Backscatter Radio Research Articles

A Winning Backscatter Modulator: A Quarter-Gram, Ultrahigh-Frequency RFID for On-Metal Operation

Wireless sensors have become commonplace in our daily lives for such tasks as tracking temperature, measuring humidity, and sensing light and gas. Many technologies capable of relaying sensor information are continually being refined, but the search for a robust and customizable technology to satisfy the exponentially increasing demand for largescale and low-cost deployment of these wireless sensor nodes while minimizing consumption has led to backscatter radio as a potential solution. Backscatter radio, first proposed in the 1940s as communication by means of reflected power [1], is a fairly mature technology that has been used in a variety of modern applications, especially those employing passive RFID technology in the ultrahigh-frequency band. The tags used in these applications are usually made up of just a transistor or switch and an antenna. These tags can be activated using only the RF from an interrogating signal and, as a result, can be operated without any external energy source. Communication ranges of up to a few meters have been reported using this technology [2], [3].

Motion-Fi: Recognizing and Counting Repetitive Motions With Wireless Backscattering

Driven by a wide range of real-world applications, several ground-breaking RF-based motion-recognition systems were proposed to detect and/or recognize macro/micro human movements. These systems often suffer from various interferences caused by multiple-users moving simultaneously, resulting in extremely low recognition accuracy. Even if the repetitive motions are fairly well detectable through the wireless signals in theory, in reality they get blended into various other system noises during the motion. Moreover, irregular motion patterns among users will lead to expensive computation cost for motion recognition. To tackle these challenges, we propose a novel wireless sensing system, called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Motion-Fi<inline-formula><tex-math notation="LaTeX">$\ ^+$</tex-math><alternatives><mml:math><mml:msup><mml:mspace width="4pt"/><mml:mo>+</mml:mo></mml:msup></mml:math><inline-graphic xlink:href="xiao-ieq2-2971996.gif"/></alternatives></inline-formula></i> , which marries battery-free wireless backscattering and device-free sensing in one clean sheet. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Motion-Fi<inline-formula><tex-math notation="LaTeX">$\ ^+$</tex-math><alternatives><mml:math><mml:msup><mml:mspace width="4pt"/><mml:mo>+</mml:mo></mml:msup></mml:math><inline-graphic xlink:href="xiao-ieq3-2971996.gif"/></alternatives></inline-formula></i> is an accurate, interference tolerable motion-recognition system, which counts repetitive motions without using scenario-dependent templates or profiles and enables multi-user performing certain motions simultaneously because of the relatively short transmission range of backscattered signals and dedicated signal separation method. We implement a backscattering wireless platform to validate our design in various scenarios for over 6 months when different persons, distances and orientations are incorporated. In our experiments, the periodicity in motions could be recognized without any learning or training process, and the accuracy of counting such motions can be achieved within 5 percent count error. With little efforts in learning the patterns, our method could achieve 95.2 percent motion-recognition accuracy for a variety of 7 typical motions. Moreover, by leveraging the periodicity of motions, the recognition accuracy could be further improved to nearly 100 percent with only three repetitions. Our experiments also show that the motions of multiple persons separating by around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ 2$</tex-math><inline-graphic xlink:href="xiao-ieq4-2971996.gif"/></inline-formula> meters cause little accuracy reduction in the counting process.

Rescue: Wireless power-enabled communication architecture for earthquake rescue operations

In a natural disaster such as an earthquake, it is vital to know the number of people trapped under the ruins. To address this problem, we propose RESCUE — wiREless backScattering CommUnication-based disastEr recovery system. RESCUE is composed of special Radio-frequency identification (RFID) readers and sensors that are used to determine the total number of people under the ruins. Passive wireless sensor nodes are placed inside the building during construction and are equipped with a camera that is activated during an earthquake. After the earthquake, communication to the passive tags of sensors is achieved by wireless power transfer from a reader located outside the ruins. Tags harvest this energy and send the image data stored by the camera. We also design an antenna structure to maximize simultaneous wireless information and power transfer (SWIPT) for devices under ruins. We analyze the communication channel between reader and sensors and derive a channel model over ruins. Furthermore, we obtain the results of experimental study where we validate the derived channel model. Results show that RESCUE can collect the desired data in a relatively short time, and hence, is a promising disaster recovery system architecture.

