Access Mobility and Security above 100 GHz

Collaborative Research

PI Lead, Daniel Mittleman, Brown University

PI, Edward Knightly, Rice University and PI, Kaushik Sengupta, Princeton University

NSF-2211616, NSF-2211617, and NSF-2211618

Overview:

The use of frequencies above 100 GHz for wireless links is rapidly emerging as a key enabler for future (beyond 5G) wireless systems. These high-frequency communications systems, which are referred to as terahertz (THz) links, offer numerous exciting possibilities, such as plentiful bandwidth for ultra-highspeed data transmission, high-resolution sensing, and enhanced resilience against malicious attacks such as eavesdropping and jamming. Yet, so far, little research has been devoted to the question of how to implement a network that can provide high bandwidth links for multiple mobile users, while still maintaining security against eavesdroppers. This joint design problem remains unaddressed.

The objective of this proposal is to realize physically secure, networked (mobile and multi-user), spectrally efficient, broadband links at frequencies above 100 GHz. Our team will develop a set of methodologies to enable (1) rapid localization of users in a mobile network environment including both friends and foes, (2) establishment of multiple directional links which simultaneously optimize data throughput and security, and (3) discovery and mitigation of eavesdropper attacks, including both attacks on data links and those which attempt to leverage control-plane functions which leak channel information.

 

Intellectual Merit:

The proposed research program will be the first comprehensive study of terahertz wireless networks which holistically incorporates considerations of network functionality and security. The overarching goal of this project is to develop a radically new node architecture which can intrinsically support multiple access for mobile clients in a broadband THz network, while also maintaining a high degree of security against malicious eavesdropping.

The first thrust of this project involves the exploration of novel antenna designs which exploit strong angular dispersion. We propose a new method to enable active fast electrical tuning of such devices. This approach, uniquely suited to the THz spectral range, will be exploited for detection of an eavesdropping attack, as well as for localization of legitimate users and mobility detection.

A second project thrust targets to develop spatio-temporal modulated array architectures for scrambling the information contained in side-lobes of the broadcast, via spectral aliasing. Combined with agile sensing functionalities, the proposed interface will selectively create secure zones for communication. We will explore the limits of performance of these systems with respect to spectral efficiency, quantify the concept of information beam width and consider the implications for various forms of malicious attacks.

In a third thrust, we will leverage the power of this new node architecture to ensure that the quality of service for multiple mobile users is maintained, even while guaranteeing that eavesdroppers are unable to access, not only the primary communication channels, but also control plane functions required to establish and maintain mobile links. We will consider a powerful adversary, with multiple eavesdroppers employing machine learning capabilities, in a multi-user mobile network. The result will provide the optimal performance for a spectrally efficient and secure THz network, even in the presence of a sophisticated attack by colluding eavesdroppers.

What was accomplished under each thrust to accomplish the goal of the project:

 Major Activities:

Thrust 1: Implemented and experimentally evaluated the integration of devices into the interior of leaky-wave antennas, including both passive (3D-printed photonic crystal) and active (electronically reconfigurable metasurface). In collaboration with a group at Princeton, we also showed how leaky-wave antennas can be used as passive tags in a multi-tag retro-reflective network with up to 2 GHz of bandwidth per tag. We also demonstrated the first data transfer using a metasurface to generate a beam that follows a curved trajectory for obstacle avoidance.

In addition, we finished designing a 46-element reconfigurable transmit/reflect array constituted with custom silicon ICs on both sides (each side being a 23-element array) packaged in 14-layer organic packaging operating in the mmWave band. Each of these ICs embed switching capabilities, allowing us to create directional modulation and spatiotemporal security. Packaging has been a major challenge, and it is well understood that US is lacking in this aspect particularly for multi-chip systems operating at very high frequencies. We consulted with several packaging companies, and we were able to identify one who can fabricate and assemble this complex mmWave systems. We also demonstrated a new low-power THz receiver operable in energy constrained and contested environment. Utilizing the analytical relationship in a Karmer-Kronig set up, the receiver utilizes only amplitude information at the receiver to extract phase information, thereby establishing complex spectrally efficient modulation schemes without any LO synchronization and frequency synthesizer requirement at the receiver side.

