Published papers

This letter presents a method for measuring rotor blade strain synchronized with the rotation angle using a battery-free wireless strain sensor. The proposed approach leverages the emerging ISO/IEC 18000-65 standard to enable concurrent data streaming from battery-free backscatter sensors in the 920 MHz band. Synchronization between strain sensing and the rotor blade rotation angle is achieved using a photoelectric sensor for rotation timing detection, time-stamped strain measurements, and an FPGA-based timestamp reset mechanism with analog reset pulse generation. By coordinating the blade rotation timing with the reset pulse in a postprocessor, the strain data are accurately aligned with the rotation angle. The feasibility of the proposed method is validated through wind tunnel experiments using a model helicopter, demonstrating a synchronization timing standard deviation of 0.4 ms resulting in about 3° angle standard deviation at high speed rotation.

This paper introduces a method to flexibly shape the sensing zone of 920 MHz band backscatter communication systems comprising battery-free wireless sensors. The shaping is realized by a passive repeater using a combination of a leaky coaxial cable (LCX) and an antenna. The passive repeater relays the wireless power and commands from an interrogator and delivers them to the backscatter sensors. The signals from backscatter sensors are relayed back to the interrogator through the passive repeater. This paper proposes a physical-distanceagnostic link model of a backscatter communication including an LCX passive repeater based on a set of experimental data. The advantage of using magnetic coupling antenna in the LCX passive repeater was also presented. A prototype of the passive repeater in 920 MHz was developed to reveal its practicality and performance by demonstrating non-line-of-sight wireless temperature sensing in an electro-magnetic shield room with less than 0.2% packet error rate.

Concurrent backscatter communication with multiple battery-free sensors suffers from the frequency instability of dedicated subcarrier caused by the process-power supply voltage-temperature (PVT) characteristic of the radio frequency integrated circuit (RFIC) in the battery-free sensor. This paper proposes an all-digital open-loop carrier and timing recovery circuit, which uses the most likelihood estimation of subcarrier frequency, and the delayed detection exploiting the differential zero-crossing points as the symbol timing recovery triggers. The proposal is implemented in a software defined interrogator. Conducted testing with a fading simulator and real world wireless testing with the battery-free backscatter sensors in 920 MHz band reveal the effectiveness of the proposal.

This letter presents a novel battery-free wireless strain sensing system consisting of an interrogator and single or multiple streaming sensors. A streaming sensor comprises a radio frequency integrated circuit (RFIC), a commercial off-the-shelf strain gauge, and an analog-to-digital converter (ADC). Backscatter communication in the 920 MHz band is employed to both power the sensor and to establish communication. The emerging ISO/IEC 18000-65 (FDIS), which supports concurrent backscatter streaming, is adopted as the communication protocol. The ADC sampling rate and the backscatter frequency channel are over-the-air configurable through a glue logic between the RFIC and the ADC. The proposed system was evaluated in an indoor propagation environment. It was experimentally demonstrated that the proposed system can achieve an error free concurrent strain measurement with a measurement resolution in the order of 10-5 and reading distance of 1.8 m using a 4-W effective isotropic radiation power software defined interrogator.

To scale to trillions of Internet-of-Things (loT) nodes, devices must be battery-less by achieving energy autonomy, having a μW power floor, few off-chip components, and a small form factor. These features are beneficial for short-range high-density loT applications such as structural health monitoring (SHM) for wings and gears, human motion capture (HMC), and air conditioning system monitoring. This work focuses on the synchronous transmission of sensor data streams, called concurrent data streaming, from multiple sensor nodes for multi-points sensing. Traditional wireless communication protocols need construct wireless networks for time synchronization, increasing system complexity [1]. However, the recent development of Multiple Subcarrier Multiple Access (MSMA), enables concurrent backscatter communication by assigning multiple uplink subcarrier (USC) channels to sensor nodes [2]-[4]. To expand the deployment in the mentioned loT applications, MSMA should be compatible with existing communication protocols and achieve real-time control of starting and stopping of concurrent data streaming without wasting extra sensing system energy. To avoid channel interference among multiple nodes and catastrophic failure of concurrent data transmission, the USC frequency must always keep stable from the influence of PVT.

