Publications & Presentations

2024

1. PASTSPC 2nd Place

   Fully Wireless Collaborative Beamforming Using A Three-Element Coherent Distributed Phased Array

   Jason M. Merlo, Naim Shandi, Matthew Dula, Ahona Bhattacharyya, and Jeffrey A. Nanzer

   In 2024 IEEE International Symposium on Phased Array Systems & Technology (PAST) , Oct 2024

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   In this work we experimentally demonstrate a wirelessly coordinated three-element coherent distributed phased array performing beamforming and beam steering to a target in the far-field over 17 m at a carrier frequency of 1.05 GHz. We build on our previous work utilizing a high accuracy two-way time transfer (TWTT) technique for inter-node time synchronization and ranging and an analog continuous two-tone frequency transfer technique to perform syntonization by moving to a fully wireless architecture, without the need for external reference signals such as global navigation satellite system (GNSS)-derived pulse-per-second (PPS), including a distributed computing system architecture, and the use of fully wireless communication links between nodes. Finally, we perform a far-field beamforming experiment with beam steering to a receiver 17 m away at a carrier frequency of 1.05 GHz and demonstrate a beamforming coherent gain of 0.95 (9.32 dB) with a beamforming inter-element timing accuracy of below 60 ps.

   @inproceedings{merlo2024past,
     address = {Boston, Massachusetts, USA},
     author = {{Merlo}, Jason M. and {Shandi}, Naim and {Dula}, Matthew and {Bhattacharyya}, Ahona and {Nanzer}, Jeffrey A.},
     booktitle = {2024 IEEE International Symposium on Phased Array Systems \& Technology (PAST)},
     month = oct,
     title = {Fully Wireless Collaborative Beamforming Using A Three-Element Coherent Distributed Phased Array},
     year = {2024}
   }

2. TCOM

   Decentralized Picosecond Synchronization for Distributed Wireless Systems

   Naim Shandi, Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Transactions on Communications, Oct 2024

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   We demonstrate a wireless, decentralized time-alignment method for distributed antenna arrays and distributed wireless networks that achieves picosecond-level synchronization. Distributed antenna arrays consist of spatially separated antennas that coordinate their functionality at the wavelength level to achieve coherent operations such as distributed beamforming. Accurate time alignment (synchronization) of the local clocks on each node in the array is necessary to support accurate time-delay beamforming of modulated signals. In this work we combine a consensus averaging algorithm and a high-accuracy wireless two-way time transfer method to achieve decentralized time alignment, correcting for the time-varying bias of the clocks in a method that has no central node. Internode time transfer is based on a spectrally-sparse, two-tone signal achieving near-optimal time delay accuracy. We experimentally demonstrate the approach in a wireless four-node software-defined radio system, with various network connectivity graphs. We show that within 20 iterations all the nodes achieve convergence within a bias of less than 12 ps and a standard deviation of less than 3 ps. The performance is evaluated versus the bandwidth of the two-tone waveform, which impacts the synchronization error, and versus the signal-to-noise ratio.

   @article{shandi2024tcom,
     author = {Shandi, Naim and Merlo, Jason M. and Nanzer, Jeffrey A.},
     doi = {10.1109/TCOMM.2024.3480993},
     journal = {IEEE Transactions on Communications},
     keywords = {Synchronization;Clocks;Accuracy;Estimation;Phased arrays;Topology;Signal to noise ratio;Delay effects;Array signal processing;Time-frequency analysis;Consensus averaging;distributed antenna arrays;distributed beamforming;distributed phased arrays;synchronization;wireless networks},
     month = oct,
     pages = {1-1},
     title = {Decentralized Picosecond Synchronization for Distributed Wireless Systems},
     year = {2024},
   }

3. GRConLightning Talk

   Distributed Networked Radio Deployments Using The DELTA Framework

   Jason M. Merlo, and Jeffrey A. Nanzer

   In GNU Radio Conference (GRCon) , Sep 2024

   Video Code Slides
4. IEEE AP-S/URSISPC Finalist

   Distributed Interferometric Radar for Radial and Angular Velocity Measurement

   Jason M. Merlo, and Jeffrey A. Nanzer

   In 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI) , Jul 2024

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   In this work we present a fully wireless coherent distributed interferometric radar for sensing the radial and angular velocities of dynamic targets. We build upon prior work on wireless coordination in both time and frequency by utilizing a high accuracy two-tone frequency transfer method for frequency syntonization and global navigation satellite system-based pulseper-second timing alignment. The interferometric radar is based on software-defined radios and uses a continuous-wave 3.3 GHz signal to measure the Doppler and interferometric frequency shifts of signals scattered from moving targets. The two nodes in the array are separated by 91 cm (10λ). Coordination between the nodes was implemented wirelessly at 4.3 GHz, and received radar signals were transferred for joint processing at one node at 5.8 GHz. We demonstrate the performance of the distributed interferometric radar by measuring the instantaneous radial and angular velocities of a pedestrian carrying a corner reflector tangentially past the array.

   @inproceedings{merlo2024aps,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI)},
     month = jul,
     pages = {1-2},
     title = {Distributed Interferometric Radar for Radial and Angular Velocity Measurement},
     year = {2024}
   }

5. AP-S/URSI

   High Accuracy Decentralized Time Synchronization Using SNR Based Weighting

   Naim Shandi, Jason M. Merlo, and Jeffrey A. Nanzer

   In 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI) , Jul 2024

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   We present a decentralized approach to wireless picosecond-level synchronization of distributed antenna array elements that weights its connections based on the signal-to-noise ratio of the links between nodes. Time synchronization is a critical aspect of distributed phased array performance, and is a principal factor limiting the bandwidth of distributed beam-forming operations. We previously demonstrated decentralized synchronization with picosecond-level accuracy using a consensus averaging approach, where nodes iteratively share information among their neighbors to converge to the global average of the time offsets between nodes. Ensuring that all nodes in a distributed array reach consensus on a common time reference depends on the accuracy of the individual links, however, and low SNR links will generate poor time estimates, increasing the residual error of the converged state. Here, we propose a new weighting strategy based on evaluating the SNR in every iteration to allow for time information sharing only between the nodes with highest received SNR. We evaluate the approach experimentally through the implementation of a six-node system using software defined radios (SDRs), yielding in an order of magnitude improvement in accuracy relative to the case of a fully connected antenna array with a static topology.

