31 results found
Li J, 2022, Performance Limits of 433 MHz Quarter-wave Monopole Antennas due to Grounding Dimension and Conductivity, Annals of Emerging Technologies in Computing, ISSN: 2516-0281
Li J, 2022, Challenges and opportunities for nematic liquid crystals in radio frequency and beyond, Crystals, Vol: 12, Pages: 1-2, ISSN: 2073-4352
Li J, 2021, Towards 76-81 GHz scalable phase shifting by folded dual-strip shielded coplanar waveguide with liquid crystals, Annals of Emerging Technologies in Computing, Vol: 5, Pages: 14-22, ISSN: 2516-0281
Unconventional folded shielded coplanar waveguide (FS-CPW) has yet to be fully investigated for tunable dielectrics-based applications. This work formulates designs of FS-CPW based on liquid crystals (LC) for electrically controlled 0-360° phase shifters, featuring a minimally redundant approach for reducing the LC volume and hence the costs for mass production. The design exhibits a few conceptual features that make it stand apart from others, noteworthy, the dual-strip structure with a simplified enclosure engraved that enables LC volume sharing between adjacent core lines. Insertion loss reduction by 0.77 dB and LC volume reduction by 1.62% per device are reported at 77 GHz, as compared with those of the conventional single-strip configuration. Based on the proof-of-concept results obtained for the novel dual-strip FS-CPW proposed, this work provides a springboard for follow-up investible propositions that will underpin the development of a phased array demonstrator.
Li J, 2021, Rethinking figure-of-merits of liquid crystals shielded coplanar waveguide phase shifters at 60 GHz, J, Vol: 4, Pages: 444-451, ISSN: 2571-8800
The demand for reconfigurable millimetre-wave (mm-Wave) components based on highly anisotropic liquid crystals (LC) is higher than ever before for the UK and worldwide. In this work, 60 GHz investigation on a bespoke shielded coplanar waveguide (SCPW) phase shifter structure filled with 16 types of microwave-enabled nematic LCs respectively indicates that the patterns of the device’s figure-of-merit (FoM, defined as the ratio of maximum differential phase shift to maximum insertion loss) reshuffle from those of the characterised LC materials’ FoM (defined as the ratio of tunability to maximum dissipation factor). To be more specific, GT7-29001- and MDA-03-2838-based phase shifters exhibit the highest FoM for devices, outperforming phase shifters based on GT5-28004 and TUD-566 with the highest FoM for materials. Such a mismatch between the device’s FoM and LC’s FoM implies a nonlinearly perturbed wave-occupied volume ratio effect. Furthermore, the relationship between insertion loss and the effective delay line length is nonlinear, as evidenced by measurement results of two phase shifters (0–π and 0–2π, respectively). Such nonlinearities complicate the established FoM metrics and potentially lead to a renewed interest in the selection and material synthesis of LCs to optimise reconfigurable mmWave devices, and promote their technological exploitation in phased array systems targeting demanding applications such as inter-satellite links and satellite internet.