Concurrent data communication based on backscat-ter is an unnoticed research domain in the past, but recently it is considered as a promising technology for Massive IoT in the B5G/6G wireless sensor network (WSN). In this domain, it is a practical multiple access solution to use multiple subcarriers. This paper proposes a frequency continuous calibration method to improve the frequency accuracy of uplink subcarrier (USC) in the concurrent data communication mode, called USC continuous calibration (USCoCa). Tag obtains a 10kHz amplitude modulation frequency reference from reader, uses a digital frequency lock loop to calibrate the on-chip digital controlled ring oscillator, and generates USC with a wide frequency range and good frequency accuracy. The tag with USCoCa is fabricated with 0.51×1.02 mm2 area size. The test result shows that US CoCa can keep continuous calibration of USC frequency, and the USC frequency deviation is less than 0.25 % within the range of 100-620kHz frequency. At the same time, the USC frequency stability can even be maintained under the +134°C /min rate temperature environment. Therefore, USCoCa can ensure stable frequency channel assignment for the multiple sub carrier UHF RFID system and guarantee robust concurrent data stream transmission.

This study utilized multimodal information using both acceleration data and coordinate data to recognize sign language motion. First, to confirm the effectiveness of recognizing sign language motion using individual acquisition information, the authors conducted an experiment to compare motion classification performance when each acquisition of data was treated independently. The performance comparison results showed differences in the tendency for false classifications in the classifiers constructed using only each data group. Therefore, high sign language recognition performance seemed more likely to be achieved if each classifier using multimodal information were to be integrated in a complementary manner. In this paper, three integration methods for multimodal information are proposed and compared against each other. Method 1 assumes that the classification results of classifiers trained individually on the acquired data are mixed with incorrect candidate classifiers with relatively low likelihood, and the classifiers are integrated after narrowing down the candidate classifiers using that likelihood. Method 2 considers that ranked information, even with a low likelihood, give a high probability of producing a correct classification result, and the authors carried out classification result integration by assigning a rejection condition using the rank sum of each classifier. Method 3 combines Method 1 and Method 2. It is an integration model that rejects each result with a low probability of being correct and then integrates the classifiers by value of the rank that is output as a significant classification result integration. An evaluation index γ was proposed for assessing recognition performance based on the correct rate, error rate, and rejection rate, and the above three proposed methods were evaluated based on that index.

This paper proposes a wireless strain sensing system for model helicopter rotor blades using implanted backscatter sensors. Strain measurements from two sensors were continuously collected via dedicated subcarrier frequency backscatter channels. While coherent combining of two IQ streams improved packet error rate (PER) in low-speed rotations, it negatively affected performance at high speeds setting due to Doppler shift. Numerical simulations clarified the degradation mechanism and suggested a practical solution: using a single receiving antenna. This approach was validated through laboratory experiments.

This article introduces a highly integrated passive wireless sensing system based on the extended electronic product code (EPC) Gen2 protocol, using sensor tags and a reader to support concurrent data streams from multiple sensors. Multiple subcarrier multiple access (MSMA) is used by assigning different subcarrier frequencies to the sensor tags for backscattering so that each tag has an independent channel for communication to achieve the simultaneous streaming of multisensor data. In order to improve the subcarrier frequency accuracy and further increase the number of communication channels, this article proposes a novel method of subcarrier frequency calibration based on the Gen2 protocol. This method enables a high-accuracy clock to be generated on-chip. At the same time, this article also proposes a method based on Gen2 protocol to start a group of tags and receive real-time data in parallel. The tag chip is fabricated in a 0.18-μm CMOS process with a die size of 850μm × 850μm, and the reader prototype is built using software-defined radio equipment. The test results show that the sensing system proposed in this article can collect and recognize concurrent data streams from multiple sensor tags synchronously. The measured sensitivity of the sensor tag is -14.9 dBm with a 3-axis accelerating sensor, and the calibration accuracy of subcarrier frequency is better than ±0.385 kHz in the range of 40-640 kHz. With this accuracy, the tags can be allocated up to 78 subcarrier communication channels theoretically.