MilliBack

Real-time handwriting tracking is important for many emerging applications such as Artificial Intelligence assisted education and healthcare. Existing movement tracking systems, including those based on vision, ultrasound or wireless technologies, fail to offer high tracking accuracy, no learning/training/calibration process, low tracking latency, low cost and easy to deploy at the same time. In this work, we design and evaluate a wireless backscattering based handwriting tracking system, called MilliBack, that satisfies all these requirements. At the heart of MilliBack are two Phase Differential Iterative (PDI) schemes that can infer the position of the backscatter tag (which is attached to a writing tool) from the change in the signal phase. By adopting carefully-designed differential techniques in an iterative manner, we can take the diversity of devices out of the equation. The resulting position calculation has a linear complexity with the number of samples, ensuring fast and accurate tracking. We have put together a MilliBack prototype and conducted comprehensive experiments. We show that our system can track various handwriting traces accurately, in some testings it achieve a median error of 4.9 mm. We can accurately track and reconstruct arbitrary writing/drawing trajectories such as equations, Chinese characters or just random shapes. We also show that MilliBack can support relatively high writing speed and smoothly adapt to the changes of working environment.

Perfectly Synchronized Streaming From Multiple Digitally Modulated Backscatter Sensor Tags

This paper proposes multiple subcarrier multiple access (MSMA), a multiple access method for perfectly synchronized concurrent data collection from a group of passive backscatter wireless sensor tags, and its experimental evaluation. By achieving less than $128~\mu \text{s}$ synchronization error, backscatter communications can be applied to the structural health monitoring of artifacts, such as civil structures and machinery. In MSMA, each sensor tag uses a dedicated subcarrier frequency to produce a modulated backscatter. The sensor data is superposed onto the subcarrier with either an analog or digital modulation. The inevitable harmonics among the subcarriers, stemming from the backscatter principle, can be rejected by numerically calculating the harmonic replicas and subtracting them from the observed signal using a software defined receiver. A frame-based signal processing in the receiver results in no relative synchronization error among subcarriers, even after the interference rejection. Because the interference rejection can be performed before the demodulation and decoding, the concurrency can be secured irrespective of the choice of modulation method. We developed a prototype of MSMA using LabVIEW communications software defined radio environment, and prototype sensor tags using discrete electrical parts. The performance and limitations of MSMA using digitally modulated subcarriers are evaluated both in wired and wireless environments with up to four backscatter sensors. It is confirmed that the interference rejection can achieve about 7 dB processing gain. An experiment to measure the vibration of a bridge under weekday traffic with four sensors revealed the results obtained by MSMA and wired sensor agree well.

Backscatter-Aided Cooperative Relay Communications in Wireless-Powered Hybrid Radio Networks

Wireless backscatter communications has been introduced as a promising technique for data transmissions without a conventional energy supply from a battery or power outlet. It shares some similarity with RF energy harvesting powered wireless transmissions. Many benefits can be achieved when both techniques are integrated. In this article, we introduce the concept of a hybrid radio that is an integrated design of wireless-powered active RF communications and backscatter communications. We then discuss a few use cases of hybrid radio networks to suggest potential applications and to improve network performance. Furthermore, we consider a specific case in which hybrid radios act as passive relays to assist active RF communications. The passive relays backscatter the RF signals from the source to the receiver which, by experiments, shows to improve the transmission rate due to the enhanced multi-path diversity gain. We also introduce an optimization to quantify the performance improvement achievable by the radios' cooperation in the hybrid radio network.

