Stewart J. Thomas

Quadrature Amplitude Modulated Backscatter in Passive and Semipassive UHF RFID Systems

Passive and semipassive UHF RF identification (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. Considering modulated backscatter in terms of complex-valued scattered fields opens the possibility of quadrature modulation of the backscatter channel. When compared with binary amplitude shift keying (ASK) or phase shift keying (PSK) based RFID systems, which transmit 1 bit of data per symbol period, and thus 1 bit per on-chip clock oscillator period, tags employing vector backscatter modulation can transmit more than 1 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 log2 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 semipassive 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 quadrature amplitude modulated (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 four-state phase shift keying (4-PSK)/four-state quadrature amplitude modulated (4-QAM) modulator are provided for a semipassive (battery-assisted) tag operating in the 850-950-MHz band. This first prototype modulator transmits 4-PSK/4-QAM at a symbol rate of 200 kHz and a bit rate of 400 kb/s at a static power dissipation of only 115 nW.

A 96 Mbit/sec, 15.5 pJ/bit 16-QAM modulator for UHF backscatter communication

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 semi-passive (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.

QAM backscatter for passive UHF RFID tags

Traditional passive UHF RFID tags employ either ASK or PSK backscatter modulation to communicate data from memory or sensors on the tag to a remotely-located reader. These simple modulation schemes transfer data at a rate of one bit per symbol period, which for an integrated CMOS tag IC requires an on-chip oscillator with a frequency at least equal to the bit rate. Motivated by the fact that most modern UHF RFID readers already employ I/Q demodulation of the backscattered signal to account for backscatter phase rotation as the tag moves with respect to the reader, we propose a QAM backscatter method using no on-chip inductors that is compatible with a single-chip CMOS tag implementation.With QAM backscatter, tags transmit more than one data bit per symbol period, permitting tag designers to employ a lower power on-chip oscillator operating at a frequency equal to the (lower) symbol rate while maintaining the same data throughput as ASK or PSK, or alternatively to send data at a higher rate for a given on-chip oscillator frequency. We present the fundamental design equations required for arbitrary QAM backscatter modulators and present simulated bit error rate (BER) and error vector magnitude (EVM) curves for the operation of an inductor-free 4-QAM and 8-QAM modulator centered at 915MHz and evaluated over the 860MHz–950MHz worldwide UHF band.

A Battery-Free Multichannel Digital Neural/EMG Telemetry System for Flying Insects

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.

Feasibility of wireless sensors using ambient 2.4GHz RF energy

We present a new system for measuring ambient RF energy in the 2.4GHz ISM band. This apparatus is intended to establish the feasibility of harvesting ambient RF energy to power emerging ultra-low-power sensors and microcontrollers. We simultaneously acquire RF measurements from a spatial and polarization diversity antenna system, with both a spectrum analyzer (frequency-selective but slow), and a log amp (wideband but fast), explain key tradeoffs in the measurement configuration, and present a post-processing algorithm which provides a reliable characterization of the RF energy available in the 2.4GHz ISM band. Preliminary results suggest enough energy is available to support a low duty cycle wireless sensor node system. An average RF power of 11nW is observed 10m away from a typical Wi-Fi access point in an office environment, suggesting the possibility of low duty cycle, wirelessly powered sensing and communication using a Bluetooth Low Energy (BLE) or another ultra low power uplink.

Rich-Media Tags: Battery-free wireless multichannel digital audio and image transmission with UHF RFID techniques

In this paper we present the first fully passive (battery-free) wireless transmission of multiple digital audio channels and images via modulated backscatter. We leverage a previously reported single chip, passive transponder that can digitize and uplink up to 10 analog input channels sampled at a rate of 26.1 kHz. Given a base station transceiver operating at a frequency of 915 MHz and a transmit power of +36 dBm EIRP, the transponder has a demonstrated operating range of ≈1.4 m. The transponder data uplink uses binary phase-shift key (BPSK) modulated backscatter operating at a total link throughput rate of 5 Mbps, with an uplink energy consumption of only 3.7 pJ/bit. The transponder was initially designed for biomedical telemetry of neural and EMG signals. We present a new application of this tag for multichannel, high fidelity digital audio recording, as well as color image transfer using a slow-scan television (SSTV) modulation (PD290) with a resolution of 640 by 493 pixels. Additionally, we demonstrate fully-passive digital recording of ambient sound using a microphone powered by the chips harvested energy at an operating range of 0.72 m. The passive, digital microphone is sensitive enough to record human speech within approximately 5 m of the device. We believe these results will serve as a first step toward media-rich battery-free (wirelessly powered) devices that take advantage of the high speed, low power nature of modulated backscatter communication links.

