Backscatter Tag Research Articles

Backscatter Multiplicative Multiple-Access Systems: Fundamental Limits and Practical Design

In this paper, we consider a novel ambient backscatter multiple-access system, where a receiver (Rx) simultaneously detects the signals transmitted from an active transmitter (Tx) and a backscatter tag. Specifically, the information-carrying signal sent by the Tx arrives at the Rx through two wireless channels: the direct channel from the Tx to the Rx and the backscatter channel from the Tx to the tag and then to the Rx. The received signal from the backscatter channel also carries the tag's information because of the multiplicative backscatter operation at the tag. This multiple-access system introduces a new channel model referred to as backscatter multiplicative multiple-access channel (BM-MAC). We analyze the achievable rate region of the BM-MAC and prove that its region is strictly larger than that of the conventional time-division multiple-access scheme in many cases, including, e.g., the high SNR regime and the case when the direct channel is much stronger than the backscatter channel. Hence, the multiplicative multiple-access scheme is an attractive technique to improve the throughput for ambient backscatter communication systems. Moreover, we analyze the detection error rates for coherent and noncoherent modulation schemes adopted by the Tx and the tag, respectively, in both synchronous and asynchronous scenarios, which further bring interesting insights for practical system design.

Practical Energy Harvesting for Batteryless Ambient Backscatter Sensors

This work studies the performance of two methods for providing power to an ultra-low power, ambient backscatter tag, omitting the need for any battery. RF energy harvesting from a dedicated source and energy harvesting from ambient light using a single photodiode are compared. Extensive measurement results from tests conducted under real world conditions are offered for both harvesting methods. It is concluded that for a total cost of under 7 Euros the need for a battery can be eliminated, by using a single photodiode element along with a suitable boost converter. The ultra-low power character of the utilized tag enables the use of multiple harvesting methods and paves the way towards truly battery-less wireless sensor systems.

System Architecture and Signal Processing for Frequency-Modulated Continuous-Wave Radar Using Active Backscatter Tags

In this paper, we study a frequency-modulated continuous-wave based radio-frequency identification system, which is composed of a reader with single transmitter and multiple receiving antennas as well as multiple active backscatter tags. In particular, our contribution is twofold in this paper. First, we construct a general architecture for achieving the functionality of wireless positioning based on the principle of the switched injection-locked oscillator (SILO). Unlike the previous SILO-based schemes, some limitations on the pulsed waveform generated by the tags are overcome. In addition, only one reader set up with multiple receiving antennas is used to precisely localize tags based on the delay and direction-of-arrival estimates. Second, the developed algorithms make use of a shift invariant property among the baseband signal snapshots of intra- and/or intermodulation intervals, i.e., snapshots in one modulation period and/or sets of snapshots in multiple modulation periods. A new joint singular value decomposition based ESPRIT algorithm is developed for determining the number of multiple active tags and their positions by implementing the matrix reconstruction and stacking techniques. The proposed method has its basic foundation on the processing of received data with global fusion scheme. In this way, this method can improve the performance of parameter estimates in terms of accuracy and resolution without suffering from the pair matching problem. System simulations and experimental validations are carried out to verify the proposed system architecture associated with the signal processing technique.

Simultaneous Imaging, Sensor Tag Localization, and Backscatter Uplink via Synthetic Aperture Radar

This paper presents an extension of synthetic aperture radar (SAR) techniques to enable simultaneous radar imaging, sensor tag localization, and backscatter-based data uplink from multiple sensor tags in a cluttered environment. A unified system model is presented that leverages coherent processing of backscattered signals gathered over the synthetic aperture for all three of these purposes. The proposed approach, using balanced orthogonal codes for SAR-based localization as well as the backscatter data uplink, is shown to have several favorable properties, including straightforward tag-vs-clutter discrimination, straightforward multiple access among tags, and improved signal-to-noise ratio during localization. A proof-of-principle indoor experiment is presented in the X-band (10–13 GHz) using two custom-designed backscatter tags interrogated by a vector network analyzer functioning as an FMCW radar. The proposed system model is validated by simultaneous imaging of a cluttered scene, tag localization with a maximum range error of 9 mm, and data demodulation from both tags telemetering temperature changes at a rate of 1 bit/s at ranges of 4.4 m and 4.7 m. The resulting point-spread functions of tags demonstrate a range resolution of 4.7 cm and a cross-range resolution of 9.1 cm.

