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Papers - Mitsugi, Jin

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A low power all-digital frequency locked loop with resource-efficient frequency detector and hysteresis lock detector

Zhao Y., Mitsugi J., Min H.

IEICE Electronics Express 21 ( 20 ) 2024

ISSN 13492543

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.

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Accurate Phase Angle Measurement of Backscatter Signal under Noisy Environment

Iwasaki T., Tokumasu O., Mitsugi J.

IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences (IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences) E106A ( 3 ) 464 - 470 2023.03

ISSN 09168508

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.

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Two-Stage Harmonics Rejection in FDMA Backscatter Communication System Using FFT Filterbank

Mitsugi J., Tokumasu O., Sakabe H.

IEEE Journal of Radio Frequency Identification (IEEE Journal of Radio Frequency Identification) 7 518 - 527 2023

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.

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Fractional Delayed Detection For Unstable Subcarrier-Coded Backscatter Link

Zhao Z., Mitsugi J.

2023 IEEE 13th International Conference on RFID Technology and Applications, RFID-TA 2023 - Proceedings 253 - 256 2023

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.

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MSN: Battery-Less Multiple Subcarrier Multiple Access Sensor Node with Full-Duplex Backscatter Concurrent Data Streaming

Zhao Y., Wang H., Xu Y., Ying Z., Lu Y., Liu J., Hu T., Mitsugi J., Min H.

2023 IEEE Asian Solid-State Circuits Conference, A-SSCC 2023 2023

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.

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USCoCa: A Subcarrier Frequency Continuous Calibration Method for Backscattered Concurrent Data Mode in UHF RFID System

Zhao Y., Jiang H., Ying Z., Gao F., Li S., Shen Z., Tang K., Wang H., Mitsugi J., Min H.

2023 IEEE International Conference on RFID, RFID 2023 66 - 71 2023

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.

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A Highly Integrated Passive Wireless Sensing System With Synchronized Data Streaming of Multiple Tags

Tang K., Wang M., Liu M., Zhang C., Xu S., Ji S., Lai S., Yan N., Min H., Mitsugi J.

IEEE Internet of Things Journal (IEEE Internet of Things Journal) 9 ( 17 ) 15525 - 15537 2022.09

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.

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Fast Blind Equalizer Exploiting Statistical Features of Symbol Constellations

Takeyama N., Mitsugi J.

Journal of Information Processing (Journal of Information Processing) 30 909 - 916 2022

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.

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Experimental Evaluation of Wireless Transport Package Test with Battery-free Backscatter Sensors

Mitsugi J., Tokumasu O., Ichikawa H., Ochiai T., Kawakita Y.

2022 IEEE International Conference on RFID, RFID 2022 58 - 63 2022

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.

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Open loop demodulator for narrowband continuous backscatter wireless links

Mitsugi J., Kawakita Y.

IEEE Wireless Communications and Networking Conference, WCNC 2022-April 1425 - 1430 2022

ISSN 15253511

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.

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Wireless Loose Bolt Detection with Multiple Battery-free Backscatter Sensors

Iwasaki T., Tokumasu O., Mitsugi J.

Proceedings of IEEE Sensors 2022-October 2022

ISSN 19300395

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.

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A novel method to improve the subcarrier frequency accuracy for UHF RFID sensing system

Tang K., Zhang C., Wang M., Xu S., Ji S., Yan N., Min H., Mitsugi J.

2021 15th Annual IEEE International Conference on RFID, RFID 2021 (2021 15th Annual IEEE International Conference on RFID, RFID 2021) 2021.04

ISSN 9781665440905

This paper presents a novel method to improve the subcarrier frequency accuracy of tag chip for UHF RFID sensing system based on EPC global Gen2 protocol. By changing the tag's response to the Write command, the tag returns the subcarrier before returning the response specified in the protocol. In this way, a frequency calibration loop will be established between reader and tag. This method not only enables a low-power oscillator with a precise frequency to be integrated into a tag chip, but also enables flexible configuration of the frequency. The tag chip containing this idea is fabricated in SMIC 0.18 μm CMOS process with a die size of 850 μm × 850 μm. Measurement results show that the proposed novel method can realize the calibration of the subcarrier frequency, and the subcarrier frequency deviation is less than ±0.385KHz in the range of 40KHz-640KHz. With this accuracy, the tag can be allocated up to 78 subcarrier communication channels.