Multi-Rate Polling: Improve the Performance of Energy Harvesting Backscatter Wireless Networks

Multi-Rate Polling: Improve the Performance of Energy Harvesting Backscatter Wireless Networks

Efficient and Sensitive Electrically Small Rectenna for Ultra-Low Power RF Energy Harvesting

A new electrically small antenna with size ka = 0.415 is presented, fabricated and measured in this work. This is intrinsically matched to 50 Ω, has omni-directional and linear-polarized radiation pattern in the horizontal plane with maximum directivity of 1.75 dBi and simulated radiation efficiency of 93%. The antenna in combination with a low-complex and co-planar rectifier with one single diode forms a high efficient and sensitive electrically small rectenna with ka = 0.53 at 868 MHz (UHF RFID-band in Europe). The latter has measured efficiency 22.5% for −19 dBm power input and sensitivity of −44 dBm (or equivalently 0.00028 μW/cm2 power density), while at 2.25 μW/cm2 is able to supply continuously, i.e., without a boost converter or use of any energy tank, a small electrical device with 118 μW. In order to increase the dc output voltage and the delivered dc power to the load for lower power density levels, rectenna-array configuration is exploited. Application to batteryless, backscatter wireless sensor node powering is discussed. Specifically, for a power density of 0.1237 μW/cm2 the RF energy harvesting system delivers 172 μW at 2.85 V every 22.5 s.

The Art of Signal Processing in Backscatter Radio for &amp;mu;W (or Less) Internet of Things: Intelligent Signal Processing and Backscatter Radio Enabling Batteryless Connectivity

Backscatter (or simply scatter) radio is based on reflection principles, where each tag modulates information on top of an illuminating signal, by simply connecting its antenna to different loads; modulation of information is based on the modifications of the tag antenna-load reflection coefficient, requiring in principle only a switch and omitting powerconsuming signal conditioning units, such as mixers, amplifiers, oscillators, and filters. The ultralow-power nature of backscatter radio, in conjunction with the recent advances in multiple access and achieved communication ranges (on the order of hundreds of meters to kilometers), due to intelligent signal processing, elevate backscatter radio as the de facto communication principle e for nW (or less)-level consumption, last-mile connectivity, and Internet of Things (IoT) networking. This article is an update to the state-of-the-art advances in the emerging backscatter radio domain, focusing on the signal processing engine, including ambient illumination from existing signals, as well as unconventional backscatter radio-based IoT technologies that could revolutionize environmental sensing and agriculture. Finally, the offered research methodology and techniques in short-packet, channel-encoded (or not), coherent (or not) sequence detection will assist researchers in radio-frequency identification (RFID)/backscatter radio as well as other domains of the telecommunications industry.

A uW Backscatter-Morse-Leaf Sensor for Low-Power Agricultural Wireless Sensor Networks

Nowadays, the monitoring of plant water stress is of high importance in smart agriculture. Instead of the traditional ground soil-moisture measurement, the leaf sensing is a new technology, which is used for the detection of plants needing water. In this paper, a novel, low-cost, and low-power system for leaf sensing using a new plant backscatter sensor node/tag is presented. The latter, can result in the prevention of water waste (water-use efficiency), when is connected to an irrigation system. Specifically, the sensor measures the temperature differential between the leaf and the air, which is directly related to the plant water stress. Next, the tag collects the information from the leaf sensor through an analog-to-digital converter, and then, communicates remotely with a low-cost software-defined radio reader using monostatic backscatter architecture. The tag consists of the sensor board, a microcontroller, an external timer, and an RF front-end for communication. The timer produces a subcarrier frequency for simultaneous access of multiple tags. The proposed work could be scaled and be a part of a large backscatter wireless sensor network. The communication protocol exploits the low-complexity Morse code modulation on a 868 MHz carrier signal. The presented novel proof-of-consent prototype is batteryless and was powered by a flexible solar panel consuming power around $20~\mu \text{W}$ . The performance was validated in an indoors environment where wireless communication was successfully achieved up to 2 m distance.

