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.

We describe a low power vector backscatter modulator capable of transmitting 16-QAM at a rate of 96 Mbps while consuming only 1.49 mW (15.5 pJ/bit). While designed around a center frequency of 915 MHz, the modulator is capable of operation over the worldwide 868 -950 MHz UHF band. We present experimental results from the modulator operating in 4-QAM/4-PSK, 4-PAM, and 16-QAM modes. Achieved data rates are comparable to WiFi (IEEE 802.11) with a measured tag-side power consumption over 50 times lower than a WiFi chipset. Potential applications for low power, high bit rate modulators include biotelemetry, high-bandwidth data transfer from camera tags or audio tags, uplink from mass storage tags, and exchange of large amounts of encryption or authentication data. Given a +36 dBm EIRP transmitter operating at 915 MHz, the semipassive (battery-assisted) prototype tag is return link limited and has a theoretical maximum operating range of 17.01 m at 96 Mbps or 21.25 m at 40 Mbps.

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This paper presents a digital neural/EMG telemetry system small enough and lightweight enough to permit recording from insects in flight. It has a measured flight package mass of only 38 mg. This system includes a single-chip telemetry integrated circuit (IC) employing RF power harvesting for battery-free operation, with communication via modulated backscatter in the UHF (902-928 MHz) band. An on-chip 11-bit ADC digitizes 10 neural channels with a sampling rate of 26.1 kSps and 4 EMG channels at 1.63 kSps, and telemeters this data wirelessly to a base station. The companion base station transceiver includes an RF transmitter of +36 dBm (4 W) output power to wirelessly power the telemetry IC, and a digital receiver with a sensitivity of -70 dBm for 10⁻⁵ BER at 5.0 Mbps to receive the data stream from the telemetry IC. The telemetry chip was fabricated in a commercial 0.35 μ m 4M1P (4 metal, 1 poly) CMOS process. The die measures 2.36 × 1.88 mm, is 250 μm thick, and is wire bonded into a flex circuit assembly measuring 4.6 × 6.8 mm.

We describe a radio frequency (RF) energy harvester and power management circuit that trickle charges a battery from incident power levels as low as -20dBm. We designed the harvester for the 2.4 GHz RF band to leverage the ubiquity of energy that is produced by Wi-Fi, Bluetooth, and other devices. This paper reports on the design and current status of the harvester and compares our performance to other published results. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. This paper describes a novel rectenna design, boost converter, and battery charger for RF energy harvesting specifically tuned to this low-power regime. At -20dBm RF input power, the harvesting system (rectenna, boost converter, and battery charger) sources 5.8µJ into a rechargeable battery after 1 hour.