   @inproceedings{shandi2024aps,
     author = {{Shandi}, Naim and {Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {2024 IEEE International Symposium on Antennas and Propagation and INC/USNC-URSI Radio Science Meeting (AP-S/INC-USNC-URSI)},
     doi = {10.1109/AP-S/INC-USNC-URSI52054.2024.10686155},
     keywords = {Wireless communication;Phased arrays;Accuracy;Limiting;Conferences;Information sharing;Topology},
     month = jul,
     pages = {1873-1874},
     title = {High Accuracy Decentralized Time Synchronization Using SNR Based Weighting},
     year = {2024},
   }

6. AP-S/URSI

   A Distributed Microwave Correlation Interferometer for Fourier Domain Imaging Using Wireless Time and Frequency Coordination

   Derek Luzano, Jason M. Merlo, Daniel Chen, Jorge R. Colon–Berrios, Ahona Bhattacharyya, and Jeffrey A. Nanzer

   In 2024 IEEE International Symposium on Antennas and Propagation and INC/USNC‐URSI Radio Science Meeting (AP-S/INC-USNC-URSI) , Jul 2024

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   Interferometric imaging, such as that used in radio astronomy and satellite remote sensing, relies on co-processing signals received by antenna elements with wide electrical separations, yielding samples of the spatial Fourier domain that are used for image formation. Traditionally, this has been done only in fixed systems, either on the ground or on a single platform. In this paper, we demonstrate the ability to sample the spatial Fourier domain using a distributed microwave receiving array with wireless coordination of time and frequency. We implement a two-element receiving system based on software-defined radios that co-process received signals via cross-correlation, yielding samples of the spatial Fourier domain. We measure the complex interference pattern at 2.3 GHz and 3.3 GHz, representing baselines of 7λ and lOλ, respectively. The results demonstrate the feasibility of distributed Fourier domain sampling, a fundamental building block for distributed Fourier-domain imaging.

   @inproceedings{luzano2024distributed,
     author = {Luzano, Derek and Merlo, Jason M. and Chen, Daniel and Colon--Berrios, Jorge R. and Bhattacharyya, Ahona and Nanzer, Jeffrey A.},
     booktitle = {2024 IEEE International Symposium on Antennas and Propagation and INC/USNC‐URSI Radio Science Meeting (AP-S/INC-USNC-URSI)},
     doi = {10.1109/AP-S/INC-USNC-URSI52054.2024.10686889},
     keywords = {Wireless communication;Microwave measurement;Wireless sensor networks;Time-frequency analysis;Satellite antennas;Satellite broadcasting;Radio interferometry},
     month = jul,
     pages = {1455-1456},
     title = {A Distributed Microwave Correlation Interferometer for Fourier Domain Imaging Using Wireless Time and Frequency Coordination},
     year = {2024},
   }

7. IEEE MWTL (IMS)Top Papers of IMS 2024SPC Finalist

   Fully Wireless Coherent Distributed Phased Array System for Networked Radar Applications

   Jason M. Merlo, Samuel Wagner, John Lancaster, and Jeffrey A. Nanzer

   IEEE Microwave and Wireless Technology Letters, Jun 2024

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   In this work, we experimentally demonstrate, for the first time, a fully wireless coherent distributed antenna array (CDA) performing distributed transmit and receive beamforming for a down-range sensing application at microwave frequencies without the need for external frequency and time references such as global navigation satellite systems (GNSSs). We build on previous methods utilizing a continuous-wave (CW) two- tone frequency transfer system and a pulsed two-tone time synchronization system to align the distributed platforms in both time and frequency. Coherent transmit and receive beamforming was performed utilizing 100 MHz linear frequency modulation (LFM) waveforms for sensing. Two experiments were performed: one imaging a static scene and one imaging a moving pedestrian holding a corner reflector. The beamforming gain is quantified in the static measurements yielding a median beamforming gain of 2.12 dB and a maximum gain of 2.86 dB (96.5% coherent beamforming gain).

   @article{merlo2024ims-mwtl,
     author = {Merlo, Jason M. and Wagner, Samuel and Lancaster, John and Nanzer, Jeffrey A.},
     doi = {10.1109/LMWT.2024.3375085},
     journal = {IEEE Microwave and Wireless Technology Letters},
     keywords = {Synchronization;Array signal processing;Time-frequency analysis;Transmitting antennas;Receiving antennas;Gain;Wireless communication;Automotive radar;distributed antenna arrays;distributed beamforming;networked radar;remote sensing;wireless sensor networks;wireless synchronization},
     month = jun,
     number = {6},
     pages = {837-840},
     title = {Fully Wireless Coherent Distributed Phased Array System for Networked Radar Applications},
     volume = {34},
     year = {2024},
   }

8. IEEE TAP

   Array Phase Center Dynamics Using Spatial Amplitude Modulation for High Efficiency Secure Wireless Communication

   Jacob R. Randall, Jason M. Merlo, Amer Abu Arisheh, and Jeffrey A. Nanzer

   IEEE Transactions on Antennas and Propagation, Jan 2024

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   We present an approach to secure wireless communication based on dynamically changing the apparent phase center of a two-element antenna array using spatial amplitude dynamics. We implement phase center dynamics by modulating the relative amplitudes of the signals fed to two antenna elements. By moving the phase center symmetrically with respect to the geometric center of the array, the far-field amplitude pattern remains mostly constant while the phase pattern changes, imparting modulation onto the transmitted or received signals that is a function of angle. The resulting directional modulation effectively scrambles the information in the radiated waveform at angles outside of the information beam, where the modulation impacts are negligible. The result is a narrow region where information is recoverable, and since the radiated power does not change, the efficiency remains high. We present the design of a 2.5 GHz two-element array with asymmetric amplitude modulation and characterize the phase center location as a function of the amplitude ratio between the elements. We demonstrate secure wireless communication experimentally in a high signal-to-noise ratio (SNR) environment, demonstrating the reduction in throughput due solely to the phase center dynamics. Finally, we analyze the information beamwidth and present a design procedure relating the amplitude ratio, SNR, element spacing, and QAM modulation format to the information beamwidth.