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Concurrent backscatter streaming with batteryless sensors for wireless motion capture applications

Mitsugi J., Kawakita Y., Ichikawa H.

IEEE Vehicular Technology Conference (IEEE Vehicular Technology Conference) 2021-April 2021.04

ISSN 9781728189642

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.

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Wireless Modal Testing with Multiple Battery-free Backscatter Sensors

Mitsugi J., Tokumasu O., Kawakita Y.

2021 IEEE International Conference on Wireless for Space and Extreme Environments, WiSEE 2021 (2021 IEEE International Conference on Wireless for Space and Extreme Environments, WiSEE 2021) 90 - 96 2021

ISSN 9781665403719

This paper reports the design, implementation and the result of real-world experiment of wireless modal testing with battery-free backscatter sensors. The battery-free backscatter sensor comprises a custom radio frequency integrated circuit (RFIC), a commodity micro electro-mechanical system (MEMS) accelerometer, and an antenna. Sensor data concurrency demanded in modal analysis is realized by allocating a dedicated subcarrier to each backscatter sensor. The orchestration of wireless communications in 920 MHz based on Gen2 protocol and the wireless power supply to the backscatter sensor are performed by a software defined interrogator built on Lab-VIEW communications and USRP. We developed an over the air (OTA) subcarrier frequency allocation with a sequence of WRITE commands of Gen2 protocol, and its performance was confirmed with the experiments in an indoor radio propagation. The accuracy of the proposed wireless structural modal testing was experimentally verified by comparing with a commercial modal test system with wired accelerometers. Although the phase characteristic measurement is disturbed by the instability of the clock source of RFIC, the gain characteristics of wireless modal testing matches well with that of a commercial wired modal testing system.

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Full duplex backscatter communication with subcarrier synchronized decimation

Mitsugi J., Kawakita Y., Ichikawa H.

IEEE Wireless Communications and Networking Conference, WCNC (IEEE Wireless Communications and Networking Conference, WCNC) 2021-March 2021

ISSN 9781728195056

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.

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Wireless modal analysis with backscatter sensors subjected to clock instability

Mitsugi J., Tanaka H.

WiSEE 2020 - 8th Annual IEEE International Conference on Wireless for Space and Extreme Environments, Proceedings (WiSEE 2020 - 8th Annual IEEE International Conference on Wireless for Space and Extreme Environments, Proceedings) 17 - 23 2020.10

ISSN 9781728164519

This paper examines the practicality of concurrent sensor data streaming with backscatter communication applied to modal analysis, which demands accurate data synchronization among streams. The major synchronization error source in concurrent backscatter streaming system is the instability of backscatter sensor clock. Synchronization of the streams by resampling at receiver is necessary but careless implementation may produce artificial delay caused by the difference of anti-Alias filters tailored to the individual clock rates of backscatter sensors. In this paper, a set of signal processing to synchronize concurrent backscatter signals is designed and implemented in a prototype system. The prototype system comprises backscatter sensors, interrogators and modal analyzer, which takes care of the generation of transfer matrix. The signal processing in the modal analyzer features a recursive least square regression to recover instantaneous sampling rate of backscatter sensor and a zero-phase-filtering in overlap-And-Add approach. The proposal is evaluated by comparing the transfer matrices generated from the prototype system and a commercial modal analysis system where wired accelerometers are used. The experimental results show that the proposed signal processing can successfully synchronize concurrent backscatter streams by eliminating the artificial delay.

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Simultaneous Gen2 Inventory and Angle of Arrival Measurement of Backscatter Signals with Multiple Commodity SDRs

Mitsugi J., Kawakita Y.

2020 IEEE International Conference on RFID, RFID 2020 (2020 IEEE International Conference on RFID, RFID 2020) 5 ( 4 ) 368 - 377 2020.09

ISSN 9781728155760

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.

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Channel Reservation by Even-odd Separation in Multiple Subcarrier Multiple Access

Takahashi K., Takazawa Y., Kamei H., Mitsugi J., Ichikawa H., Kawakita Y.