Thrust 2: We evaluated the previously proposed scheme, Random Meta-atoms to send Directional Misinformation (RMDM), using an electronically tunable metasurface, proving a practical implementation of the static metasurfaces we used in our prior design. We observe a clear improvement compared to the previous results: the bit error rate (BER) of the eavesdropper is rapidly increasing as the angular separation between eavesdropper Eve and legitimate receiver Bob increases, with Eve observing completely random bits (BER=0.5) with only 2 degrees angular separation from Bob.

In addition, we explored the design space for spatio-temporal active surfaces with gain. These surfaces have the ability to create directional modulation in space for enhanced security. We invented new antenna geometries for low-coupling, EM-circuit co-design to allow arrays of embedded chips on both sides of the surface, which can serve as a wireless gateway as a programmable reflect-transmit array. We also investigated new sub-THz receiver architectures utilizing Kramer-Kronig principles and analog/mixed-signal processing for low-power and low-latency operation in a contested environment.

Thrust 3: We showed how a randomly generated metasurface configuration can result in distinct constellation points received at various user locations, allowing the transmission of simultaneous and independent symbols to users at different locations with a single configuration. Furthermore, we demonstrated that by generating a large number of such metasurface configurations, we can achieve a wide range of symbol options for different users’ locations and their corresponding modulation rate. Consequently, we leverage this pre-characterized set of configurations to enable fast (per-symbol) reconfiguration during downlink data transmission. We also showed how an on-device metasurface can be used to thwart an eavesdropper employing a millimeter-wave radar eavesdropping approach, by imposing a false phase modulation on top of the acoustic signal that she aims to detect.

Specific Objectives:

Thrust 1: Since there is no prior work of scalable mmWave active arrays with embedded gains, we needed to explore all the fundamental trade-offs in the space. To create the spatio-temporal modulated surfaces, we found that inter-element coupling can be a major source for creating an instability for active surfaces with gains. We invented new low-coupling antenna structures where the inter-element coupling was reduced from 20 to 25 dB allowing additional 5dB gain from the surfaces. In addition to the inter-element coupling, we discovered that coupling from the Tx antennas to all the remaining Rx antennas severely distorts the antenna patterns. We carefully created new antenna structures with modified grounds and coupling prevention walls, that minimize both radiative and surface-wave coupling. One important challenge has been the tight spacing between the antenna and the chip elements due to the small wavelengths. This makes the design of the antennas and the matching networks to interface with the antenna extremely challenging. We have been able to navigate this tight design space, and now we have a design that can embed this chip-array on both sides to act as a reconfigurable transmit-reflect array on demand. 

Thrust 2: Wireless links in the sub-THz band inherently require directional beams to overcome the path loss within the system’s power budget. By extension, limiting the angular range of the wireless broadcast limits the physical region where eavesdropping is possible, improving security. However, it is shown in prior work that directional beams are not a complete security measure, and eavesdroppers can also exploit directional reception to successfully perform an attack. In previous results, we showed that RMDM is capable of drastically improving link security compared to the directional beams-only scenario by both increasing Eve’s BER and narrowing the low-BER region around Bob down to only 8 degrees. These results were obtained through an emulated setup, where multiple static metasurfaces were used in place of a real reconfigurable surface. Our recent experiments using a reconfigurable metasurface aim to demonstrate RMDM with a fully realized architecture, showing the viability of directional misinformation in future transmitter architectures in the sub-THz region for enabling secure links.

Thrust 3: With the rapid growth in demand for wireless capacity, multi-user (MU) multiplexing will be a key tool for scaling data rates to terabits per second (Tb/sec). MU multiplexing enables simultaneous transmission of multiple independent data streams towards multiple receivers within the same broadcast sector, thereby increasing spectral efficiency and network capacity. In this thrust, we demonstrate a new approach to sub-THz downlink MU multiplexing, which requires no RF chains and no antenna arrays. Our approach employs a switchable metasurface that reconfigures at the symbol rate to transform a monochromatic sub-THz input into a high-entropy wavefront that results in angularly diverse amplitude and phase responses. Through this PHY-layer design, we aim to enhance sub-THz link security against advanced adversaries, e.g., nodes equipped with complex learning algorithms and spatially distributed sensors.

