1Quadrature Amplitude Modulated Backscatter in Passive and Semi-Passive UHF RFID Systems

Abstract—Passive and semi-passive UHF RFID systems have traditionally been designed using scalar-valued differential radar cross section (DRCS) methods to model the backscattered signal from the tag. This paper argues that scalar-valued DRCS analysis is unnecessarily limiting because of the inherent coherence of the backscatter link and the complex-valued nature of load-dependent antenna-mode scattering from an RFID tag. Consider-ing modulated backscatter in terms of complex-valued scattered fields opens the possibility of quadrature modulation of the backscatter channel. When compared with binary ASK or PSK based RFID systems which transmit one bit of data per symbol period, and thus one bit per on-chip clock oscillator period, tags employing vector backscatter modulation can transmit more than one bit per symbol period. This increases the data rate for a given on-chip symbol clock rate leading to reduced on-chip power consumption and extended read range. Alternatively, tags employing an M-ary modulator can achieve log2M higher data throughput at essentially the same DC power consumption as a tag employing binary ASK or PSK. In contrast to the binary ASK or PSK backscatter modulation employed by passive and semi-passive UHF RFID tags, such as tags compliant with the widely used ISO18000-6c standard, this paper explores a novel CMOS-compatible method for generating M-ary QAM backscatter modulation. A new method is presented for designing an inductorlessM-ary QAM backscatter modulator using only an array of switched resistances and capacitances. Device-level simulation and measurements of a 4-PSK/4-QAM modulator are provided for a semi-passive (battery-assisted) tag operating in the 850-950MHz band. This first prototype modulator transmits 4-PSK/4-QAM at a symbol rate of 200kHz and a bit rate of 400kbps at a static power dissipation of only 115 nW. Index Terms—QAM backscatter, Passive RFID tags, CMOS integrated circuits, Backscatter, Quadrature amplitude modula