BLE-Backscatter: Ultralow-Power IoT Nodes Compatible With Bluetooth 4.0 Low Energy (BLE) Smartphones and Tablets

Backscatter communication promises significant power and complexity advantages for Internet of Things devices such as radio frequency identification (RFID) tags and wireless sensor nodes. One perceived disadvantage of backscatter communication has been the requirement for specialized hardware such as RFID readers to receive backscatter signals. In this paper, we show how backscatter signals can be designed for compatibility with the Bluetooth 4.0 low energy (BLE) chipsets already present in billions of smart phones and tablets. We present a prototype microcontroller-based “BLE-Backscatter” tag that produces bandpass frequency-shift keying modulation at 1 Mb/s, enabling compatibility with conventional BLE advertising channels. Using a +23-dBm equivalent isotropically radiated power continuous wave (CW) carrier source, we demonstrate a range of up to 13 m between the tag and an unmodified Apple iPad Mini as well as a PC with the Nordic Semiconductor nRF51822 chipset. With the tag 1 m from the receiver, we demonstrate a range of up to 30 m between the CW carrier source and the tag. In both cases, the existing Bluetooth stack was used, with no modifications whatsoever to hardware, firmware, or software. The backscatter tag consumes only 1.56 nJ/b, over $6\times $ less than the lowest power commercial Bluetooth transmitters.

Determination Scheme for Detection Thresholds Using Multiple Antennas in Wi-Fi Backscatter Systems

In this paper, a detection scheme for the uplink transmission of Wi-Fi backscatter system is proposed. In the uplink transmission, a backscatter tag modulates signals from a Wi-Fi access point by backscattering. Since the reflection occurs, the backscattered signal experiences more attenuation than the direct Wi-Fi signals to a Wi-Fi reader. Most recent studies focus on effective detection techniques to increase the coverage of the backscatter tag in the poor signal to interference plus noise ratio environment. This paper proposes an improved detection scheme using multiple antennas at the reader. At the reader, the backscattered signals are received by multiple antennas and thresholds are determined appropriately for improvement of detection performance. Typically the methods using multiple antennas require channel information. However the channel estimation for the uplink of backscatter system is difficult since the backscattered signals are weak. The detection method of this paper does not require the channel information for the uplink. Simulation results show improved performance and possibility for the increase of coverage.

Dominant Channel Occupancy for Wi-Fi Backscatter Uplink in Industrial Internet of Things

This paper presents a dominant channel occupancy (DCO) mechanism for the Wi-Fi backscatter uplink in the industrial Internet of things (IIoT). The DCO provides high-priority channel access and reliable burst transmission to the Wi-Fi backscatter devices, thereby enabling the Wi-Fi backscatter tag to deliver its tag information to the Wi-Fi reader without interference from neighboring legacy Wi-Fi devices to guarantee the timeliness and reliability of the IIoT system. For the former, we consider three types of medium access control (MAC) configurations: “carrier sense multiple access with collision avoidance (CSMA/CA) starting with short inter-frame space (SIFS)”, “freezing of the backoff period”, and “reduced CWmin.” In addition, the DCO uses the SIFS between burst packets to guarantee reliable burst transmission. To verify the effectiveness of DCO and determine a proper value for MAC parameters, we conduct experimental simulations under IEEE 802.11n PHY/MAC environments. The simulation results show that the reduced CWmin has the most significant effect on the channel occupancy. The Wi-Fi backscatter devices achieve much higher throughput than the separate cases when two or more configurations are used simultaneously. Moreover, the results exhibit that the use of SIFS between consecutive packets supports reliable burst transmission regardless of the transmission of the legacy Wi-Fi devices in the vicinity.