   @article{randall2023tap,
     author = {Randall, Jacob R. and Merlo, Jason M. and Arisheh, Amer Abu and Nanzer, Jeffrey A.},
     doi = {10.1109/TAP.2023.3323141},
     journal = {IEEE Transactions on Antennas and Propagation},
     month = jan,
     number = {1},
     pages = {487-496},
     title = {Array Phase Center Dynamics Using Spatial Amplitude Modulation for High Efficiency Secure Wireless Communication},
     volume = {72},
     year = {2024},
   }

2023

1. EuRAD

   A Dual-Carrier Linear-Frequency Modulated Waveform for High-Accuracy Localization in Distributed Antenna Arrays

   Ahona Bhattacharyya, Jason M. Merlo, and Jeffrey A. Nanzer

   In 2023 20th European Radar Conference (EuRAD) , Sep 2023

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   In this paper we propose the use of a combined two-tone LFM ranging waveform for node localization of distributed antenna arrays (DAAs). Coherent operation in a distributed array requires accurate synchronization of the phase, frequency, and time on each node, and the precise localization of the transmitting nodes, which is challenging at microwave frequencies, especially for nodes that are spatially dynamic. The spatially sparse two-tone waveform and the linear frequency modulation (LFM) ranging waveform have been individually studied for two-way time synchronization and ranging extensively. In this paper we experimentally demonstrate the use of the combined two-tone LFM waveform to estimate the distances between the nodes and accurately calculate their locations. The accuracy of the combined waveform was measured experimentally and demonstrated localization accuracy of less than 1 cm. Based on these measurements, the system can theoretically support beamforming frequencies of up to 4.4 GHz.

   @inproceedings{bhattacharyya2023eurad,
     author = {Bhattacharyya, Ahona and Merlo, Jason M. and Nanzer, Jeffrey A.},
     booktitle = {2023 20th European Radar Conference (EuRAD)},
     doi = {10.23919/EuRAD58043.2023.10289274},
     month = sep,
     pages = {331-334},
     title = {A Dual-Carrier Linear-Frequency Modulated Waveform for High-Accuracy Localization in Distributed Antenna Arrays},
     year = {2023},
   }

2. GRCon

   High Accuracy Wireless Timing Synchronization Using Software Defined Radios

   Jason M. Merlo, and Jeffrey A. Nanzer

   In GNU Radio Conference (GRCon) , Aug 2023

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   Timing synchronization plays a critical role in many high performance software defined radio applications. However, currently, to achieve the level of synchronization required to perform distributed open loop beamforming, cabled techniques such as precision PPS fanout buffers or White Rabbit must be used to align the system clocks to within a small fraction of a sample. However, we have recently presented techniques to achieve picosecond-level synchronization wirelessly for distributed phased array beamforming accomplished using the RF front-end on Ettus X300 software defined radios. This talk will address the waveform design, time delay estimation and refinement process, and software implementation strategies used to achieve this high level of performance using host-controlled processing in GNU Radio.

   @inproceedings{merlo2023grcon,
     address = {Tempe, Arizona, USA},
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {GNU Radio Conference (GRCon)},
     link = {https://events.gnuradio.org/event/21/contributions/413/},
     month = aug,
     title = {High Accuracy Wireless Timing Synchronization Using Software Defined Radios},
     url = {https://events.gnuradio.org/event/21/contributions/413/},
     year = {2023},
   }

3. URSI GASSYoung Scientist Award

   Picosecond Non-Line-of-Sight Wireless Time and Frequency Synchronization for Coherent Distributed Aperture Antenna Arrays

   Jason M. Merlo, and Jeffrey A. Nanzer

   In International Union of Radio Scientists General Assembly , Aug 2023

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   Picosecond-level timing synchronization with wireless frequency transfer is presented in a non-line-of-sight (NLoS) environment using two software-defined radios (SDRs). Using a spectrally sparse pulsed two-tone waveform, high accuracy timing synchronization is achieved. Frequency synchronization is achieved via a continuous 10 MHz two- tone waveform transmitted between nodes which is demodulated by a self-mixing circuit to recover the 10 MHz reference signal used to discipline the local oscillator on the receiving SDR. Total time synchronization, beamforming, and phase accuracies of 8.84 ps, 23.17 ps, and 10\,^∘, respectively, were achieved enabling modulation bandwidths of up to 4.3 Gb/s at a carrier frequency of up to 1.33 GHz.

   @inproceedings{merlo2023ursigass,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {International Union of Radio Scientists General Assembly},
     doi = {10.23919/URSIGASS57860.2023.10265506},
     month = aug,
     title = {Picosecond Non-Line-of-Sight Wireless Time and Frequency Synchronization for Coherent Distributed Aperture Antenna Arrays},
     year = {2023},
   }

4. IEEE AWPL

   A Dynamic Array Using Spatial Amplitude Modulation with an Asymmetric Wilkinson Power Divider for Secure Wireless Applications

   Jacob R. Randall, Amer Abu Arisheh, Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Antennas and Wireless Propagation Letters, Aug 2023

