due to both the movement of the vehicle itself and surrounding high-speed vehicles. However, as the number of distributed antennas increase, the number of candidate antennas becomes enormous (hereinafter, "Problem 1"). In addition, switching among distributed antennas during high-speed movement causes rapid changes in the Doppler frequency⁷ and propagation delays, resulting in a significant degradation of communication quality (hereinafter, "Problem 2").

2. Demonstration Details

■ Distributed Antennas Simultaneous Beam-search Technology (Technology 1)

To solve Problem 1, NTT and DOCOMO have developed a beam-search technology that can rapidly detect the optimum combination of distributed antennas and beams using the same beam identification signal⁸ at the same time and frequency for multiple distributed antennas. When a vehicle traveling at approximately 50 km/h shields a stationary mobile terminal and a distributed antenna, the time required to detect the optimum combination using a conventional search of all combinations is proportional to the number of distributed antennas (four times longer in the demonstration), resulting in the degradation of communication quality. However, Technology 1 enabled the optimum combination to be detected in the same time as for one distributed antenna, thereby suppressing the degradation of communication quality.

Overview of Technology:
　For high-frequency distributed MIMO transmission, it is necessary to detect the optimal combination of distributed antennas and beams in response to changes in the radio communication environment for each mobile terminal. For this purpose, it is necessary to periodically observe the radio communication quality of each combination of distributed antennas and beams. Conventionally, the mobile terminal observes the radio communication quality of all combinations of distributed antennas and beams by transmitting beam identification signals while switching the beams for each distributed antenna to avoid interference between distributed antennas. However, this method slows the detection of an optimal antenna and beam due to increased measurement time as the number of distributed antennas increases. In response, NTT and DOCOMO devised a method in which distributed antennas switch beams and transmit the same beam identification signal, both at the same time, thereby keeping the measurement time constant even if the number of distributed antennas increases. Since the beam identification signals do not interfere with each other, a mobile terminal can identify the beams and select the appropriate distributed antenna and beam based on the combined reception quality of the beam identification signal from each distributed antenna. (Fig. 1)

This method is expected to enable the appropriate beam to be selected for multiple distributed antennas in the same time as a search for one distributed antenna.

Figure 1: Distributed Antenna Simultaneous Beam-search Technology

Test and Result:
　Technology 1 was demonstrated on a test track of the National Institute of Land, Infrastructure, Technology and Policy, which is under the Japanese Ministry of Land, Infrastructure, Transport and Tourism (Figure 2-1). The frequency band of the equipment was 40 GHz, which, together with higher frequency bands,⁹ is assumed to be used in the future, in addition to the 28 GHz band currently used for millimeter-wave 5G in Japan. Other physical specifications were based on a 5G NR (New Radio), OFDM¹⁰ system with a signal band of 100 MHz and a subcarrier interval of 60 kHz. In addition, the base station equipment was connected to multiple distributed antennas using analog – radio over fiber (A-RoF), which transmits intermediate frequency (IF) signals¹¹ from the base station equipment in analog via optical fiber. Four distributed antennas were installed on both sides of the road positions No. 1 to No. 4, and mobile terminals were installed on a vehicle's ceiling, with two antennas facing forward as shown in Figure 2-1. In addition, a large vehicle moving approximately 50 km/h was introduced as a fast-moving shield, as shown in the same figure.

Throughput characteristics were evaluated in the case of using Technology 1 for antenna and beam selection, and the case of each distributed antenna being selected by observing the communication quality of each beam in turn (hereinafter referred to as "conventional technology") (Fig. 2-2). The number of candidate beams on the distributed antenna side was 16, and the number of candidate beams on the mobile terminal side was 9. The antenna and beam selection period was set to 120 ms for Technology 1 and 480 ms for the conventional technology based on the measurement time. In Fig. 2-2, the horizontal axis of the graph shows the time and the vertical axis shows downlink throughput. With conventional technology, which takes time to observe the communication quality of all beams, there was a section where throughput dropped significantly by about 350 ms until the antenna and beam were switched, after being shielded by a large vehicle. On the other hand, Technology 1 observed all antennas and beams in the time equivalent to one distributed antenna, which avoided throughput dropping below 100Mbps. Also, the drop time was reduced to about 100 ms, or about 1/4 of the conventional time. Technology 1 was demonstrated to reduce the selection time of antennas and beams regardless of the number of distributed antennas, so it is expected to be more effective when more distributed antennas are deployed.

Figure 2-1: Experimental Environment for Technology 1

Figure 2-2: Demonstration Results - Conventional Technology vs. Technology 1 Throughputs

■ Frequency and Timing Pre-Compensation Technology (Technology 2)

To solve Problem 2, NEC developed a technology for distributed antennas on the base station side to cooperate in order to pre-compensate the reception frequency and timing of the mobile terminal, thereby realizing user-centric communications so that any change in Doppler frequency or propagation delay due to high-speed movement was not noticeable on the mobile terminal side. Using Technology 2, throughput reduction was suppressed during antenna switching, and stable high-capacity communications were achieved even when the mobile terminals were traveling at 100 km/h.