Pulse Shaping: The Missing Piece of Backscatter Radio and RFID

The increasing use of backscatter radio for pervasive Internet-of-Things systems as a low-power and low-cost communication scheme will result in dense deployments of tags that need to operate under bandwidth (BW) constraints. Typical backscatter tags and computational radio frequency (RF) identification systems are unnecessarily limited to modulating data with “ON–OFF” switching front-ends, which leads to extensive spectrum occupancy, due to the rectangular pulses. This paper sets the foundations to overcome these limitations and proposes designs of RF front-ends that are capable of varying the tag reflection coefficient in a continuous way over time. Arbitrary waveforms can be generated to perform pulse shaping on the backscattered signals, which require significantly reduced BW. The principles presented in this paper will enable sophisticated tags to perform complex modulation schemes under BW constraints, while maintaining a very low RF front-end complexity, using a single nonlinear component such as a p-i-n diode or field-effect transistor. Migrating the idea of pulse shaping from complex power-demanding active radios to such minimal RF front-ends will complete backscatter radio as a communication scheme for simple RF tags and sensors in large-scale deployments. Each tag becomes a miniature software-defined radio that implements diverse communication schemes with increased spectral efficiency.

Comparison of Phase-Based 3D Near-Field Source Localization Techniques for UHF RFID.

In this paper, we present multiple techniques for phase-based narrowband backscatter tag localization in three-dimensional space with planar antenna arrays or synthetic apertures. Beamformer and MUSIC localization algorithms, known from near-field source localization and direction-of-arrival estimation, are applied to the 3D backscatter scenario and their performance in terms of localization accuracy is evaluated. We discuss the impact of different transceiver modes known from the literature, which evaluate different send and receive antenna path combinations for a single localization, as in multiple input multiple output (MIMO) systems. Furthermore, we propose a new Singledimensional-MIMO (S-MIMO) transceiver mode, which is especially suited for use with mobile robot systems. Monte-Carlo simulations based on a realistic multipath error model ensure spatial correlation of the simulated signals, and serve to critically appraise the accuracies of the different localization approaches. A synthetic uniform rectangular array created by a robotic arm is used to evaluate selected localization techniques. We use an Ultra High Frequency (UHF) Radiofrequency Identification (RFID) setup to compare measurements with the theory and simulation. The results show how a mean localization accuracy of less than 30 cm can be reached in an indoor environment. Further simulations demonstrate how the distance between aperture and tag affects the localization accuracy and how the size and grid spacing of the rectangular array need to be adapted to improve the localization accuracy down to orders of magnitude in the centimeter range, and to maximize array efficiency in terms of localization accuracy per number of elements.

Reducing the phase cancellation effect in augmented RFID system

This paper proposes to use the spatial diversity of dual-antenna to counter the phase cancellation effect in an augmented radio frequency identification (RFID) system, which is developed for accurate indoor localization. In the augmented RFID system, a tag-like semi-passive RFID component (referred as sensatag) senses the backscattering communication between standard ISO 18000-6C compliant RFID reader and passive tags within its proximity. However, due to the superposition of reader's carrier wave (CW), the amplitude difference in the tag's backscattering signal is significantly attenuated at the sensatag's antenna. We refer to such effect as phase cancellation. To counter the destructive phase cancellation effect, two techniques are proposed based on dual-antenna diversity. In the first one called simple combination technique, simple addition of signals from two antennas is performed; in the second one called receiving path selection technique, signals from both antennas are converted into baseband and then conduct the selection in digital domain. Then we demonstrate the performance of these two techniques in a newly developed Proximity-detection-based augmented RFID system simulator (PASS) framework. The result shows that dual-antenna-based implementations are better than the simple antenna implementation in reducing phase cancellation effect. Moreover, receiving path selection technique reduces the probability of phase cancellation significantly with higher hardware complexity compared with simple combination technique.

