Novel Distance Measurement Method Using Inband-RFID-Technology

For many RFID applications it is desirable to get more information about an item than the unique ID, e.g. the position of a unique item often is of great interest. In this paper we present first simulation results of a novel method for measuring the distance between a reader and commercial off-the-shelf tags. This implies that the communication between a reader and a tag or a bulk of tags is not influenced or interrupted by this novel distance measuring method. It is expected to achieve a distance measuring accuracy of about 1% referred to the maximum communication distance between a reader and a tag. Additionally to the state-of-the-art RFID communication principle, which uses a well-known amplitude modulated (amplitude modulated due to the data transmission from reader to tag) RFID carrier, this novel measuring method uses an overlaid phase modulation. For the measuring principle to work it is important that the wavelength corresponding to the data rate of the phase modulation is not higher than twice the distance between reader and tag. If this condition is truly fulfilled, the phase information of the phase modulated signal will be clearly corresponding to the distance between reader and tag. The reader transmits the phase information to the tag which is returned to the reader due to the backscatter modulation. In a coherent reader structure the phase difference between the transmitted and received baseband signal can be used to calculate the distance from reader to tag. This distance calculation could be realized by correlation. Therefore the cross-correlated of the transmitted and received phase information can be subtracted from the auto correlation of the transmitted phase information. In case of this the distance can be calculated.