Significant Results:

 Thrust 1: To operate in energy constrained and contested environment, we demonstrated the first Kramer-Kronig based receiver that extracts phase information from the amplitude through low-power Hilbert filtering in the mixed-signal domain. This eliminates the need for complex frequency synthesis, LO synchronization, and allows low-latency link establishment in a secure fashion. 

Thrust 2: We implemented RMDM using an electrically programmable metasurface. We generated a set of random configurations that allows Alice to send relative misinformation to Eve. We showed that with the programmable metasurface, Alice can increase Eve’s BER to 0.5 with an angular separation between Eve and Bob of around 2 degrees, narrowing the “threat zone” by half compared to our previous results with static metasurfaces (4 degrees). Effectively, this enforces Eve to be co-located with Bob to successfully eavesdrop.

Thrust 3: We utilized an electrically programmable metasurface to generate high entropy wavefronts with spatially diverse responses to enable concurrent transmission of distinct information symbols to multiple users at different angular locations. We generated and tested a set of random configurations and evaluated the available symbol options achievable at different receiver locations. We studied the achievable SU time-shared and MU rates achieved for different signal-to-noise ratio (SNR) values. Our results show that in moderate SNR scenarios with sufficient angular separation, maximal MU gains of twofold are achievable when two users are served simultaneously. In addition, we study the relation between the two users’ angular separation and the achievable network MU rate. Our results show that significant MU Gain is attainable even if the two users have angular separation as slight as 2°, while the maximum gain of twofold is achievable with 10° angular separation.

Key outcomes or other achievements

Over the past year, our research has made significant advancements across various key areas of this project. Some highlights include:

  • the integration of both active and passive devices into leaky-wave antennas, for enhanced functionality and wavefront engineering, directly addressing the goals of the project to integrate active functionality into leaky-wave architectures;
  • the first experimental demonstration of an Airy beam carrying modulated data around an obstacle;
  • an experimental evaluation of a proposed scheme to use random wavefronts, engineering via switchable metasurfaces, as a counter-measure against eavesdroppers;
  • a study of the use of passive leaky-wave antennas in a retro-reflection network as secure tags with unprecedented bandwidth;
  • an exploration of spatio-temporal active surfaces with gain, with the ability to create directional modulation;
  • a study of new sub-THz Kramer-Kronig receiver architectures, together with analog/mixed-signal processing for low-power and low-latency operation in a contested environment;
  • a new multi-user architecture employing random wavefronts to enhance the downlink aggregate data rate by twofold, compared to conventional single-user access.

 

We have also actively engaged in educational initiatives including the recruitment of new Ph.D. students, the provision of post-doctoral fellowships, offering internships to undergraduate students, organizing in-person workshops, and actively participating in international conferences.

What opportunities for training and professional development has the project provided?

Outreach Activities:

In-person Workshops among Rice, Brown, Princeton research groups

During the previous year, we successfully organized two two-day workshops, held in-person. The first workshop took place in November 2023 at Brown University, followed by the second workshop in April 2024 at Princeton University. These workshops represent the continuation of a bi-annual meeting series that we have organized for the last several years, which also include our collaborators from Northeastern University and MIT (who interact with all of the three PIs through other synergistically related research programs funded by NSF and other agencies).

The two workshops have been attended by all of the students and post-docs who have participated in aspects of the research related to this grant. These workshops encompassed a variety of activities, including tutorials, demonstrations, project presentations, and brainstorming sessions. The majority of student participants had the opportunity to present their work, and all attendees actively engaged in discussions. The brainstorming sessions and idea exchanges are a key source of new ideas; these have sparked the exploration of new project directions and fostered collaborations. Notably, the workshops facilitated connections among the students, encouraging ongoing discussions and collaborations beyond the workshop environment. The next workshop is planned for February 2025 at Rice University.