Reducing Phase Cancellation Effect with ASK-PSK Modulated Stamp in Augmented UHF RFID Indoor Localization System

Abstract In this paper, we propose exploiting ASK-PSK modulated stamp in receiving path selection technique to lessen the phase cancellation effect in augmented ultra-high-frequency (UHF) radio frequency identification (RFID) indoor localization system (AURIS). In AURIS, a tag-like semi-passive RFID component (referred as sensatag) can capture backscatter signal of other proximal tags with presence of RF source. According to the principle of backscatter radio link, the received signal at sensatag antenna is the superposition of backscatter signal of tags and continuous carrier wave (CW) from RF source. However, due to phase difference between tag's backscatter signal and RF CW, the modulated backscatter signal could be cancelled. We refer this effect as phase cancellation effect. Exploiting the spatial diversity of dual-antenna's two receiving paths, the likelihood of phase cancellation occurrence could be reduced. But the developed technique with two co-operating paths is not energy efficient. Therefore, this paper proposes to inject a ASK-PSK modulated stamp sequence in the pilot tone of backscatter signal as a signature of phase cancellation for ASK modulated data frame, which could be identified by the receiving sensatag. With the knowledge of occurrence of phase cancellation, sensatag could activate the alternative receiving path. This technique fully exploits the space diversity of dual-antenna, and also reduces the power consumption by reducing one receiving path in operation. We demonstrate the performance of stamp based receiving path selection technique with data obtained from computer simulation.

Enhancement of RF Tag Backscatter Efficiency With Low-Power Reflection Amplifiers

Increasing backscatter tag communication ranges is crucial for the development of low-power long-range wireless sensor networks. A major limitation for increasing the signal-to-noise ratio (SNR) for RF identification tags lies in the fact that tag antennas are terminated with passive loads for modulation, which yields reflection-coefficient values less than unity. Recent work in the field has exploited reflection amplifiers that achieve reflection-coefficient values larger than unity to increase the communication range. However, most of these systems rely on increasing the reflection coefficient at one modulation state only, which is suboptimal. In this paper, an analysis is given for the optimal way to utilize a reflection amplifier and how this compares to suboptimal practices. To demonstrate the concept, a tag is designed that achieves reflection-coefficient values higher than unity for both states in the 900-930-MHz band. The two values are antipodal, thus maximizing the tag SNR for a given amplifier. The system comprises of an ultra-low-power reflection amplifier with up to 10.2-dB gain and sub-milliwatt power consumption, and a phase-shift modulator that selectively alternates the phase of the backscatter signal between 0 ° and 180 ° . The reflection amplifier-phase modulator system is experimentally characterized in terms of gain, power consumption, and backscatter efficiency.

Analysis of UWB Tag Backscattering and Its Impact on the Detection Coverage

The joint adoption of RFID and UWB technologies is a promising solution to overcome most of the limitations of current RFID systems. In particular, UWB RFID based on the modulation of the backscatter signal represents a good option when tags cost, size and power consumption are stringent requirements. In this context, while for current UHF RFID systems an extensive literature is present, only few studies have concentrated on the backscattering of UWB signals. In the present work, we deal with the electromagnetic analysis of the UWB RFID tag backscattering, which is adopted to characterize the interaction of the tag with a close object. We investigate tag backscattering first from an analytical perspective, then through simulations and measurements. Measured data is then post-processed, and performance in terms of detection coverage analysis is finally assessed to evaluate the effects of an object in proximity to the antenna.

Accurate Indoor Ranging by Broadband Harmonic Generation in Passive NLTL Backscatter Tags

Millimeter-precision meter-distance real-time indoor ranging capability is challenging due to multipath reflections in a rich scattering environment. Traditional continuous wave (CW) phase-based ranging methods, although simple and flexible, are vulnerable to phase offsets and interferences. We improve the previous CW approach by passive broadband harmonic nonlinear-transmission-line (NLTL) tags. Since phase information is now contained within the second harmonic rather than the fundamental frequency, interferences and phase errors caused by direct reflections of the interrogating signal are greatly reduced. By the broadband property of NLTL, a heuristic multi-frequency CW method is formulated to resolve the phase integer ambiguity and to further improve ranging accuracy and robustness even under large phase errors. We present theoretical and simulation analyses, followed by experimental verification.