Li J, 2021, An Efficient Mixed-signal Dielectric-partitioning Model of Liquid Crystals based Shielded Coplanar Waveguide for Electronically Reconfigurable Delay Lines Design, Integrated Optics: Design, Devices, Systems and Applications VI, Publisher: Society of Photo-optical Instrumentation Engineers, ISSN: 0277-786X
Li J, 2021, Hybrid Propulsion Motor Drives Model based on Multi-level Inverters with Optimised Fuel Economy, IEEE Vehicular Power and Propulsion Conference (IEEE VPPC 2020)
Li J, 2021, Wideband PCB-to-Connectors Impedance Adapters for Liquid Crystal-based Low-loss Phase Shifters, 50th European Microwave Conference (EUMC)
Li J, 2021, Managing 60 GHz Field Peaking of an Liquid Crystal Enclosed Coplanar Waveguide by Core Edge Shaping, 2020 IEEE Asia-Pacific Microwave Conference (APMC)
Li J, 2021, Optically Steerable Phased Array Enabling Technology Based on Mesogenic Azobenzene Liquid Crystals for Starlink Towards 6G, 2020 IEEE Asia-Pacific Microwave Conference (APMC)
Li J, 2021, Figure-of-Merits Mismatch in Liquid Crystals mmWave Phase Shifters, 46th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Publisher: IEEE, ISSN: 2162-2027
Li J, 2020, Performance Limits of Liquid Crystals Coplanar Phase Shifters beyond 60 GHz due to Fabrication, IEEE International Conference on Computing, Networking, Telecommunications & Engineering Sciences Applications 2020 (IEEE CoNTESA)
Li J, 2020, Optically Steerable Phased Array Enabling Technology Based on Mesogenic Azobenzene Liquid Crystals for Starlink Towards 6G, 2020 IEEE Asia-Pacific Microwave Conference (APMC 2020), Publisher: IEEE
Li J, 2020, Design and control optimisation of a novel bypass-embedded multilevel multicell inverter for hybrid electric vehicle drives, IEEE 11th International Symposium on Power Electronics for Distributed Generation Systems (PEDG 2020), Publisher: IEEE
A novel bypass-embedded multilevel multicell inverter is proposed with manipulation of bidirectional flow of currents targeting electric motor drives and regenerative braking (battery recharging) for hybrid electric vehicles. To balance switching losses, maximise DC supply utilisations, balance the state of charge, and minimise total harmonic distortions, the inverter control method is optimised by incorporating phase disposition PWM with variable carrier frequency, third harmonic injection, as well as carrier reordering and reduction schemes. The proposed control scheme is investigated on an 11-level inverter for motor drives of a parallel hybrid electric vehicle with regenerative braking capability. The total harmonic distortion is 1.23% for line voltage, and 1.04% for stator current without filtering networks, i.e. a comparable performance to the states of the arts.
Li J, 2020, Bias tees integrated liquid crystals inverted microstrip phase shifter for phased array feeds, 21st International Conference on Electronic Packaging Technology (ICEPT), Publisher: IEEE
Microwave phase shifters based on tunable dielectrics such as nematic liquid crystals (NLC) have attracted considerable research attention in the last two decades. To electronically drive the permittivity variations based on the NLC’s molecular anisotropy, conventional device configuration is based on externally wiring three-port bias tees as diplexers for frequency-domain multiplexing, i.e. isolating the RF port from the low-frequency high-amplitude bias field. The multi-stage bulky solution significantly impedes its applicability for phased array feeds. To the best of the author’s knowledge, limited research has attempted the integration of compact bias tees into the phase shifter. To this end, this work demonstrates a bias-tees-embedded standalone phase shifter integrated on a double-sided RO4003 substrate using advanced fabrication and assembly techniques (photolithography, surface mounting, reflow soldering). The measured differential phase shift reports 391.75° at 10 GHz with the insertion loss up to -5.66 dB (including bias tees) for an optimised design. A figure-of-merit of 69°/dB is achieved, which even outperforms recent advances in NLC-based planar phase shifters without bias tees.
Li J, 2020, Millimetre-wave beam steering with analog-resolution and minimised distortion based on liquid crystals tunable delay lines with enhanced signal-to-noise ratios, SPIE Security + Defence: Millimetre Wave and Terahertz Sensors and Technology XIII, Publisher: Society of Photo-optical Instrumentation Engineers, ISSN: 0277-786X
Millimetre-wave beam-steering distortions are due mainly to unbalanced feeding amplitudes among the delay lines network. To compensate for this, active amplifiers or attenuators are employed, increasing the circuit complexity and costs, whilst compromising the passive array advantage. This work presents a passive millimetre-wave distortion-free beam steering solution with analog resolution in liquid crystal planar delay line technology. Enclosed coplanar waveguide (ECPW) tunable delay lines with 0-180˚ and 0-360˚ phase-shifting ranges are prototyped respectively. A novel impedance matching and mismatching approach for amplitude compensation is proposed and verified, as evidenced by the 67 GHz measured maximum insertion loss of –4.37 dB (variation up to 5%) for the 0-180˚ delay line, and –8.28 dB (variation up to 8%) for the 0-360˚ device. Based on a smart combination of the 0-180˚ and 0-360˚ ECPW, an optimised delay lines deployment scheme is proposed for two-dimensional beam steering with a fault tolerance capability and a minimised systematic insertion loss in total on the feeding network.
