Differential RCS of RFID tag

Introduction: Radio frequency identification (RFID) can be traced back to the late 1940’s work on using RF modulated backscatter for communications [1]. Fig. 1 illustrates the operation of a modern passive RFID system which includes an RFID reader and an RFID tag, composed of an antenna and an integrated circuit chip. The reader signal alternates between a continuous wave (CW) and modulated transmissions. The tag sends data during one of the CW periods by switching its input impedance between two states, effectively changing its radar cross-section (RCS) and thus modulating the backscattered field. While the general theory of scattering from loaded antennas [2–5] and modulated probes [6] has been well established, and scalar RCS of RFID tags has also been recently analysed [7], such an important tag parameter as vector differential RCS has not been explicitly discussed in RFID literature and compared with experiment. If the receiver is non-coherent, it can only register a magnitude difference between two scalar RCS values (scalar differential RCS). If the receiver is coherent (such as an RFID reader where transmitter and receiver are phase locked), it detects both amplitude and phase of the signal and hence can register a vector difference between two RCS values (vector differential RCS). This difference depends on relative phases of the field scattered in different chip impedance states and is not a mere difference of two scalar RCS values. Two chip impedances may result in the same scalar RCS values for the tag but produce nonzero modulated backscattered power.