Word-Fi: Accurate Handwrite System Empowered by Wireless Backscattering and Machine Learning

Word-Fi is a handwriting input system, driven by wireless backscattering technology and machine learning methods. It could effectively mitigate the surrounding noise and extract the weak signals incurred by tiny writing gestures accurately. Leveraging our customized wireless backscattering system, Word-Fi could be noise tolerant across relatively complex environments, especially when multiple persons are presented around, which significantly differs from status quo wireless sensing systems that suffer from multi-user presentation. For weak signal extraction, Word- Fi incorporates an efficient feature selection scheme for classification and improves the classifier by fully exploiting the physical layer information. After using the word suggestion module, it could recognize writing words with fairly high accuracy (above 90 percent) across different volunteers (7-10).

Backscatter Relay Communications Powered by Wireless Energy Beamforming

The integration of wireless power transfer (WPT) with the low-power backscatter communications provides a promising way to sustain battery-less wireless networks. In this paper, we consider a backscatter communication network wirelessly powered by a power beacon station (PBS). Each backscatter radio uses the harvested energy to power its data transmissions, in which some other radios can help as the wireless relays with an aim to improve throughput performance by cooperative transmission. Under this setting, we formulate a throughput maximization problem to jointly optimize WPT and the relay strategy of the backscatter radios. An iterative algorithm with reduced complexity and communication overhead is proposed to decompose the original problem into two sub-problems distributed at the PBS and the backscatter receiver. Moreover, we take uncertain channel information into consideration and formulate robust counter-parts of the throughput maximization problem when either the backscatter or relay channel is subject to estimation errors. The difficulty of the robust counter-part lies in the coupling of the PBS’ power allocation and relay strategy in matrix inequalities, which is addressed by alternating optimization with guaranteed convergence. Numerical results reveal that the cooperative relay strategy of the backscatter radios significantly improves the throughput performance.

Wireless Powered Asynchronous Backscatter Networks With Sporadic Short Packets: Performance Analysis and Optimization

In the fifth generation era, the pervasive applications of Internet of Things and massive machine-type communications have initiated increasing research interests on the backscatter wireless powered communication (B-WPC) technique due to its ultrahigh energy efficiency and low cost. The ubiquitous B-WPC network is characterized by nodes with dynamic spatial positions and sporadic short packets, of which the performance has not been fully investigated. In this paper, we give a comprehensive analysis of a multiantenna B-WPC network with sporadic short packets under a stochastic geometry framework. By exploiting a time-space Poisson point process model, the behavior of the network is well captured in a decentralized and asynchronous transmission way. We then analyze the energy and information outage performance in the energy harvest and backscatter modulation phases of the backscatter network, respectively. The optimal transmission slot length and division are obtained by maximizing the network-wide spatial throughput. Moreover, we find an interesting result that there exists the optimal tradeoff between the durations of the energy harvest and backscatter modulation phases for spatial throughput maximization. Numerical results are demonstrated to verify our analytical findings and show that this tradeoff region gets shrunk when the outage constraints become more stringent.

Game-Theoretic Modeling of Backscatter Wireless Sensor Networks Under Smart Interference

In this letter, we study an interference avoidance scenario in the presence of a smart interferer, which can rapidly observe the transmit power of a backscatter wireless sensor network (WSN) and effectively interrupt backscatter signals. We consider a power control with a sub-channel allocation to avoid interference attacks and a time-switching ratio for backscattering and RF energy harvesting in backscatter WSNs. We formulate the problem based on a Stackelberg game theory and compute the optimal parameters to maximize a utility function against the smart interference. We propose two algorithms for the utility maximization using Lagrangian dual decomposition for the backscatter WSN and the smart interference to prove the existence of the Stackelberg equilibrium. Numerical results show that the proposed algorithms effectively maximize the utility, compared to that of the algorithm based on the Nash game, so as to overcome smart interference in backscatter communications.