Li J, 2020, 60 GHz optimised nickel-free gold-plated enclosed coplanar waveguide liquid crystal phase shifter, IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IEEE MTT-S IMWS-AMP), Publisher: IEEE
A novel nickel-free gold-plated enclosed coplanar waveguide is experimentally demonstrated for a liquid crystal tunable delay line phase shifter at 60 GHz. By numerical investigations, we find that conventional surface impedance model (without meshing inside metals) fails to accurately characterise the effect of gold-plating thickness on conductor loss, especially when the plating thickness is smaller than 4 times of the gold's skin depth. To address this, adaptive meshing inside metals is performed for accurate modelling of the device. The gold-plating thickness is then experimentally optimised and tailored for the liquid crystal-based device-making procedures, i.e. taking into account the accelerated copper migration into gold problem during the alignment agent baking process at 200°C. Measurement results of assembled phase shifters at 60 GHz verify that a direct gold plating of 2μm on Cu can reduce the insertion loss by 1.79 dB and 2.85 dB as compared with that with a thinner plating of 50 nm, and no plating, respectively. The maximum insertion loss of -2.996 dB demonstrated at 60 GHz for the 0-π phase shifter delivers a figure-of-merit of 60°/dB, an improvement of 42°/dB as compared with up-to-date literature.
Wang Y, LI J, 2020, From Reflecting Brownian Motion to Reflected Stochastic Differential Equations: A Systematic Survey and Complementary Study, Publisher: Elsevier BV
Li J, 2020, Optically Steerable Phased Array Enabling Technology Based on Mesogenic Azobenzene Liquid Crystals for Starlink Towards 6G, Publisher: TechRxiv. Preprint, Institute of Electrical and Electronics Engineers (IEEE)
Paper for 2020 Asia-Pacific Microwave Conference (APMC 2020).
Li J, 2020, All-optically Controlled Microwave Analog Phase Shifter with Insertion Losses Balancing, ENGINEERING LETTERS, Vol: 28, Pages: 663-667, ISSN: 1816-093X
Li J, 2020, Low-loss tunable dielectrics for millimeter-wave phase shifter: from material modelling to device prototyping, Publisher: IOP Publishing, Pages: 012057-012057, ISSN: 1757-8981
<jats:title>Abstract</jats:title> <jats:p>Passively controllable millimeter-wave phase shifters based on continuously tunable dielectrics offer promising alternatives to conventional switch-based topologies (e.g. active semiconductor devices and MEMS) with digital resolution. However, only a handful of tunable dielectrics have thus far been identified with high tunability and low loss, among them, liquid crystal (LC) in the nematic phase is of research and development interest. This work presents the modelling of LC materials at 66 GHz based on a novel shielded coplanar device structure in place of traditional waveguides. Phase shifters of 0-π tunability are fabricated and measured, with an insertion loss of -4 dB and return loss lower than -15 dB demonstrated.</jats:p>
Li J, 2020, Vulnerabilities Mapping based on OWASP-SANS: A Survey for Static Application Security Testing (SAST), Annals of Emerging Technologies in Computing, Vol: 4, Pages: 1-8, ISSN: 2516-0281
<jats:p>The delivery of a framework in place for secure application development is of real value for application development teams to integrate security into their development life cycle, especially when a mobile or web application moves past the scanning stage and focuses increasingly on the remediation or mitigation phase based on static application security testing (SAST). For the first time, to the author’s knowledge, the industry-standard Open Web Application Security Project (OWASP) top 10 vulnerabilities and CWE/SANS top 25 most dangerous software errors are synced up in a matrix with Checkmarx vulnerability queries, producing an application security framework that helps development teams review and address code vulnerabilities, minimise false positives discovered in static scans and penetration tests, targeting an increased accuracy of the findings. A case study is conducted for vulnerabilities scanning of a proof-of-concept mobile malware detection app. Mapping the OWASP/SANS with Checkmarx vulnerabilities queries, flaws and vulnerabilities are demonstrated to be mitigated with improved efficiency.</jats:p>
