A wireless distance-measuring method is proposed, which is a radio communication device of the license-free 2.4-GHz band spread spectrum (SS) incorporated distance-measuring function. Spread spectrum communication is a transmission system that enables sharing of broad band by converting narrow-band signals into broad-band signals using spread codes as identification codes. The system is used for portable telephones and other transmission media because of its high-security and disturbance-withstanding capability, and is also used in GPS as it enables to measure distance based on the delay time of radio wave propagation.
Figure 1 shows the principle of the distance-measuring method. Radio communication device A transmits spread modulated information to radio communication device B, which extracts the information in inverse-spread modulating the received signals. Device B is then synchronized with device A. Device B then returns the information necessary to send to device A, which receives the information and determines the distance between A and B based on the time lag from the original synchronization signal. Since this method is made according to the time lag of the round-trip transmission, the distance can be measured only by device A. The devices however do not require high-precision clocks. The resolution of the distance measuring is determined by the time width of one spread code, the unit of time lag measurement. It can be 11.6m in this case.
In car running tests, it is confirmed that the maximum measurable distance of 760 m when it is clear between two radio communication devices, with errors smaller than the resolution of distance-measuring. This system is therefore applicable to the detection of train position.
A multi-channel system using different spread codes for each channel is also studied, and it is estimated that approximately 20 channels can be used according to the S/N ratio and other data obtained from field tests. This means that the system can work with multiple trains running in the same radio telecommunication area, including quadruple track sections. Some applications using this system along railway line are proposed, for example, level crossing system for precise control of the alarm start time, and ATS system for continuous control without ground coils (Fig. 2).