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   A new method of obtaining directional modulation by implementing antenna array dynamics is proposed. An asymmetric switching feed structure is implemented in a two-element array that supports a static antenna pattern in a desired direction while adding sufficient complex modulation at other angles to mitigate the transfer of information. The dynamic antenna array feeds are calibrated to be in-phase and have a signal feed amplitude ratio of approximately 6.15\pm0.1 dB through the design of an asymmetrical Wilkinson divider. The antenna system comprises of a two-state switching matrix using a double pole double throw (DPDT) RF switch. Secure communication capability is demonstrated in simulation and experiment by a 2.5 GHz two-element patch array. The communication system uses a 16-QAM single-carrier signal transmitting 48 kbits in a pseudo-random bit sequence (PRBS) at a rate of 4 Mbits/s. The DPDT switch was synchronized to the transmitter symbol rate of 1 MHz. To isolate the effects of the phase dynamics, the communication system was operating at an SNR of 33 dB, thus transmitting high power to all directions. A low bit error ratio (BER) of <10^−3 is demonstrated at the desired transmission direction 13, with higher BER outside this region. A measurement of a static antenna yielded BER =0 in all directions; thus, the bit errors and narrow 13 information beamwidth were due exclusively to the antenna array dynamics. Performance metrics of this directional modulation technique are compared against previous literature for the reader. Further insights to spurious signals and mitigation are taken into consideration for the equipment and instrumentation of a narrowband signal to validate the technique as a viable “black-box” system implementation.

   @article{randall2023awpl,
     author = {Randall, Jacob R. and Arisheh, Amer Abu and Merlo, Jason M. and Nanzer, Jeffrey A.},
     doi = {10.1109/LAWP.2023.3310911},
     journal = {IEEE Antennas and Wireless Propagation Letters},
     month = aug,
     pages = {1-5},
     title = {A Dynamic Array Using Spatial Amplitude Modulation with an Asymmetric Wilkinson Power Divider for Secure Wireless Applications},
     year = {2023},
   }

5. IEEE AP-S/URSISPC Honorable Mention

   Wireless Time and Phase Alignment for Wideband Beamforming in Distributed Phased Arrays

   Jason M. Merlo, and Jeffrey A. Nanzer

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2023

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   We present a two node wideband wireless distributed antenna array operating from 3.1–3.3 GHz on software-defined radios (SDRs). To synthesize the total bandwidth of 300 MHz, a 100 MHz bandwidth signal was used over multiple frequency steps, with the carrier frequency re-tuned between each pulse. The nodes were wirelessly synchronized using a high-accuracy two-way time transfer technique which was also used to determine the inter-node range for beamforming. A total beamforming bias \pm standard deviation of 62.46 \pm 45.74 ps and 0.27 \pm 2.28\,^∘at the target beamforming angle at a range of 35.5 m was achieved.

   @inproceedings{merlo2023aps,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/USNC-URSI52151.2023.10238024},
     month = jul,
     pages = {365-366},
     title = {Wireless Time and Phase Alignment for Wideband Beamforming in Distributed Phased Arrays},
     year = {2023},
   }

6. IEEE AP-S/URSI

   Sub-Millimeter Ranging Accuracy for Distributed Antenna Arrays Using Two-Way Time Transfer

   Naim Shandi, Jason M. Merlo, and Jeffrey A. Nanzer

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2023

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   Distributed antenna arrays can provide unique capabilities in applications such as distributed communication and sensing. However, in order to coordinate distributed arrays, accurate positioning information of all nodes in the array is required to ensure coherent operations at the target location. Other works have shown that two-way time transfer using spectrally sparse two-tone ranging waveforms provides high accuracy timing synchronization. In addition to time correction, these waveforms may provide inter-node ranging with high accuracy. In this paper the accuracy of ranging via two-way time transfer for a two-node time synchronization system using a two-tone waveform is evaluated. Results are compared to ranging performance using a standard linear frequency modulated waveform (LFM). Experimental results show an accuracy of 0.5 mm using a 40 MHz bandwidth two-tone waveform with a 36 dB signal-tonoise ratio (SNR), supporting a beamforming frequency of up to 40 GHz.

   @inproceedings{shandi2023aps,
     author = {{Shandi}, Naim and {Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/USNC-URSI52151.2023.10238119},
     month = jul,
     pages = {517-518},
     title = {Sub-Millimeter Ranging Accuracy for Distributed Antenna Arrays Using Two-Way Time Transfer},
     year = {2023},
   }

7. IEEE MWTL

   Coherent Distributed Bistatic Radar Using Wireless Frequency Syntonization and Internode Ranging

   Anton Schlegel, Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Microwave and Wireless Technology Letters, Jul 2023

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   We present a distributed microwave radar system operating at 4.8 GHz that uses wireless frequency transfer and internode ranging. We implement wireless frequency alignment (syntonization) using a sparse two-tone signal transmitted from the primary node and a self-mixing receiver on the secondary node. The primary node acts as a distributed radar transmitter, sending a linear frequency-modulated (LFM) waveform downrange, toward the target. There is also a repeater that retransmits an LFM from the secondary node that is used to estimate the separation of the node to correct for the phase rotation of the frequency signal due to propagation. The secondary node also locks its oscillator to the primary node and captures the signal reflected from the scene. We demonstrate the ability of the fully wireless distributed radar system to perform bistatic ranging in an outdoor environment at 4.8 GHz using software-defined radios (SDRs). Measurements to a wall yielded a maximum standard deviation of 1.7 cm and a maximum bias of 14.9 cm.

   @article{schlegel2023coherent,
     author = {Schlegel, Anton and Merlo, Jason M. and Nanzer, Jeffrey A.},
     doi = {10.1109/LMWT.2023.3283918},
     journal = {IEEE Microwave and Wireless Technology Letters},
     number = {9},
     pages = {1393-1396},
     title = {Coherent Distributed Bistatic Radar Using Wireless Frequency Syntonization and Internode Ranging},
     volume = {33},
     year = {2023},
   }

8. IEEE IMSSPC 1st Place

   High Accuracy Wireless Time-Frequency Transfer For Distributed Phased Array Beamforming

   Jason M. Merlo, Anton Schlegel, and Jeffrey A. Nanzer

   In IEEE/MTT-S International Microwave Symposium (IMS) , Jun 2023

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   Wirelessly coordinated distributed arrays will have a significant impact in next generation communications and sensing systems in coming years. However, to achieve high modulation bandwidths and carrier frequencies, timing and frequency transfer must be synchronized to small fractions of the modulation and carrier periods respectively. In this work, we demonstrate coordination and beamforming of a wireless distributed antenna array at 1 GHz in a cluttered outdoor environment achieving a coherent gain of 95.4% and beamforming accuracy of 27.6 ps over a 41 m channel.

