We propose a new ground-to-train communication system using free-space optics between a train and the ground. In the proposed system, a cylindrical concave lens spreads the incoming beam from transmitter (Laser Diode: LD) horizontally to form a wide fan-shaped beam. The fan-shaped beam is projected to a train and the width of the projected beam is equal to the length of a typical bullet-train car. This transmitter with cylindrical concave lens and a receiver (Avalanche Photo Diode: APD) are installed at a train and the ground, and the horizontally spread beam is received continuously by corresponding receiver. The train can keep a communication link continuously to the ground thanks to this spread beam. We analyzed the performance of the proposed system by experiments. The experimental results show that a sufficient received Signal-to-Noise Ratio (SNR) can be obtained when a train is not moving; therefore, the signal can be received continuously even when the train is moving. Furthermore, in order to keep a continuous communication link even when the train vibrates or shifts vertically, the vertical spread angle of a laser beam is adjusted between 0.1 degrees and 0.5 degrees. These experimental results lead to the conclusion that this system is expected as a Giga-bit class high speed communication technology between the train and the ground.
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