Student Training

PI Mittleman’s team at Brown University

In Mittleman’s lab, two senior PhD students (Zhaoji Fang and Yaseman Shiri) are now entering the final year of their graduate studies. Both are working on final experiments and beginning to work on thesis writing. Meanwhile, new PhD student Chloe Stults has joined the group in the fall of 2024, and is beginning to learn the operation of measurement apparatus and other important background material. Her projects will include the use of active metasurface devices in various implementations. Three new undergraduate researchers have also joined the group this fall.

PI Knightly’s team at Rice University

In Knightly’s lab, participating student Zhambyl Shaikhanov successfully defended his PhD thesis and graduated, starting his new position as an Assistant Professor at University of Maryland. The senior students were trained to operate the newly acquired electrically tunable metasurface, which was a key component in demonstrating the significant results reported above. The lab welcomed two new students, PhD student Caroline Spindel and MS student Mars Akishev. They successfully completed their qualifier research projects and were trained by senior students to operate the newly acquired Keysight 4×4 D-band system, as well as the existing J-band system, enabling extensive experiments with modulated data transmission in the bands 140-160 GHz and 220-260 GHz respectively. Over the summer, Research Experiences for Undergraduates (REU) intern Elaine Ren participated in the ongoing projects of the lab with hands-on experimentation, analysis and presentation experience.

PI Sengupta’s team at Princeton University

In Sengupta’s group, students get training in industry standard Cadence design software or IC design including experience with state-of-the-art silicon IC processes including GF 22nm and TSMC 65 nm for RFIC design. They also get trained in electromagnetic design tools such as HFSS and Keysight’s Advanced Design Systems. For this project, they also got trained in design of complex mmWave systems packaging. In addition, the students recently got trained in Keysight’s state of the art mmWave/sub-THz equipment including 110 GHz vector signal generator, 110 GHz signal analyzer, 128 GS/s arbitrary waveform generators and 256 GS/s real-time 10-bit oscilloscope and the VSA software. We have four undergraduate students working on senior thesis on this topic.

 

Have the results been disseminated to communities of interest?

Invited Presentations

PI Mittleman, Brown University, gave the following invited presentations:

  • Near-field terahertz networking, Second National Terahertz Communication Technology Forum, Shanghai, May 2024.
  • Physical-layer security in terahertz wireless networks, Plenary lecture, 18th European Conference on Antennas and Propagation, Glasgow, March 2024.
  • Secure terahertz communications, 2nd International Workshop on Terahertz Technology (IWTT2), remotely via Zoom presentation, Delhi India, December 2023.
  • Near-field terahertz networking, IEEE Montreal Section THz Science and Technology Seminar (TSTS) Series, Zoom seminar series, Montreal, December 2023.
  • Near-field terahertz networking, Plenary lecture, European Microwave Week, Berlin, September 2023.
  • Near-field terahertz networking, Plenary lecture, International Symposium on Ultrafast Phenomena and THz Waves (ISUPTW 2023), Qingdao China, September 2023.

PI Knightly, Rice University, gave the following invited presentations:

  • Curved Beams, Flying Metasurfaces, and Emerging Capabilities for 6G, WONS 2024, Chamonix, France, January 2024
  • Curved Beams, Flying Metasurfaces, and Emerging Capabilities for 6G, Texas A&M, February 2024.
  • Curved Beams, Flying Metasurfaces, and Emerging Capabilities for 6G, Inria, Lyon, France, May 2024.
  • New Spectrum Access Capabilities above 100 GHz, NSF Spectrum Week, Washington DC, May 2024.
  • Sub-THz Mobile Access, 36th Seminar on the Mobile Communications Thematic Network (RTCM), Leiria, Portugal, July 2024.

PI Sengupta, Princeton University, gave invited presentations at the following:

  • IEEE APS, Florence, Jul. 2024.
  • IEEE Distinguished Microwave Lecture, Shanghai and Kore chapters, Oct. 2023
  • IEEE Distinguished Microwave Lecture, McMaster University, Oct. 2023,
  • IEEE IMOC Barcelona, Nov. 2023.
  • Plenary, IEEE MAPCON, Delhi, Dec 2023.
  • Oxford and Cambridge University, Mar. 2024.
  • IEEE IMS Workshop, Washington DC, Jun. 2024.