   @inproceedings{merlo2023wirelessly,
     author = {{Merlo}, Jason M. and {Schlegel}, Anton and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE/MTT-S International Microwave Symposium (IMS)},
     doi = {10.1109/IMS37964.2023.10188022},
     month = jun,
     pages = {109-112},
     title = {High Accuracy Wireless Time-Frequency Transfer For Distributed Phased Array Beamforming},
     year = {2023},
   }

9. IEEE TAP

   Design of a Single-Element Dynamic Antenna for Secure Wireless Applications

   Amer Abu Arisheh, Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Transactions on Antennas and Propagation, Jun 2023

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   We introduce a new technique for secure wireless applications using a single dynamic antenna. The dynamic antenna supports a constantly and rapidly changing current distribution that generates a radiation pattern that is static in a desired direction and dynamic elsewhere. This imparts additional modulation on the signal and obscures information transmitted or received outside of the information beam, thus achieving directional modulation. Dynamic currents are supported by a single feed that is switched between separate ports on a single antenna, generating two different radiation patterns without changing the physical shape of the antenna. We introduce the theoretical concept by exploring an ideal complex dynamic radiation pattern that remains static in a narrow desired direction and is dynamic elsewhere. The impact on the transmission of information is analyzed, showing that the information beam narrows as the modulation order increases, and design constraints on the spatial width of the information beam as a function of modulation format are determined. We design and analyze a 2.3 GHz two-state dynamic dipole antenna and experimentally demonstrate secure wireless transmission. We demonstrate the ability to change the information beamwidth and steer the information beam experimentally in real time, and to maintain high throughput in the information beam while obscuring the information elsewhere. In contrast to multiport or array-based antennas, our approach introduces a novel rapidly switched single-element technique for secure wireless applications that can be used independently from the rest of the wireless system, essentially operating as a “black box” for an additional layer of security.

   @article{arisheh2023tap,
     author = {Arisheh, Amer Abu and Merlo, Jason M. and Nanzer, Jeffrey A.},
     doi = {10.1109/TAP.2023.3288013},
     journal = {IEEE Transactions on Antennas and Propagation},
     number = {10},
     pages = {7715-7727},
     title = {Design of a Single-Element Dynamic Antenna for Secure Wireless Applications},
     volume = {71},
     year = {2023},
   }

10. IEEE RadarConf

    Multi-Pass Automotive Synthetic Aperture Radar Image Fusion

    Jason M. Merlo, and Jeffrey A. Nanzer

    In IEEE Radar Conference (RadarConf) , May 2023

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    In this work we demonstrate an efficient technique for the registration and compositing of multiple automotive synthetic aperture radar (SAR) images as a step towards distributed SAR imaging. Through the use of an efficient sub-pixel Fourier-based iterative correlation and refinement technique, sub-sections of the overall SAR images are aligned. Because only small sub-regions of the full SAR images are aligned at a time, this technique inherently compensates for accelerations experienced when sampling the synthetic aperture, by assuming a quasi-constant velocity over small sub-regions of the image. Because the image fusion is performed after image formation, this technique lends itself well to a distributed architecture where the image formation is performed on-vehicle and transferred to other vehicles or a centralized cloud processor where images from other vehicles can be aggregated. Finally, because the registration of sub-regions of the SAR images are performed, the registration alignment vector can be used as a odometry provider to improve vehicle odometry estimates; if a metrology grade SAR map is used as reference, a globally reference position estimate could be performed.

    @inproceedings{merlo2023joint,
      author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
      booktitle = {IEEE Radar Conference (RadarConf)},
      doi = {10.1109/RadarConf2351548.2023.10149757},
      month = may,
      title = {Multi-Pass Automotive Synthetic Aperture Radar Image Fusion},
      year = {2023},
    }

11. IEEE MWTL

    Multiobjective Distributed Array Beamforming in the Near Field Using Wireless Syntonization

    Ahona Bhattacharyya, Jason M. Merlo, Serge R. Mghabghab, Anton Schlegel, and Jeffrey A. Nanzer

    IEEE Microwave and Wireless Technology Letters, Jun 2023

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    We demonstrate the efficacy of distributed microwave multiobjective beamforming at ranges near field to the array using an optimization algorithm and wireless frequency alignment (syntonization). While considerable research has recently been devoted to distributed phased array coordination, the ability to steer signals to locations close to the array in open-loop (feedback-free) systems has not been demonstrated. In this work, we apply a traditional far-field beamforming algorithm to a set of distributed antennas that are wirelessly syntonized. We demonstrate multiobjective beamforming at 0.9 GHz using software-defined radios (SDRs) in a distributed array steering either two beams (focii) or one beam and one null. This work demonstrates the feasibility of a critical part of future distributed beamforming systems that, when combined with other coordination technologies, will support coherent beamforming in widely distributed wireless systems.

    @article{bhattacharyya2023multiobjective,
      author = {Bhattacharyya, Ahona and Merlo, Jason M. and Mghabghab, Serge R. and Schlegel, Anton and Nanzer, Jeffrey A.},
      doi = {10.1109/LMWT.2022.3231183},
      journal = {IEEE Microwave and Wireless Technology Letters},
      month = jun,
      number = {6},
      pages = {775-778},
      title = {Multiobjective Distributed Array Beamforming in the Near Field Using Wireless Syntonization},
      volume = {33},
      year = {2023},
    }

12. IEEE T-MTT

    Wireless Picosecond Time Synchronization for Distributed Antenna Arrays

    Jason M. Merlo, Serge R. Mghabghab, and Jeffrey A. Nanzer

    IEEE Transactions on Microwave Theory and Techniques, Apr 2023

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    Distributed antenna arrays have been proposed for many applications ranging from space-based observatories to automated vehicles. Achieving good performance in distributed antenna systems requires stringent synchronization at the wavelength and information level to ensure that the transmitted signals arrive coherently at the target, or that scattered and received signals can be appropriately processed via distributed algorithms. In this paper we address the challenge of high precision time synchronization to align the operations of elements in a distributed antenna array and to overcome time-varying bias between platforms due to oscillator drift. We use a spectrally sparse two-tone waveform, which obtains approximately optimal time estimation accuracy, in a two-way time transfer process. We also describe a technique for determining the true time delay using the ambiguous two-tone matched filter output, and we compare the time synchronization precision of the two-tone waveform with the more common linear frequency modulation (LFM) waveform. We experimentally demonstrate wireless time synchronization using a single pulse 40MHz two-tone waveform over a 90cm 5.8GHz wireless link in a laboratory setting, obtaining a timing precision of 2.26ps.

