A classification algorithm is applied to UHF radio frequency identification (RFID) system to estimate the indoor position of a passive tag utilizing a received signal strength indicator (RSSI). Passive tag signals are collected by conventional UHF RFID reader antenna located at two different positions. K-NN and curve fitting are used for distance estimation and examined at different frequencies within the range 902-928 MHz to find suitable frequencies which can minimize the average error. In the proposed method, the measured RSSI values are compared with the fingerprint database in different frequencies to find the nearest neighbor. The best accuracy achieved at frequency range 926-928 MHz is 18.3 cm. The monitoring system is composed of a reader and tags under test, which makes the proposed system robust, easy to set up, and with low cost. The limitation of the proposed method is also discussed.
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