Backscatter communication is an emerging wireless access technology to realize ultra-low power terminals exploiting the modulated reflection of incident radio wave. This paper proposes a method to measure the phase angle of backscatter link using principal component analysis (PCA). The phase angle measurement of backscatter link at the receiver is essential to maximize the signal quality for subsequent demodulation and to measure the distance and the angle of arrival. The drawback of popular phase angle measurement with naive phase averaging and linear regression analysis is to produce erroneous phase angle, where the phase angle is close to ±π 2 radian and the signal quality is poor. The advantage of the proposal is quantified with a computer simulation, a conducted experiment and radio propagation experiments.

Trillions of Internet of Things devices require rigorous design trade-offs regarding size, cost, and energy efficiency, leading to design challenges for clock modules. This paper proposes a low-power all-digital frequency locked loop (ADFLL) featuring a digital frequency detector with minimum D-type flip-flop usage and a lock detector with hysteresis lock/unlock decision zone, suitable for system-on-chip applications. The ADFLL is verified in the field-programmable gate arrays (FPGA) and is implemented as an on-chip clock generator in a radio frequency identification tag using 130 nm CMOS technology with 0.185 × 0.26 mm2 size. The generated clock has ±0.25% frequency accuracy with good temperature and process robustness.

Transport package test represents the integrity check of packaged commercial products exposed to an emulated transportation environment such as vibrations and shocks. This paper examines the practicality of wireless transport package test with a backscatter streaming system with battery-free backscat-ter sensors by extensive experiments. The system bases on Gen2 protocol with a set of streaming extensions. Random vibration and drop tests, defined in an ISO standard, were performed on a test subject PC. The measurement accuracy of the backscatter streaming system was compared with that of wired sensor to reveal that the accuracy of the system is sufficient for basic transport package test conditions. It was confirmed that the wireless, battery-less and lightweight natures of the backscatter sensor facilitate the test preparation and configuration changes, and is advantageous in continuous wireless streaming. In addition to the merits, the limitations of the system in transport package test are also discussed.

This paper proposes a carrier recovery method for subcarrier-coded backscatter links using a fractional, less than one-symbol, delayed detection. The fractional delayed detection extends the permissible subcarrier frequency errors but results in an unbalanced distribution of delayed symbols. This paper reveals the contribution of fractional detection in RFID sensing systems, particularly the system with an FDMA backscatter to collect streaming data. The unbalanced symbol distribution problem is resolved using a principal component analysis (PCA) of delayed symbols. The fractional delayed detection was evaluated both with a MATLAB simulation and a combination of a software-defined interrogator and a prototype wireless sensor. Both evaluations showed the effectiveness of the proposal.

This paper proposes a loose bolt detection system with battery-free wireless sensors based on backscatter communication. Loose bolt detection has been performed using wired or optical sensors. However, wired sensors demand temporary attachment of sensors and the routing of wires. Optical sensors demand precise alignment and the line of sight to the measurement points. The proposed backscatter system essentially eliminates those problems and can be used even when the target machine or structures are in their operation if the backscatter sensors are implanted to the target structure. On the other hand, the proposed system involves the challenges of data synchronization and the limited bandwidth. In this study, a prototype wireless loose bolt detection system based on transmittance function and damage index is developed and is comparatively evaluated with wired accelerometers by a series of experiments. The data synchronization problem was tackled by preprocessing of the collected data. It is also revealed that sufficient bandwidth in the wireless communication channels is important for accurate detection.

This paper proposes a fast blind equalization algorithm realized by equating the higher-order statistics of transmitted and received signals’ IQ constellations over wireless channels. The idea is motivated by the fact that the transmitter and the receiver usually pre-share the modulation scheme, which determines the statistical distribution of the IQ constellation. Conventional blind equalizer algorithms are not practical because these optimize the equalizer coefficients with the gradient descent method demanding from hundreds to thousands of iterations to converge. This paper proposes a dimensionless loss function that can be extended to various modulation schemes and applies the Levenberg-Marquardt (LM) algorithm to regularize the Hessian matrix of the loss function automatically. This regu-larization compensates the singularity of the Hessian matrix which enables fast equalizing. The proposed method is evaluated in a MATLAB simulation to reveal that the algorithm can converge within dozens of iterations, achieving about a hundred times faster convergence and about a 30 times faster computation time than conventional blind equalization. The evaluation also reveals that the cause of the singularity of the Hessian matrix is the eigenvector space of equalizer coefficients which works to amplify inter-symbol-interference.