    @article{merlo2023wireless,
      author = {{Merlo}, Jason M. and {Mghabghab}, Serge R. and {Nanzer}, Jeffrey A.},
      doi = {10.1109/TMTT.2022.3227878},
      journal = {IEEE Transactions on Microwave Theory and Techniques},
      month = apr,
      number = {4},
      pages = {1720-1731},
      title = {Wireless Picosecond Time Synchronization for Distributed Antenna Arrays},
      volume = {71},
      year = {2023},
    }

2022

1. IEEE JMW

   A Microwave Tomography System Using Time-Reversal Imaging for Forestry Applications

   Saptarshi Mukerjee, John Doroshewitz, Jason M. Merlo, Christopher Oakley, Lalita Udpa, David MacFarlane, Emily Huff, and Jeffrey A. Nanzer

   IEEE Journal of Microwaves, Sep 2022

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   A healthy urban forest is important to improve both human health and overall environmental quality. There is currently a lack in technology that has the ability to evaluate living trees in their natural environment without invasive destructive sampling. This work presents a cost-effective, real-time microwave tomography system for practical forestry applications. The scattering measurement system is designed using wide band microstrip monopole antennas (1–5 GHz) and coupled to a switching matrix and a controlling software for automated real-time data collection. A time reversal signal processing algorithm is developed for performing imaging, based on the measurements. The imaging system is initially validated by imaging simple cylindrical targets and finally utilized for imaging defects in different tree trunk samples. Preliminary experimental results demonstrate the practicality, novelty and benefits of this approach for forestry imaging applications.

   @article{mukerjee2022microwave,
     author = {Mukerjee, Saptarshi and Doroshewitz, John and Merlo, Jason M. and Oakley, Christopher and Udpa, Lalita and MacFarlane, David and Huff, Emily and Nanzer, Jeffrey A.},
     doi = {10.1109/JMW.2022.3199194},
     journal = {IEEE Journal of Microwaves},
     month = sep,
     number = {4},
     pages = {614-625},
     title = {A Microwave Tomography System Using Time-Reversal Imaging for Forestry Applications},
     volume = {2},
     year = {2022},
   }

2. IEEE AP-S/URSI

   High Accuracy Wireless Time Synchronization for Distributed Antenna Arrays

   Jason M. Merlo, and Jeffrey A. Nanzer

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2022

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   A method for high accuracy time synchronization of nodes in a distributed antenna array that achieves time alignment accuracy of less than 10 ps is demonstrated. This technique utilizes a two-step process, first performing coarse time synchronization using a conventional pulse-per-second-based time alignment to distribute a global timestamp on device startup, then performing a refinement process using a pulsed dual-tone waveform that is matched filtered and interpolated for fine delay estimation. This process is demonstrated experimentally using software-defined radio nodes separated by 1.2 m using a carrier of 5.8 GHz and a dual-tone waveform with 50 MHz bandwidth.

   @inproceedings{merlo2022aps,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/AP-S/USNC-URSI47032.2022.9887217},
     month = jul,
     pages = {1966-1967},
     title = {High Accuracy Wireless Time Synchronization for Distributed Antenna Arrays},
     year = {2022},
   }

3. IEEE AP-S/URSI

   A Dynamic Pattern Dipole Antenna for Secure Wireless Communications

   Amer Abu Arisheh, Jason M. Merlo, and Jeffrey A. Nanzer

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2022

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   We present the design and measurement of a switched dipole antenna for directional modulation in a single antenna via radiation pattern dynamics. The antenna is fed by a single port, the output of which is fed to two locations on the dipole. By switching between the two states, the complex radiation pattern is quasi-static within a secure region in the mainlobe, and dynamic outside of the secure region. We show that the complex pattern dynamics outside the secure region impart additional modulation onto the radiation pattern that will distort transmitted or received data. We present the design of the dynamic pattern antenna and measured radiation patterns of the two antenna states. We numerically characterize the secure wireless capability through calculation of the bit-error-ratio on various digital modulations in communications, demonstrating the ability to maintain a small secure region.

   @inproceedings{arisheh2022aps,
     author = {{Arisheh}, Amer Abu and {Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/AP-S/USNC-URSI47032.2022.9887173},
     month = jul,
     pages = {1470-1471},
     title = {A Dynamic Pattern Dipole Antenna for Secure Wireless Communications},
     year = {2022},
   }

2021

1. IEEE TVT

   A C-Band Fully Polarimetric Automotive Synthetic Aperture Radar

   Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Transactions on Vehicular Technology, Jul 2021

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   Due to the rapid increase in 76 GHz automotive spectrum use in recent years, wireless interference is becoming a legitimate area of concern. However, the recent rise in interest of automated vehicles (AVs) has also spurred new growth and adoption of low frequency vehicle-to-everything (V2X) communications in and around the 5.8 GHz unlicensed bands, opening the possibility for communications spectrum reuse in the form of joint radar-communications (JRC). In this work, we present a low frequency 5.9 GHz side-looking polarimetric synthetic aperture radar (SAR) for automotive use, utilizing a ranging waveform in a common low frequency V2X communications band. A synthetic aperture technique is employed to address the angular resolution concerns commonly associated with radars at lower frequencies. Three side-looking fully polarimetric SAR images in various urban scenes are presented and discussed to highlight the unique opportunities for landmark inference afforded through measurement of co- and cross-polarized scattering.

