High-speed comms for high-speed trains
Ultrafast remote radio station switching can provide smooth communications for trains moving at 500 km/h or more.
The demand for smooth, high-speed communications is rapidly increasing, especially from users on fast-moving vehicles such as high-speed trains, due to the popularity of smartphones and other devices.
In current cellular networks, however, connections to internet networks during high-speed travel are frequently interrupted because of radio station handovers.
To overcome this limitation, researchers at Japan’s NICT Network System Research Institute developed a high-speed, handover-free communication network for high-speed trains using a seamless wavelength-division-multiplexing (WDM) radio-over-fibre (RoF) and wireless network in EHF bands.
In particular, the proof-of-concept demonstration adopted a combination of a linear cell network configuration, a high-speed seamless fibre-wireless system in the millimetre-wave (mmWave) band and an ultrafast optical-path switching technique.
In this work, NICT developed a technology to transmit 20 Gbps radio signals in the 90 GHz band from a central station to 50 remote radio stations using the switchable WDM-RoF and mmWave wireless network.
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.
To reduce fibre 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.
The system comprised 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 the train control centre. By distributing signals to radio stations appropriately, a smooth and uninterrupted (handover-free) communication system can be achieved.
In addition, owing to the use of a centralised network, remote radio stations can be greatly simplified and, thus, the cost and power consumption of the system can be significantly reduced.
The use of seamless convergence of fibre-optic and wireless networks in high-frequency mmWave bands shows that the challenge of high-speed transport communications can be overcome, even when trains are moving at speeds of 500 km/h or faster.
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, NICT aims to conduct field tests on real railway systems.
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