Spread-Spectrum Based Ranging of Passive UHF EPC RFID Tags

This letter introduces a ranging method which can be applied to off-the-shelf backscatter RFID tags, like standardized EPC Gen 2 tags. For this purpose a low power direct sequence spread spectrum (DSSS) sequence is superimposed onto the interrogator´s signal during tag to reader communication. The chip rate and sequence length is chosen to fit an entire DSSS sequence into each backscattered bit. During the tag to reader communication, the bits sent by the tag are decoded and a time domain recording with the length of one DSSS period is made for each bit. By cyclic alignment of each recording and subtracting the averaged recording of a “0”-bit from the one of a “1”-bit, static echos and antenna coupling effects are perfectly canceled. In addition, noise variance is reduced, improving the SNR. Finally, by applying a cyclic correlation with the transmitted DSSS sequence and locating the correlation maximum, the distance to the RFID tag can be determined.