   @article{merlo2021sar,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     doi = {10.1109/TVT.2021.3138348},
     journal = {IEEE Transactions on Vehicular Technology},
     number = {3},
     pages = {2587-2600},
     title = {A C-Band Fully Polarimetric Automotive Synthetic Aperture Radar},
     volume = {71},
     year = {2021},
   }

2. IEEE T-MTT

   A Point Target Model for Interferometric Radar Angular Velocity Estimation

   Eric Klinefelter, Jason M. Merlo, Hayder Radha, and Jeffrey A. Nanzer

   IEEE Transactions on Microwave Theory and Techniques, Jul 2021

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   In this work, we derive the exact response of an interferometric correlation radar to an arbitrary scene containing N independent point scatterers. The response of an interferometric radar to multiple targets can be complicated by the nonlinear processing, which generates intermodulation responses in addition to the desired fundamental frequency responses. An accurate signal model is critical for predicting and mitigating these intermodulation products and allows for model-based parameter estimation methods to be used to estimate target angular velocity. We derive a general response to an arbitrary number of independent targets and validate both the exact model and a simpler approximate model via simulation and experimental measurement with a 38-GHz continuous-wave interferometric radar. The experiment contains two targets traveling perpendicular to the radar’s line-of-sight on linear stages at speeds of 0.36 and 0.5 m/s. We use an error metric to describe the similarity between the measured or simulated time-frequency response and the time-frequency response of a signal generated using the frequency estimates of either the exact or approximate models and show that in regimes where the small-angle approximation holds, both models provide an accurate representation of the measured or simulated interferometric response, demonstrating the feasibility of these models.

   @article{klinefelter2021point,
     author = {Klinefelter, Eric and Merlo, Jason M. and Radha, Hayder and Nanzer, Jeffrey A.},
     doi = {10.1109/TMTT.2021.3136265},
     journal = {IEEE Transactions on Microwave Theory and Techniques},
     number = {3},
     pages = {1562-1570},
     title = {A Point Target Model for Interferometric Radar Angular Velocity Estimation},
     volume = {70},
     year = {2021},
   }

3. IEEE TAP

   Distributed Antenna Array Dynamics for Secure Wireless Communication

   Sean M. Ellison, Jason M. Merlo, and Jeffrey A. Nanzer

   IEEE Transactions on Antennas and Propagation, Jul 2021

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   We demonstrate a new approach to implementing directional modulation that leverages distributed array dynamics to change the antenna array pattern over time. With appreciable spacing between the elements, the spatial phase pattern varies rapidly, such that small physical motions result in large phase variations. We define an average radiation pattern that characterizes the space-time modulation of the array pattern and calculate a spatial information filter based on the additional modulation imparted on transmitted data. The concept is demonstrated experimentally using a two-element 1.5 GHz transmit array with relative motion between the elements. The array consists of two nodes, each with one dipole antennas transmitting BPSK data, and each with two log-periodic antennas used to estimate the relative separation of the nodes and correct the beamsteering phase of the transmitters. The node separation is varied dynamically over a range of 0.98λ– 4.7λ. We investigate the use of linearly- and sinusoidally-varying internode dynamics, and demonstrate a low bit-error-ratio of approximately 10-5 at the desired transmission direction, indicating high throughput, and a high error ratio of approximately 10-0.3 at angles outside of the mainbeam, indicating corrupted data, at a signal-to-noise ratio of 12 dB.

   @article{ellision2021distributed,
     author = {Ellison, Sean M. and Merlo, Jason M. and Nanzer, Jeffrey A.},
     doi = {10.1109/TAP.2021.3137449},
     journal = {IEEE Transactions on Antennas and Propagation},
     number = {4},
     pages = {2740-2749},
     title = {Distributed Antenna Array Dynamics for Secure Wireless Communication},
     volume = {70},
     year = {2021},
   }

4. IEEE T-MTT

   Three Dimensional Velocity Measurement Using a Dual Axis Millimeter-Wave Interferometric Radar

   Jason M. Merlo, Eric Klinefelter, and Jeffrey A. Nanzer

   IEEE Transactions on Microwave Theory and Techniques, Jul 2021

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   In this work, a method for directly measuring target velocity in three dimensions using a dual axis correlation interferometric radar is presented. Recent advances have shown that the measurement of a target’s angular velocity is possible by correlating the signals measured at spatially diverse aperture locations. By utilizing multiple orthogonal baselines and using conventional Doppler velocity methods to obtain radial velocity, a full three-dimensional velocity vector can be obtained using only three receive antennas and a single transmitter, without the need for tracking. A 41.8 GHz dual axis interferometric radar with a 7.26λ antenna baseline is presented along with measurements of a target moving parallel to the plane of the radar array, and of a target moving with components of both radial and tangential velocity. These experiments achieved total velocity root-mean-square errors of 41.01 mm/s (10.5%) for a target moving along a plane parallel to the array, and 45.07 mm/s (13.5%) for a target moving with components of radial and tangential motion relative to the array; estimated trajectory angle RMSEs of 10.42\,^∘ and 5.11\,^∘ were achieved for each experiment respectively.

   @article{merlo2021dimensional,
     author = {{Merlo}, Jason M. and {Klinefelter}, Eric and {Nanzer}, Jeffrey A.},
     doi = {10.1109/TMTT.2021.3124251},
     journal = {IEEE Transactions on Microwave Theory and Techniques},
     number = {3},
     pages = {1674-1685},
     title = {Three Dimensional Velocity Measurement Using a Dual Axis Millimeter-Wave Interferometric Radar},
     volume = {70},
     year = {2021},
   }

5. IEEE MWCL

   A Multiple Baseline Interferometric Radar for Multiple Target Angular Velocity Measurement

   Jason M. Merlo, Eric Klinefelter, Stavros Vakalis, and Jeffrey A. Nanzer

   IEEE Microwave and Wireless Components Letters, Jul 2021

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   We present a novel technique for directly estimating the angular velocities of multiple targets using multi-baseline millimeter-wave interferometric radar to significantly reduce nonlinear signal distortion caused when multiple targets are present. We show that through the multiplication of the normalized instantaneous frequency measurements across different baselines, nonlinear intermodulation products resulting from dual-antenna interferometric angular velocity measurements can be mitigated, producing only the terms corresponding to the angular velocity of the targets in the scene. To validate this, simulations were performed demonstrating the close agreement between the proposed method and an ideal correlation (without intermodulation distortion). Near-field errors resulting from far-field approximations are analyzed. Finally, experimental results of a three-antenna, three-baseline 38 GHz interferometric radar are presented that demonstrate the recovery of the motion of two oscillating pendulums of differing angular frequencies.

