Question

I am in charge of the information communication in transportation network and interested in the mobile communication network using Infrared Wireless Communication. What is the maximum speed of mobile vehicle (car, railway train, etc.) that can maintain the stable communication link with the fixed hub on the ground?  I think that the speed of 300km/h is necessary in the case of the railway new line.

Answer

Infrared Communication has the sharp directivity (between 0.1and 20 degree in commercial products) and the communication is performed in the physically restricted area. In the case of Infrared Beacon between road and vehicle, Infrared Beam is radiated just like spotlight and the communication is performed while the vehicle passes over in the beam. It is the burst type communication like ETC (Electronic Toll Collection System).

Mobile Communication using Infrared has the portability, but the narrow band communication is the main type of the signal transmission because of the above mentioned reason.

The relative factors of the vehicle speed are as follows.

• Amount of information
• Transmission speed
• Transmission time (the time decided by the beam size and the vehicle speed)

In the case of Infrared Beacon 70km/h of the vehicle speed is the specification of the system, but the speed more than 300km/h is possible by reducing the amount of information. The explanation is shown in the following figure.

＊	Vehicle speed
	V (km/h)=V･1000/3600 (m/s)= V･5/18 (m/s)
＊	Pass-over time in the beam
	t=X/(V･5/18) (s)
＊	Amount of information that vehicle can transmit during pass-over time in the beam
	P=Y･t=X･Y/(V･5/18) (Mbit)
	（X: Communication area size (m), Y: Transmission speed (Mbps)）

for example Vehicle speed:100km/h, Communication area size:10m, Transmission speed:10Mbps

Amount of information P=10 (Mbps)×10 (m)／(100･5/18) (m/s)=3.6 (Mbit)