2019 12th International Conference on Mobile Computing and Ubiquitous Network, ICMU 2019 (2019 12th International Conference on Mobile Computing and Ubiquitous Network, ICMU 2019) 2019.11

ISSN 9784907626419

© 2019 IPSJ. Authors propose a method for improving channel reservations in multi-subcarrier multiple-access wireless communications schemes. This scheme is designed for use with wide-area synchronous sensing zones that use functionally extended multi-subcarrier multiple-access subcarrier communication schemes for passive radio-frequency identification (RFID). When the sensing target is larger than the communication distance of the sensor, the sensing zone must be divided. Overlapping zones invariably occur in this case. Sensors in overlapping zones transmit signals to their respective receivers over the same channels and interference follows. If three or more zones are placed side by side, middle zone's communication capacity is lower than else zones. Because, middle zone can not use nearly center frequency. However, the communication capacity and its variations have not been examined for how they are affected by the zones in which overlapping sensors are used. This study investigated communications between sensors in overlapping zones by assigning channels to receivers in odd zones. The channel reservation method and the conventional method were simulated in MATLAB an compared. By allocating channels to sensors in odd zones, communication capacity can be improved and variations suppressed.

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Statistical carrier phase recovery for narrow-band backscatter wireless link

Sato Y., Mitsugi J., Kawakita Y., Ichikawa H.

Proceedings of 2019 25th Asia-Pacific Conference on Communications, APCC 2019 (Proceedings of 2019 25th Asia-Pacific Conference on Communications, APCC 2019) 142 - 147 2019.11

ISSN 9781728136790

© 2019 IEEE. This paper proposes a carrier recovery method tailored to narrow-band multicarrier backscatter wireless links, which employ subcarrier modulation. Because backscattered subcarrier is usually produced by low accuracy and low rate internal clock in a low power consumption or batteryless, wireless sensor, the corresponding receiver needs to compensate the subcarrier frequency drift and fluctuation dynamically for demodulation and decoding. Although a backscattered subcarrier is produced by changing the antenna reflection coefficient of wireless sensor, the backscattered signal is usually converted to a zero-crossing signal to enhance signal resolution, and edge detection based demodulator is applied at the receiver. However, in a multicarrier backscatter system, the edges at symbol boundaries of each backscattered subcarrier become indistinct because of the narrow bandwidth. Consequently, subcarrier frequency estimation with existing carrier recovery technique may results in error. This paper solves the problem by removing outliers of temporal subcarrier frequency based on the statistical distribution and applying a feedback control. The statistical carrier recovery method is examined experimentally with a prototype wireless sensor and a software defined receiver to reveal the method can automatically compensate unstable subcarrier with about 10-2 order drift and outperforms the popular frequency doubler based carrier recovery. The optimal feedback gain, 1.0, is also revealed.

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Real-time separation of collided signals in multiple zones backscatter communication system

Mitsugi J., Kamei H., Kawakita Y., Ichikawa H.

7th IEEE International Conference on Wireless for Space and Extreme Environments, WiSEE 2019 - Conference Proceedings (7th IEEE International Conference on Wireless for Space and Extreme Environments, WiSEE 2019 - Conference Proceedings) 87 - 92 2019.10

ISSN 9781728121406

© 2019 IEEE. Backscatter communication is essential in applications where wireless and batteryless sensors are demanded. Backscatter communication system may suffer short communication range because its sensors transmit signals by means of the reflection of incident radio wave that is provided by counterpart interrogator. The use of multiple interrogators, forming multiple interrogation zones, is the usual practice to extend the range of backscatter communication system. However, if a sensor receives more than one radio waves-one from its counterpart interrogator and others from neighbor interrogators-the sensor produces modulated backscatters to all the interrogators, causing interference to neighbor interrogators. This inter-zone interference is particularly problematic when multiple sensors send stream data concurrently. This paper proposes a real-time separation of collided backscatter signals based on their statistical independence. The proposal uses a priori probability of backscatter signal to extract the independent carrier phase angles and the independent amplitudes from IQ signal observation. The measurement of channel state information with predefined symbol pattern in packet is not needed. The proposal is evaluated both in numerical simulations and a physical experiment using prototype backscatter sensors and a software-defined interrogator. The results show that collided backscatter signals can be separated in real-time without degrading the packet error rate of each signal.

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