Quadrature Amplitude Backscatter Modulator for Passive Wireless Sensors in IoT Applications

Fully passive wireless networks are a key-enabling technologies for the internet of things. Backscatter radios are a hypothesis to design these passive wireless networks. Backscatter modulation allows a remote device to wirelessly transfer information without requiring a traditional transceiver. Instead, a backscatter device leverages a carrier transmitted by an access point or base station. Nevertheless, the traditional approach is to use amplitude shift keying or phase shift keying backscatter solutions which limits the data rate of the sensors, since it transfers only one bit per symbol period. We propose a novel modulator, which employs a Wilkinson power divider with a phase shift in one of the design branch and two transistors acting as switches in order to generate M-quadrature amplitude modulation backscatter modulation. The design strategy for high level order backscatter modulations will be explained and a design approach will be presented in this manuscript.

Implementation of Bistatic Backscatter Wireless Communication System Using Ambient Wi-Fi Signals

Implementation of Bistatic Backscatter Wireless Communication System Using Ambient Wi-Fi Signals

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.

Physical Layer Security in MIMO Backscatter Wireless Systems

Backscatter wireless communication is an emerging technique widely used in low-cost and low-power wireless systems, especially in passive radio frequency identification (RFID) systems. Recently, the requirement of high data rates, data reliability, and security drives the development of RFID systems, which motivates our investigation on the physical layer security of a multiple-input multiple-output (MIMO) RFID system. In this paper, we propose a noise-injection precoding strategy to safeguard the system security with the resource-constrained nature of the backscatter system taken into consideration. We first consider a multi-antenna RFID tag case and investigate the secrecy rate maximization (SRM) problem by jointly optimizing the energy supply power and the precoding matrix of the injected artificial noise at the RFID reader. We exploit the alternating optimization method and the sequential parametric convex approximation method, respectively, to tackle the non-convex SRM problem and show an interesting fact that the two methods are actually equivalent for our SRM problem with the convergence of a Karush-Kuhn-Tucker (KKT) point. To facilitate the practical implementation for resource-constrained RFID devices, we propose a fast algorithm based on projected gradient. We also consider a single-antenna RFID tag case and develop a low-complexity algorithm which yields the global optimal solution. Simulation results show the superiority of our proposed algorithms in terms of the secrecy rate and computational complexity.

Continuously Power Delivering for Passive Backscatter Wireless Sensor Networks

Nowadays wireless sensor networks (WSNs) depend on the battery duration and there is a lot of interest in creating a passive sensor network scheme in the area of Internet of Things (IoT) and space-oriented WSN systems. Radio frequency (RF) energy harvesting enables the control and the delivery of wireless power to RF devices. All the devices made with this technology can be sealed, embedded with these structures, or made mobile, so they can be battery-independent. In this paper, we show that it is possible to have a complete passive WSN that could potentiate the IoT area and space craft sensing. This WSN should be capable of receiving a continuous flow of energy, so that the sensing mechanisms can be continuously powered. For this, we propose a combination of backscatter modulation with wireless power transfer, using the same diode based approach as in a traditional RF identification circuit. This way the cost is maintained low, but the amount of gathered energy is maximized during the backscatter communication. This paper also presents all the steps that are needed in order to implement such a circuit.

An NFC-Enabled CMOS IC for a Wireless Fully Implantable Glucose Sensor.

This paper presents an integrated circuit (IC) that merges integrated optical and temperature transducers, optical interface circuitry, and a near-field communication (NFC)-enabled digital, wireless readout for a fully passive implantable sensor platform to measure glucose in people with diabetes. A flip-chip mounted LED and monolithically integrated photodiodes serve as the transduction front-end to enable fluorescence readout. A wide-range programmable transimpedance amplifier adapts the sensor signals to the input of an 11-bit analog-to-digital converter digitizing the measurements. Measurement readout is enabled by means of wireless backscatter modulation to a remote NFC reader. The system is able to resolve current levels of less than 10 pA with a single fluorescent measurement energy consumption of less than 1 μJ. The wireless IC is fabricated in a 0.6-μm-CMOS process and utilizes a 13.56-MHz-based ISO15693 for passive wireless readout through a NFC interface. The IC is utilized as the core interface to a fluorescent, glucose transducer to enable a fully implantable sensor-based continuous glucose monitoring system.