Products

Journals:

Vitaly Petrov, Hichem Guerboukha, Daniel M. Mittleman, and Arjun Singh, “Wavefront Hopping: An Enabler for Reliable and Secure Near Field Terahertz Communications in 6G and Beyond,” IEEE Wireless Communications Magazine, 31, 48-55 (2024).

Hichem Guerboukha, Masoud Sasaki, Rabi Shrestha, Jingwen Li, Niels Benson, and Daniel M. Mittleman, “3D-printed photonic crystal sub-terahertz leaky-wave antenna,” Advanced Materials Technologies, 9, 2300698 (2024).

A. Singh, V. Petrov, H. Guerboukha, I.V.A.K. Reddy, E. Knightly, D. Mittleman, and J. Jornet, “Wavefront Engineering: Realizing Efficient Terahertz Band Communications in 6G and Beyond,” IEEE Wireless Communications, 31(3):133-139, June 2024.

H. Guerboukha, B. Zhao, Z. Fang, E. Knightly, and D. Mittleman, “Curving THz Wireless Data Links around Obstacles,” Communications Engineering 3, 58 (2024).

Zhambyl Shaikhanov, Mahmoud Al-Madi, Hou-Tong Chen, Chun-Chieh Chang, Sadhvikas Addamane, Daniel M. Mittleman, and Edward W. Knightly, “Audio Misinformation Encoding via an On-Phone Sub-Terahertz Metasurface,” Optica 11, 1113-1114 (2024)

C.-Y Yeh, Y. Ghasempour, Y. Amarasinghe, D. Mittleman, and E. Knightly, “Security and Angle- Frequency Coupling in Terahertz WLANs,” IEEE/ACM Transactions on Networking, 32(2):1524-1539, April 2024.

Conference Papers and Presentations:

Fahid Hassan, Jeffrey Lei, Hichem Guerboukha, Hou-Tong Chen, Chun-Chieh Chang, Sadhvikas Addamane, Michael Lilly, Edward Knightly, and Daniel M. Mittleman, “Metasurface enabled THz multi-user communications,”  in Proceedings of the 48th International Conference on Infrared Millimeter and Terahertz Waves (Montreal, 2023), Th-P2-50.

Hichem Guerboukha, Masoud Sakaki, Rabi Shrestha, Jingwen Li, Niels Benson, and Daniel M. Mittleman, “Photonic crystal THz leaky-wave antenna 3D-printed in alumina,” in Proceedings of the 48th International Conference on Infrared Millimeter and Terahertz Waves (Montreal, 2023), Fr-AM-5-5.

F. Hassan, Z. Shaikhanov, H. Guerboukha, D. Mittleman, K. Sengupta and E. Knightly, “RMDM: Using Random Meta-Atoms to Send Directional Misinformation to Eavesdroppers,” in Proceedings of IEEE Conference on Communications and Network Security (CNS), October 2023.

Z. Shaikhanov, S. Badran, H. Guerboukha, J. Jornet, D. Mittleman, and E. Knightly, “MetaFly: Wireless Backhaul Interception via Aerial Wavefront Manipulation,” in Proceedings of the 45th IEEE Symposium on Security and Privacy, May 2024.

S. Ghozzy, M. Allam, E. A. Karahan, Z. Liu and K. Sengupta, “A mm-Wave/Sub-THz Synthesizer-Free Coherent Receiver with Phase Reconstruction Through Mixed-Signal Kramer-Kronig Processing,” 2024 IEEE International Solid-State Circuits Conference (ISSCC), San Francisco, CA, USA, 2024, pp. 220-222.

 

Thesis:

Zhambyl Shaikhanov, April 2024, Rice University

Title: Metasurface-in-the-Middle Attacks: Wavefront Manipulation Threats and Countermeasures

Impacts

What is the impact on the development of the principal discipline(s) of the project?

