Japan's NICT realizes high-speed rail seamless large-capacity communication with fiber-optic radio technology

Japan, April 26, according to the Japan Information and Communications Research Institute (NICT), the NICT network system laboratory uses wireless base station and fiber-optic radio technology along the railway line to superimpose wireless signals on optical fibers for development and research. Stable communication during high speed movement. The lab developed a method to properly switch wireless base stations, achieve uninterrupted communication during high-speed mobility, and successfully transmit 20 Gbps wireless signals, approximately 20 times faster than current mobile phone line rates.

The success of this experiment shows that even in moving objects such as ultra-high-speed railways, smooth high-speed communication can be achieved at a speed of more than 500 kilometers by continuously switching wireless base stations. This research paper was selected for the most popular title paper in OFC2018 with very high value.

(Thesis: High-Speed ​​and Handover-Free Communications for High-Speed ​​Trains Using Switched WDM Fiber-Wireless System)

Experimental background

As smartphones become more popular, there is a growing demand for stable communications on high-speed rail travel or similar environments. However, during high-speed travel, when the connected stations frequently switch, the signal connection often encounters interference. Japan's NICT jointly undertakes research and development projects for wireless spectrum resource expansion - "Research and Development of High-Speed ​​Mobile Backhaul Technology in Millimeter Bands" (research representative: Hitachi International Electric) and network technology research and development to continuously promote high-speed rail uninterrupted signals. At the same time, NICT has embarked on the development of a network with optical fiber communication capabilities and high-capacity wireless transmission capabilities with millimeter-wave wireless wave characteristics.

Development result

Figure 1 Schematic diagram of high-speed railway communication system

NICT developed a high-speed rail communication system (Figure 1) that is essential in fiber-optic wireless networks and successfully transmits 20 Gbps wireless signals between wireless base stations. The new technology is as follows:

1. As a technique for seamlessly switching wireless base stations, it is necessary to assign optical signals of different wavelengths to each wireless base station, and switch the wavelength of light to be transmitted at a high speed according to the position of the train (Fig. 2). One method is to control the optical signal, transmit it to a radio base station and reduce radio signal interference from the two radio base stations. (image 3)

Second, high-capacity wireless communication technology uses millimeter wave (MMW) signals (Figure 4)

In general, in the high-speed rail system, since the high-speed rail position information is controlled by the driving command room, it is possible to determine the wireless base station distribution signal based on the position information. It is possible to construct a communication system with an uninterrupted signal by the wireless base station to distribute the signal to the train close to the train in time, just as the wireless base station follows the train movement.

This basic technology, if the millimeter-wave radio base station is separated by 1000 meters, when the running speed reaches 500 km/h (the two base stations pass the time is about 7 seconds), it is expected to transmit 20 Gbits per second when switching the radio base station. .

Future prospects

In the future, the Ministry of Internal Affairs and Communications of Japan will improve the fiber-optic wireless network technology to expand the wireless spectrum through the “Research and Development Project of Wireless Spectrum Expansion High-Speed ​​Mobile Backhaul Technology”. Other institutions such as Hitachi International Electric, Public Interest Foundation Comprehensive Railway Technology Laboratory, and National Research Cooperate with the development company's marine/port/aerospace laboratory/aerospace laboratory and the National Aeronautical and Maritime Research Laboratory to conduct demonstration tests on actual railway lines, accelerate joint research and development, and achieve social production-scholar-official cooperation.

Description of research results

1. High-speed wireless base station switching mode using fiber-optic wireless technology

Figure 2 Schematic diagram of wireless base station handover

FIG. 2 is a schematic diagram of the development of the high-speed wireless base station switching technology, in which an antenna corresponding to the wavelength of light emitted from the laser is set in the wavelength switch.

1 When moving to the wireless base station 1

Under the control of the controller, the laser transmits the optical wavelengths of the radio base station 1 and the base station 2 and transmits the same information.

2 Transfer to the area of ​​the wireless base station 2

Under the control of the controller, the laser stops transmitting the wavelength of the radio base station 1, changes the wavelength of the optical signal of the base station 3, and transmits the same information.

Second, a way to reduce the interference of wireless signals from two wireless base stations

Figure 3 Schematic diagram of the experimental setup and experimental results

Figure 3 is a schematic diagram of the structure of the experiment, the result of which is a received waveform. By performing the transmission control, the timings of the radio waves from the antenna 1 and the antenna 2 are made uniform, and the original waveform can be reproduced without distortion even when the two waveforms overlap. The result of this experiment is that radio wave interference from two radio base stations is greatly reduced, and interference-free communication is expected to be realized.

Third, the use of millimeter wave (MMW) high-capacity wireless communication technology

Figure 4 Schematic diagram of transmitting millimeter waves using fiber-optic wireless technology

Figure 4 shows NICT transmitting images of millimeter waves using the fiber optic radio technology we have developed so far. The difference between the two optical frequencies is set equal to the frequency of the radio wave, and an optical signal having a smaller frequency fluctuation is generated and transmitted in the optical fiber. In a wireless base station, an electrical signal in a millimeter wave band is obtained by interference of two light components, and after amplification, it is emitted as a radio wave from an antenna. Instead, it is also possible to receive radio waves in the millimeter band, convert them into optical signals, and transmit them to the fiber. Since fiber optics have very small propagation losses compared to copper cables, fiber-optic radio technology has the feature of transmitting high-frequency electrical signals over long distances.

Glossary

Base station (cell) mode

The wireless base stations are arranged in a straight line in their coverage area. In a conventional mobile radio, it is necessary to place it on a plane to cover the surface, but railroads, highways, etc., will line a number of elongated units in a straight line. By limiting the radiation direction of the radio signal and increasing the directivity of the antenna, a cell of several hundred meters or more can be realized even if a millimeter wave of a short distance is usually reached. In addition, since the direction of movement is known in the railway, the next mobile cell is easier to predict than conventional mobile radios, thereby achieving efficient signal distribution.

Fiber optic radio technology

A technique in which an optical signal is modulated by a fiber using a radio signal to directly transmit a radio signal. This technology has been used in mobile and terrestrial digital broadcasting systems in areas with poor radio coverage.

Wireless base station handover

In mobile communication and the like environment, when the terminal moves from the area covered by the radio base station to the next base station area, the information exchange of the terminal occurs on the upstream side of the radio base station. In switching, data communication interruptions typically occur between a few milliseconds and a few seconds, and effective data communication speeds are reduced. In addition, in order to realize high-capacity wireless communication in the future, it is necessary to use high-frequency radio waves, but usually, the arrival distance of high-frequency radio waves is short, so it is necessary to match a plurality of base stations. Especially in high-speed railways, frequent switching of radio base stations leads to an increase in communication interruption time and a decrease in connectivity. In this system, since the location information is known in advance, communication interruption can be minimized by performing appropriate signal distribution based on the information.