CHAPTER 18
FADING STATISTICS FOR MULTI-ANTENNA RF TAGS
18.1 INTRODUCTION
Backscatter radio systems—that is, systems that communicate wirelessly by modulating scattered electromagnetic fields—have found widespread use in ultra-high-frequency (UHF) and microwave radio-frequency identification (RFID) and sensor applications. In most RFID systems, backscatter communication occurs when an interrogator, or reader, transmits a continuous-wave (CW) signal to a small, low-power transponder, or radio-frequency (RF) tag. A portion of the CW signal is scattered from the RF-tag’s antenna and load-modulated that is, the scattered signal is modulated as the RF tag changes the reflection coefficient seen by the tag antenna by changing the antenna's load impedance.
Many factors can affect the operating range and reliability of a backscatter radio system, including mulitpath fading. To understand how multipath fading occurs, consider a wave launched from a transmitting antenna. If the transmitting or receiving antenna is in the vicinity of conducting or dielectric objects, part of the transmitted signal will be scattered off the objects. These scattered waves travel different paths to the receiving antenna and, as a result, arrive with different amplitudes, phases, and angles of arrival. At some points in space, the multipath ...