Overview of Technology:
　With conventional technology, a mobile terminal communicates with a base station by aligning the frequency and timing of signals from the base station. However, if a terminal is in a fast-moving automobile, train, etc., the Doppler frequency and propagation delay on the mobile terminal side differs with each distributed antenna at the time of antenna switching. The result is degraded communication quality. With Technology 2, the base station side measures the change in the Doppler frequency and propagation delay for each distributed antenna using the uplink reference signal, and cooperates with the various distributed antennas to compensate transmission frequency and transmission timing in advance, allowing reception frequency and timing on the mobile terminal side to remain constant (Fig. 3). Technology 2, by suppressing changes in Doppler frequency and propagation delay at the time of antenna switching, allows the mobile terminal to adjust the frequency and timing of radio signals from the base station as before, thereby achieving user-centric wireless communications that eliminates the perceptible effects of high-speed movement on the mobile terminals side.

Figure 3: Frequency and Timing Pre-Compensation Technology Cooperating Among Antennas

Test and Results:
　The Technology 2 demonstration was carried out on the same test track as Technology 1, using the same experimental equipment and mobile-terminal vehicles. As shown in Figure 4-1, a high-speed mobile environment was simulated by installing three distributed antennas at positions No.1 to No.3, in 150 m intervals on one side of the road. Using A-RoF to connect the base station to the distributed antennas, as with Technology 1, a downlink transmission was conducted by driving a mobile-terminal vehicle at 100 km/h.

Downlink throughput using conventional technology, in which frequency and timing are compensated only on the mobile terminal side, was compared with throughput using Technology 2, in which frequency and timing are compensated in advance among cooperating antennas on the base station side. As shown in Figure 4-2, conventional technology throughput dropped significantly to about 10Mbps when the antenna was switched, but with Technology 2, throughput drop during antenna switching was suppressed to maintain throughput at about 100Mbps. The result was stable high-capacity communication using millimeter waves even during high-speed travel.

In the future, point-cloud data from the demonstration will be used to evaluate radio performance in a system-level simulation, based on which the integration of real and cyber space will be further studied.

Figure 4-1: Experimental Environment for Technology 2

Figure 4-2: Demonstration Results - Conventional Technology vs. Technology 2 Throughputs

Video of Technology 2 demonstration: https://www.nec.com/en/global/onlinetv/en/emmwave.html.

3. Future Outlook

In the future, demonstrations of real service environments will be conducted with automobiles, buses, railways, etc., as promising uses of the increased capacity of millimeter waves. In addition, R&D will be conducted to confirm additional applications of high-frequency band distributed MIMO systems and to promote the use of millimeter waves in society.

¹Distributed MIMO (Multiple-Input Multiple-Output) is technology in which a large number of antennas are distributed from a single base station within an area for MIMO transmission between each antenna and a mobile terminal within the area.

²Beam transmission technology that transmits the same signal from multiple antenna elements and adjusts the phase difference between each element to increase the level of transmission and reception in a specific direction. This specific direction can also be changed by changing the phase difference.

³A frequency band from 7.1 GHz to 24.25 GHz, which lies between sub-6 GHz and millimeter waves used in 5G, and is being considered for use in 6G.

⁴White Paper on 5G Advancement and 6G, NTT DOCOMO, INC. November 2022 (5.0 Edition)
https://www.docomo.ne.jp/corporate/technology/whitepaper_6g/

⁵https://group.ntt/en/newsrelease/2022/10/31/221031a.html

⁶https://group.ntt/en/newsrelease/2023/10/31/231031a.html

⁷Frequency that changes when a moving terminal emits radio waves, increasing when approaching a receiving point and decreasing when moving away.

⁸Signal used to identify a beam and observe its communication quality. In 5G NR, SSB and CSI-RS are used.

⁹Analog beamforming ICs for 40 GHz band distributed antennas used in the demonstration were developed under a research project sponsored by the Ministry of Internal Affairs and Communications, "Research and Development for Further Advancement of Generation 5 Mobile Communication Systems (JPJ000254)," conducted by NEC Corporation.

¹⁰Orthogonal Frequency Division Multiplexing.

¹¹When converting frequencies between baseband and radio frequency bands during digital signal processing, frequency conversion is often performed on the intermediate frequency band to determine the implementation impact.

About NTT

NTT contributes to a sustainable society through the power of innovation. We are a leading global technology company providing services to consumers and businesses as a mobile operator, infrastructure, networks, applications, and consulting provider. Our offerings include digital business consulting, managed application services, workplace and cloud solutions, data center and edge computing, all supported by our deep global industry expertise. We are over $93B in revenue and 330,000 employees, with $3.6B in annual R&D investments. Our operations span across 80+ countries and regions, allowing us to serve clients in over 190 of them. We serve over 75% of Fortune Global 100 companies, thousands of other enterprise and government clients and millions of consumers.

About NTT DOCOMO

NTT DOCOMO, Japan's leading mobile operator with over 90 million subscribers, is one of the global leaders in 3G, 4G and 5G mobile network technologies. Under the slogan "Bridging Worlds for Wonder & Happiness," DOCOMO is actively collaborating with global partners to expand its business scope from mobile services to comprehensive solutions, aiming to deliver unsurpassed value and drive innovation in technology and communications, ultimately to support positive change and advancement in global society.
https://www.docomo.ne.jp/english/

About NEC Corporation

NEC Corporation has established itself as a leader in the integration of IT and network technologies while promoting the brand statement of "Orchestrating a brighter world." NEC enables businesses and communities to adapt to rapid changes taking place in both society and the market as it provides for the social values of safety, security, fairness and efficiency to promote a more sustainable world where everyone has the chance to reach their full potential. For more information, visit NEC at https://www.nec.com.