Towards Uninterrupted Communication for Users Moving at 500 km Per Hour:

Highlights

High-capacity transmissions of 20 Gbit/s over a converged fiber-wireless network in the 90-GHz band.
Ultra-fast remote radio station switching in less than 10 μs to realize a handover-free communication network.
Possibility for developing smooth communications for high-speed trains, even while they move at high speeds of 500 km/h or more.

Summary

NICT Network System Research Institute performed a proof-of-concept demonstration of an uninterrupted communication system for high-speed trains by adopting a combination of a linear cell network configuration, a high-speed seamless fiber-wireless system in the millimeter-wave (mmWave) band, and an ultra-fast optical-path switching technique. We successfully transmitted data at a rate of 20 Gbit/s from each remote radio station using a wavelength-division-multiplexing fiber-wireless network for the transmission of mmWave signals in the 90-GHz band. Ultra-fast switching of the optical paths to the remote radio stations in less than 10 μs was also successfully demonstrated using ultra-fast wavelength-tunable lasers, indicating that a smooth and uninterrupted communication (handover-free) network can be realized for high-speed railways.

Handover-free communication has been considered a big challenge for avoiding significant degradation of the throughput of high-mobility users, which includes users on high-speed trains, due to the frequently interrupted connections with radio stations. The use of seamless convergence of fiber-optic and wireless networks in high-frequency mmWave bands shows that such a challenge can be overcome, even when the trains are moving at high speeds of 500 km/h or faster.

The results of this demonstration were published as a post-deadline paper at the 41st Optical Fiber Communication Conference and Exhibition (OFC2018).

Background

The demands for high-speed and smooth communications are rapidly increasing, even from users who are on highly moving vehicles, such as high-speed trains, because of the explosive popularization of smartphones and other personal multimedia devices. In current cellular networks, however, connections to internet networks during high-speed movement are frequently interrupted because of radio station switching (handover). To overcome this limitation, we developed a high-speed and handover-free communication network for high-speed trains using a seamless wavelength-division-multiplexing (WDM) radio-over-fiber (RoF) and wireless network in the high-frequency bands. This work was conducted as a part of a project titled “Research and development of millimeter-wave backhaul technology for high-speed vehicles”, funded by the Ministry of Internal Affairs and Communications (MIC), Japan (Research representative: Hitachi Kokusai Electric Inc.).

Achievements

Fig. 1. Image of a communication system for high-speed railways.

In this work, NICT developed a technology to transmit 20-Gbit/s radio signals in the 90-GHz band from a central station to 50 remote radio stations using a switchable WDM-RoF and mmWave wireless network. The switching of the remote radio stations in accordance with the movement of trains can be controlled from the central station, and a switching time of less than 10 μs was achieved using high-speed wavelength-tunable lasers.

The system consists of the following principal technologies:

► High-speed wavelength tunable laser sources.

► 16-QAM multilevel modulation/demodulation technology with a sampling speed of 5 GHz.

► High-speed optical-to-electrical converter for mmWave signal generation.

► Linear cell configuration for signal distribution to railway tracks.

In high-speed railways, the remote radio station that should be activated to communicate with a train can be determined precisely using information about the location of the train from a train operation direction center. By distributing signals to radio stations appropriately, a smooth and uninterrupted (handover-free) communication system can be realized. In addition, owing to the use of a centralized network, remote radio stations can be greatly simplified and, thus, the cost and power consumption of the system can be significantly reduced.

Future Prospects

In the future, in collaboration with Hitachi Kokusai Electric Inc., the Railway Technical Research Institute, the Electronic Navigation Research Institute (part of the National Institute of Maritime, Port and Aviation Technology), and other related parties in the aforementioned MIC-funded project, we will implement field test demonstrations on actual railway lines.

Reference

Pham Tien Dat, Atsushi Kanno, Keizo Inagaki, Toshimasa Umezawa, Fançoir Rottenberg, Jérome Louveaux, Naokatsu Yamamoto, Tetsuya Kawanishi, “High-Speed and Handover-Free Communications for High-Speed Trains Using Switched WDM Fiber-Wireless System,” in Proc. 41st Optical Fiber Communication Conference and Exhibition (OFC), March 2018, paper Th4D. 2.

Appendix

1. High-speed linear cell communication system

Fig. 2 shows a conceptual diagram of the proposed system using a linear cell network and the convergence of a WDM RoF and wireless system in the mmWave band. mmWave radio stations are located linearly along the railway track and communicate with the trains via mmWave backhaul links. These signals are generated and transmitted from a central station via a WDM RoF system. In high-speed railways, the radio stations that the trains are approaching can be precisely predicted using train information such as train location and velocity, which is available at the train operation center. Thus, appropriate signal distribution to the corresponding radio stations can be performed by means of high-speed optical switching technologies, such as high-speed wavelength-tunable lasers. With this network configuration, the development of an uninterrupted network for high-mobility users can be implemented in an easier way than in standard cellular networks because the necessary control signals are available at central stations.

Fig. 2. Conceptual diagram of the proposed communication system for high-speed railways.

2.　Experimental results

Fig. 3. Spectra of the modulated optical signals and performance of 16-QAM signals.

Fig. 4. Radio station switching and performance of 16-QAM signals. — Fig. 4. Radio station switching and
performance of 16-QAM signals.

In this demonstration, 20-Gbit/s signals (16-QAM, in the microwave band with a sampling rate of 5 GHz) were transmitted over a WDM RoF network and distributed to 50 remote radio stations by switching wavelengths from laser sources (See Fig. 3). To reduce fiber dispersion effects, single-sideband optically modulated signals were transmitted over the WDM RoF system. At the remote radio stations, the signals were directly up-converted to the 90-GHz band using reference signals, which could be generated and distributed from the central station.

In addition, by using wavelength-tunable lasers that can switch their wavelength at high speed, we demonstrated that the signal distribution to radio stations can be switched in less than 10 μs (See Fig. 4). This indicates that an uninterrupted communication system for high-speed railways can be constructed, even for trains moving at 500 km/h or faster.