   @article{merlo2021multiple,
     author = {Merlo, Jason M. and Klinefelter, Eric and Vakalis, Stavros and Nanzer, Jeffrey A.},
     doi = {10.1109/LMWC.2021.3079842},
     journal = {IEEE Microwave and Wireless Components Letters},
     number = {8},
     pages = {937-940},
     title = {A Multiple Baseline Interferometric Radar for Multiple Target Angular Velocity Measurement},
     volume = {31},
     year = {2021},
   }

2020

1. IEEE RadarConf

   Joint Measurement of Target Angle and Angular Velocity Using Interferometric Radar with FM Waveforms

   Jason M. Merlo, and Jeffrey A. Nanzer

   In IEEE Radar Conference (RadarConf) , Sep 2020

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   An architecture for a direct-downconversion complex correlation interferometric radar capable of the direct, joint measurement of angle and angular velocity of a point-target is presented. Due to the simplicity of the system, this technique can be implemented on existing radars with distributed receivers and a transmitter capable of frequency modulation. Derivations for the interferometric measurement of angle and angular velocity and guidelines for the design of such a system are presented. Simulations are provided for the proposed system with varying pulse-width, antenna baseline, and pulse center frequency. The results for all simulations show a low root-mean-square error which demonstrates the feasibility of the proposed system.

   @inproceedings{merlo2020joint,
     author = {{Merlo}, Jason M. and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE Radar Conference (RadarConf)},
     doi = {10.1109/RadarConf2043947.2020.9266419},
     month = sep,
     title = {Joint Measurement of Target Angle and Angular Velocity Using Interferometric Radar with FM Waveforms},
     year = {2020},
   }

2. IEEE AP-S/URSI

   A Dual-Axis Interferometric Radar for Instantaneous 2D Angular Velocity Measurement

   Jason M. Merlo, Eric Klinefelter, and Jeffrey A. Nanzer

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2020

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   A complex correlation interferometric radar utilizing two orthogonal baselines to simultaneously measure the angular velocity of a moving target in two dimensions is presented. Measurements were conducted using a 40.5 GHz continuous-wave radar achieving an accuracy of 0.55 to 2.55%

   @inproceedings{merlo2020dualaxis,
     author = {{Merlo}, Jason M. and {Klinefelter}, Eric and {Nanzer}, Jeffrey A.},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/IEEECONF35879.2020.9330294},
     month = jul,
     pages = {1661-1662},
     title = {A Dual-Axis Interferometric Radar for Instantaneous 2D Angular Velocity Measurement},
     year = {2020},
   }

3. IEEE AP-S SDCSDC Second Place

   5.8-GHz FMCW Radar System for Drone Tracking

   Jason M. Merlo, Stavros Vakalis, Cory Hilton, and Jacob Randall

   In IEEE International Symposium on Antennas and Propagation Student Design Competition , Jul 2020

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   The design of a portable frequency-modulated continuous-wave (FMCW) radar system for detecting and localizing drones is discussed in this report. The system utilized the 5.8 GHz industrial, scientific and medical band and occupied a 100 MHz bandwidth. The system is able to localize drones at a range of 10 m or more. The theory behind FMCW radar localization is explained, and an algorithm for detecting the drone response is presented. The design considerations of a radar board and transmit and receive patch antenna arrays are discussed. We include experimental measurements of tracking a drone.

   @inproceedings{merlo2020fmcw,
     author = {{Merlo}, Jason M. and {Vakalis}, Stavros and {Hilton}, Cory and {Randall}, Jacob},
     booktitle = {IEEE International Symposium on Antennas and Propagation Student Design Competition},
     month = jul,
     title = {5.8-GHz FMCW Radar System for Drone Tracking},
     year = {2020}
   }

2019

1. IEEE AP-S/URSI

   A Microwave Tomography System Using Time-Reversal Imaging

   John Doroshewitz, Jason M. Merlo, Christopher Oakley, Lalita Udpa, Jeffrey A. Nanzer, David MacFarlane, Emily Huff, and Saptarshi Mukherjee

   In IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting , Jul 2019

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   A 16-antenna-element microwave tomographic imaging system using a fast switching matrix is presented. This system uses frequency domain sampling and time-reversal image processing to image its environment. The use of time-reversal provides a more efficient computational method than traditional inverse techniques, creating a faster processing time. This introduces a potential for near-real-time image processing. To reduce the need for precise time-domain equipment, the short pulses required for time-reversal simulations are synthesized using frequency domain sampling over a broad bandwidth. A 1-5 GHz system using 16 planar antennas controlled by a switch matrix is presented. Measured results and reconstructed images are presented for both a PEC and dielectric cylinder as well as a wood sample and compared to their optical images.

   @inproceedings{doroschewitz2019microwave,
     author = {{Doroshewitz}, John and {Merlo}, Jason M. and {Oakley}, Christopher and {Udpa}, Lalita and {Nanzer}, Jeffrey A. and {MacFarlane}, David and {Huff}, Emily and {Mukherjee}, Saptarshi},
     booktitle = {IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting},
     doi = {10.1109/APUSNCURSINRSM.2019.8889157},
     month = jul,
     pages = {583-584},
     title = {A Microwave Tomography System Using Time-Reversal Imaging},
     year = {2019},
   }