This paper presents a harmonics rejection method for the sensor-to-interrogator link of frequency division multiple access (FDMA) backscatter communication system exploiting an FFT filterbank. It is well known that backscatter signals produce undesired outband emissions and harmonics because of the absence of a pulse shaping filter. Harmonics, produced by introducing subcarrier coding, are particularly problematic because they interfere with particular frequency channels. The proposed method features the generation of harmonics replicas from the baseband signal in an allocated channel produced by a FFT filterbank. The generation of a harmonics replica from the baseband signal demands a delicate handling of the carrier and the subcarrier phases. This paper presents this problem and proposes a two-stage harmonics replica generation exploiting the statistical independence among the interfering and interfered baseband signals. The proposal was implemented in a software defined interrogator, and was evaluated with a wired experiment with various carrier-to-interference ratios and two real-world wireless propagation experiments using prototype battery-free backscatter sensors. These evaluations revealed that the proposed harmonic rejection method could deliver about 6 dB of processing gain.

Simultaneous inventory and angle of arrival (AoA) measurement of UHF RF tags is strongly demanded for item searching in warehouses and offices. Existing high accuracy AoA measurements of RF tags mainly employ a radio comprised of multiple antennas and coherent receivers to produce synchronized multiple IQ streams. Such radio is usually realized as a dedicated hardware and software, which is not suitable for prototyping new ideas. This paper proposes a method to realize simultaneous inventory and AoA measurement with multiple commodity software defined radios (SDRs). In the proposal, the cancellation of mutual phase offsets, an well known problem of multiple commodity SDRs, is done by physically switching the combination of receiving antennas and receivers with an external RF switch and subtracting the two phase differences. This way, the calibration before AoA measurement can be done during the process of phase measurement with no human intervention. The antenna switching is integrated into a SDR Gen2 interrogator developed on a dual RF-ports USRP. The external RF switch is synchronously controlled with Gen2 inventory progress using Gen2 protocol features called action and target. The accuracy of phase offset cancellation was evaluated with wired and wireless experiments with commercial Gen2 RF tags. The experiments revealed that AoA measurement can be simultaneously performed with Gen2 inventory without any intervention achieving less than 3 degree of phase offset cancellation error.

This paper reports the experimental verification of concurrent backscatter streaming system comprising batteryless backscatter sensors and a software defined interrogator. The backscatter sensor is composed of a newly developed Radio Frequency Integrated Circuit (RFIC) and a commodity three axis microelectro mechanical system (MEMS) accelerometer. The concurrency is realized by allocating a dedicated subcarrier to each backscatter sensor, and using novel interference rejection signal processing in the interrogator. The communication protocol bases on ISO/IEC 18000-63 protocol with the extension of channel allocation and streaming control. The prototype system is evaluated in an experiment in where three backscatter sensors are installed on three emulated robotic mechanisms for wireless motion capture. It is confirmed that over-the-air subcarrier allocation and the concurrent capture of three robotic mechanism accelerations can be done despite the 10-4 order frequency instability of RFICs.

Backscatter communication system, such as RFID, usually establishes bilateral communication link with time-wise half-duplex. Emerging data streaming using backscatter communication with low power or even batteryless sensors demands full duplex to stop streaming while a group of sensors backscatter. This paper introduces a full duplex method in backscatter communication using a simple decimation in backscatter sensors referred to as subcarrier synchronized decimation (SSD). Numerical simulations reveal that SSD can reconstruct the original reader-to-sensor link waveform subjected to backscatter modulation sensor-to-reader link, although the proposed method inevitably produces artificial noise because of the unequal time interval decimation. The contribution of artificial noise is investigated by using a sampling jitter model. This paper also reports a prototype implementation of SSD with USRP and LabVIEW NXG combined with a commodity DPDT switch to prove the feasibility of the proposal.

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.