LI J, Guo X, 2020, Global Deployment Mappings and Challenges of Contact-tracing Apps for COVID-19, Publisher: Elsevier BV
Li J, Guo X, 2020, COVID-19 Contact-tracing Apps: a Survey on the Global Deployment and Challenges
To address the massive spike in uncertainties triggered by the coronavirusdisease (COVID-19), there is an ever-increasing number of national governmentsthat are rolling out contact-tracing Apps to aid the containment of the virus.The first hugely contentious issue facing the Apps is the deployment framework,i.e. centralized or decentralized. Based on this, the debate branches out tothe corresponding technologies that underpin these architectures, i.e. GPS, QRcodes, and Bluetooth. This work conducts a pioneering review of the abovescenarios and contributes a geolocation mapping of the current deployment. TheApps vulnerabilities and the directions of research are identified, with aspecial focus on the Bluetooth-inspired decentralized paradigm.
Li J, 2020, Optically Inspired Cryptography and Cryptanalysis: A Survey and Research Directions, Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering, Publisher: Springer International Publishing, Pages: 98-110, ISBN: 9783030600358
Guo X, Li J, 2019, A novel Twitter sentiment analysis model with baseline correlation for financial market prediction with improved efficiency, 2019 Sixth International Conference on Social Networks Analysis, Management and Security (SNAMS), Publisher: IEEE
A novel social networks sentiment analysis model is proposed based on Twitter sentiment score (TSS) for real-time prediction of the future stock market price FTSE 100, as compared with conventional econometric models of investor sentiment based on closed-end fund discount (CEFD). The proposed TSS model features a new baseline correlation approach, which not only exhibits a decent prediction accuracy, but also reduces the computation burden and enables a fast decision making without the knowledge of historical data. Polynomial regression, classification modelling and lexicon-based sentiment analysis are performed using R. The obtained TSS predicts the future stock market trend in advance by 15 time samples (30 working hours) with an accuracy of 67.22% using the proposed baseline criterion without referring to historical TSS or market data. Specifically, TSS's prediction performance of an upward market is found far better than that of a downward market. Under the logistic regression and linear discriminant analysis, the accuracy of TSS in predicting the upward trend of the future market achieves 97.87%.
Li J, Chu D, 2019, Liquid Crystal-Based Enclosed Coplanar Waveguide Phase Shifter for 54-66 GHz Applications, CRYSTALS, Vol: 9, ISSN: 2073-4352
Li J, 2019, Structure and Optimisation of Liquid Crystal based Phase Shifter for Millimetre-wave Applications
Yontem AO, Li J, Chu D, 2018, Imaging through a projection screen using bi-stable switchable diffusive photon sieves, OPTICS EXPRESS, Vol: 26, Pages: 10162-10170, ISSN: 1094-4087
Cai L, Xu H, Li J, et al., 2017, High figure-of-merit compact phase shifters based on liquid crystal material for 1-10 GHz applications, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol: 56, ISSN: 0021-4922
Li JF, Xu H, Chu DP, 2016, Design of Liquid Crystal Based Coplanar Waveguide Tunable Phase Shifter with no Floating Electrodes for 60-90 GHz Applications, 46th European Microwave Conference (EuMC), Publisher: IEEE, Pages: 1047-1050, ISSN: 2325-0305
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