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 loaddependent antenna-mode scattering from an RFID tag. Considering 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 Mary modulator can achieve log 2 M 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 inductorless M-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 850-950 MHz 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.

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