Multipath fading measurements for multi-antenna backscatter RFID at 5.8 GHz

UHF and microwave backscatter RF-tag systems, including radio frequency identification (RFID) and sensor systems, experience multipath fading that can be more severe than that found in a conventional transmitter-to-receiver channel. Previous work has shown that multipath fading can be reduced on the modulated-backscatter signal received from the RF tag in a non-line-of-sight (NLOS) channel if more than one RF-tag antenna is used to modulate backscatter. This paper presents the first multipath fading measurements for backscatter tags using multiple antennas at 5.79 GHz - the center of the 5.725-5.850 GHz, unlicensed industrial, scientific, and medical (ISM) frequency band that may offer reliable operation for future, miniature RF tags. NLOS measurement results are presented as cumulative density functions (CDF) and fade margins for use in backscatter radio link budget analysis and a detailed description of the custom backscatter testbed used to take the measurements is provided. The measurements show that gains are available for multiple-antenna RF tags and results match well with gains predicted using the analytic fading distributions derived previously.