A 2.4GHz ambient RF energy harvesting system with −20dBm minimum input power and NiMH battery storage

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.

Modulated backscatter for ultra-low power uplinks from wearable and implantable devices

Wearable and implantable wireless biomedical devices are often constrained by the limited bandwidth and high power consumption of their communication links. The VHF or UHF transceivers (e.g. MICS radios) traditionally used for this communication function have relatively high power consumption, on the order of mW, due to the high bias currents required for the analog sections of the radio. To reduce overall power consumption, both the data rate and the duty cycle of the radio are usually minimized, because the lifetime of the device is limited by the energy density of available battery technologies. Recent innovations in modulated backscatter techniques offer the possibility of a radical reduction in the power cost and complexity of the data uplink, while significantly improving data rate. This is achieved by a re-partitioning of the communication link. Backscatter techniques shift the burden of power cost and complexity from the remote device to a base station. Instead of actively transmitting an RF signal, the remote device uplinks data to the base station by modulating its reflected field. We present two ultra-low power biotelemetry systems that leverage modulated backscatter in both the near-field and far-field propagation regimes. The first example operates in the far field and is designed to telemeter multiple channels of neural/EMG signals from dragonflies in flight. This device has a mass of 38 mg, a data rate of 5 Mbit/s, and a range of approximately 5 m. The second example operates in the near field and is designed to be implanted in mice. The sensor has a maximum implant depth of 6cm and can transmit at data rates of up to 30 Mbit/s. The power cost of the animal side of both data links is 4.9 pJ/bit and 16.4 pJ/bit respectively.

SmartHat: A battery-free worker safety device employing passive UHF RFID technology

In many safety-critical applications, battery performance is a significant limiting factor that affects the feasibility of electronic safety devices intended to alert workers to hazardous situations. In particular, battery capacity and lifetime are difficult to predict when safety devices are exposed to extremes of temperature, humidity, shock, and vibration that are common in construction, excavation, drill rigs, and mining work sites. Because battery failure is unacceptable in safety devices, periodic preventative maintenance is required, adding to device cost and labor cost and reducing acceptance of electronic safety devices. Energy harvesting and communications techniques based on passive UHF RFID technology may offer an alternative to battery power for some types of safety alert devices, particularly where hazardous conditions are created by powered heavy equipment. We present a worker safety device designed around a passive UHF RFID platform that derives its operating power from specialized interrogators mounted on heavy equipment. This device is designed to be integrated with plastic hard hats that are commonly used in the construction industry to yield an intelligent hard hat, called a “SmartHat”, that delivers an audible alert directly to workers in proximity to a particular piece of equipment. It is addressible using an ASK interrogator-to-tag link, and backscatters confirmation that an alert has been delivered to the worker. We present the design of the SmartHat tag, including a compact printed-circuit vee style antenna, an RF-to-DC power harvesting circuit, and a microprocessor-driven alert speaker. The tags average operating power while delivering a pulsed alert is 1.8V at 61μA, or 110μW (−9.6 dBm). Its power-up threshold when not delivering an alert is 1.8 V at ≈ 10μA. We also present a specialized interrogator device operating under FCC Part 18 rules in the 902–928 MHz band that is mounted to a piece of construction equipment to power and communicate with nearby SmartHats. In outdoor testing of the SmartHat tag and its companion interrogator device, +35 dBm transmitter output power feeding a 9dBi Yagi antenna (+44 dBm EIRP) allows for safety alerts to be delivered at distances of up to 16.46 m.

Waveform-aware ambient RF energy harvesting

In this paper we suggest a new class of RF energy harvesters, which we call “waveform aware harvesters”. In contrast to traditional rectenna designs, which are usually designed for high efficiency with continuous wave (CW) signals, waveform aware harvesters are RF to DC converters which are optimized for their performance with non-CW signals. We suggest that waveform aware harvesters may have significant advantages in ambient energy harvesting, where the available RF energy is in the form of communication waveforms of a variety of types. We present an initial proof-of-concept demonstration of a waveform aware harvester optimized for harvesting energy from 2.4 GHz Wi-Fi (802.11b/g) signals with a realistic traffic model. Under realistic traffic conditions, 802.11b/g client transmissions are bursty, with a high peak-to-average ratio and a low duty cycle. We demonstrate optimized recovery of harvested energy from single 802.11b/g transmission bursts on the order of 1 ms in duration. We present an expression for maximizing usable energy stored in an energy reservoir given a signal model and parameters of the energy-harvester circuit. In contrast to other work where assumptions of CW sources lead to the desirability of a large storage capacitor, our approach considers the existing communication signal model and optimizes capacitor size to maximize the stored usable energy for a short transmission burst.