A microwave switchable low‐noise power amplifier for an active backscatter tag

ABSTRACTThis article reports the design of a switchable low‐noise power amplifier (LNPA) intended for an active backscatter tag used in microwave localization and positioning applications. Due to system specifications, the backscattering transponder requires an amplifier having both low‐noise and high‐output power characteristics at the same time. Low‐noise amplifier and PA design considerations are both taken into account to realize a single structure achieving the required performances. The proposed LNPA is operating in the frequency range of 4.5–5.8 GHz. It achieves a gain of 21 dB with a ripple of ±1 dB, a noise figure of less than 2.5 dB, and an output compression point at −1 dB of 11 dBm over the operational bandwidth. This structure can be switched on and off by a control signal to modulate the tag radar cross section and enable distance measurement. A packaged prototype of the LNPA was fabricated and measured. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1251–1255, 2014

Dynamic interference suppression for chipless wireless sensors based on an out-of-band channel estimation method

A channel estimation method for chipless wireless sensors is presented. The method is developed to suppress interference signals in radio frequency backscatter systems. It uses two adjacent frequency bands to estimate and suppress the disturbing signal of a dynamic interferer. Afterwards a correction of the sensor tag's backscatter signal is achieved. The method has been tested in indoor measurements with a chipless strain sensor and a chipless temperature sensor. A metal block has been deployed as an interferer. In the given scenario, the method has enabled a determination of the sensors' resonance frequencies with relative errors of <2% in average. A general dependence of the estimation robustness on the peak bandwidth is observed.

A Millimeter-Wave Low-Power Active Backscatter Tag for FMCW Radar Systems

In this paper, a fully integrated active backscatter transponder based on the switched injection-locked oscillator (SILO) principle for frequency-modulated continuous-wave radar applications is presented. Furthermore, a method to characterize a SILO amplifier is extended and utilized to measure the system parameters of the presented backscatter tag that operates at 34.45 GHz. It is digitally tunable from 32.7 to 35.4 GHz and reaches an unpulsed output power of 5 dBm. Above injection power levels of $-\hbox{53 dBm}$ , the SILO tag responds phase coherent at its maximum output power. In system measurements, the SILO backscatter transponder was used to perform distance measurements at ranges from 0.7 to 11.5 m. A remarkably good mean distance measurement error of 7 cm with a standard deviation of 10 cm was achieved in a strong multipath environment. The single measurement precision is below 3 mm.

UHF 대역 RFID 리더 반송파 누설 억압 연구

RFID technologies, which allow collecting, storing, processing, and tracking information by wirelessly recognizing the inherent ID of object through an attached electronic tag, have a variety of application areas. This paper presents a novel carrier leakage suppression RF(CLS-RF) front-end for ultra-high-frequency RF identification reader. The proposed reader CLS-RF front-end structure generates the carrier leakage replica through the nonlinear path that contains limiter. The limiting function only preserves the frequency and phase information of the leakage signal and rejects the amplitude modulated tag signal in the envelope. The carrier leakage replica is then injected into the linear path that contains phase shifter. Therefore, the carrier leakage signal is effectively cancelled out, while not affecting the gain of the desired tag backscattering signal. We experimentally confirm that the prototype shows a significant improvement in the leakage to signal ratio by up to 36 dB in 910 MHz, which is consistent with our simulation results.

Active V-band modulated backscatter tag

We report the measurement of modulated backscatter produced by a Schottky diode and a printed patch antenna array at V-band (58.75 GHz). The tag was fabricated on a liquid crystalline polymer substrate that is used as the dielectric film and as a carrier for the diode that is wire bonded to the surface of the array. An inexpensive commercial V-band transceiver chipset was successfully used to detect modulated backscatter when the diode was switched at kHz frequencies, demonstrating that the effect is strong enough to be used in commercial applications requiring lightweight, wearable transponders. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26067

Analysis and measurement for passive tag modulationperformance of backscatter link

Optimal conditions for maximizing the effectively received power of reader receiver in passive ultra high frequency radio-frequency identification system are analyzed in this paper. The mismatched impendence affecting modulation index of tag backscatter link is discussed. Expressions of backscattered modulation indexes for calculating normalized effective received power of reader receiver, lower boundary of signal-to-noise ratio for demodulated output signal, and bit error rate are derived. Modulation indexes of backscatter link under different parameters are measured in open indoor environment. The measurement results show that the tag can be detected successfully when the modulation index of backscatter link is in a range from 5% to 10%.