Outdoor UHF RFID: Phase Stabilization for Real-World Applications

This paper investigates meteorological factors that affect the phase of radio-frequency identification (RFID) passive tags at 868 MHz, in outdoor conditions. The study identifies the effect of water on the tag and base antennas, the effect of temperature on the cables, tags, and base antenna, the effect of the tag support moisture, and the effect of atmospheric conditions on wave velocity. Combined, these effects could lead to over 8.1 radians phase drift over a year, in a typical environment. In a tag location tracking application, that would correspond to an error of 22 cm. This paper proposes techniques to correct these effects and to increase the phase stability. These techniques are applied to a new RFID system, which is tested in outdoor conditions, for five months. The new system improves the phase stability for rainy days, dry days, and long-term drift by a factor of 3, 12, and 5, respectively. After corrections, the long-term drift was reduced to below 0.05 radians per month, which corresponds to 1.5 millimeter per month.