The research program at all three institutions has engaged multiple Ph.D. students and undergraduates.  At Rice, notable accomplishments include Zhambyl Shaikhanov defending his Ph.D., titled “Metasurface-in-the-Middle Attacks: Wavefront Manipulation Threats and Countermeasures”. Additionally, new PhD student Caroline Spindel and MS student Mars Akishev have been welcomed and started new research projects on THz sensing with advanced wavefront engineering and THz Multi-user uplink access, respectively. At Brown, post-doc Hichem Guerboukha has completed his appointment and moved to a new position as a tenure-track assistant professor at the University of Missouri Kansas City. Also, new PhD student Chloe Stults has joined the group in the fall of 2024, aiming to work on metasurface-enabled active devices. Two undergraduate researchers have also recently joined the group. At Princeton, the graduate student Muhamed Allam is working on this project, along with four undergraduate students working on this in their senior thesis.  

 What is the impact on the development of human resources?

As envisioned in our BPC plan, this project includes numerous efforts to encourage younger learners, especially those from traditionally under-represented groups, to pursue higher education and careers in STEM disciplines.

Broadening participation. 

At Brown University, the Summer@Brown program offers a range of non-credit-bearing week-long topics to over 4000 high school students annually. This program attracts a population of students with a diverse range of backgrounds and experiences, offering a unique opportunity to engage with these young scholars at an early pre-college stage. In previous years, this population has comprised approximately 17% students from historically underrepresented groups, and over 60% female students.

In the summers of 2017 and 2018, Summer@Brown offered a terahertz science course, in which roughly 10 high school students per year were exposed to the concepts of THz science and the methods of THz spectroscopy. The course mixed interactive lectures with hands-on laboratory experience using a commercial time-domain spectrometer, which the students used to make a few simple measurements of material properties, or a characterization of the dispersion of simple waveguides. End-of-course evaluations provided strong evidence of the impact of this course. Averaged over the two years, the course received an overall rating of 1.5 out of 5 (with 1 being the highest score). All but one student respondent stated that the course contributed positively to their overall education and/or made them interested in further study in the field. One student stated that, “This course was a fantastically rewarding experience… Taking this class has certainly made me more interested in the process of scientific research and in the advancement of this technology for its real-world applications.”

The faculty member who offered that course has now left Brown University and is no longer available. For the summer of 2024, PI Mittleman has revived and refreshed this course. This summer, it was offered in the 2nd week of July. New course materials included a small research project in which students used a terahertz time-domain spectrometer to make simple measurements and analyze the resulting data to extract, for example, the refractive index of a slab of material or to identify an unknown white powder based on its terahertz vibrational resonances. The PI worked strategically with the Brown Pre-College Programs (who partner with over 40 organizations – schools, non-profits and foundations) to identify, recruit, and enroll students from low-income backgrounds, historically underrepresented groups, and/or students who may be the first in their families to attend college. Sixteen high-school students from all over the US took the course, including 6 female students.  In addition to collecting feedback from the student participants, the number of participants in the program who subsequently join a STEM major in college will be tracked, whether at Brown or elsewhere.

At Rice University in 2024, summer internship BPC recruiting efforts yielded one female and one URM intern of six total. In addition to their crucial hands-on contributions in achieving the described results of the project’s thrusts, the interns were tasked with presenting their research not only to lab members but also to professionals from Army Research Labs and Cisco with several talks throughout their internship to enhance their communication skills. Throughout these external meetings, the interns were encouraged to build connections that extend towards their career in the future.

At Princeton University, PI Sengupta co-directs the Princeton NextG program, through which we further BPC activities including encouraging URM and women students to the field of semiconductors and wireless.  In the annual symposium, we host poster sessions where students participate, and organize networking sessions with the leading communication and semiconductor industry partners. Students who presented the top three posters are  given awards.  We have also launched a new student lunch series where we invite students broadly in the space of wireless, communication and hardware to give presentations and share results with the broader community. This allows students to not only get feedback from their peers in an informal setting, but also to acquire the skills to present in front of a broader audience. This builds confidence in students, particularly from URM groups who may not have experience in public presentations. We also work with the leading wireless and semiconductor industry players to curate training, internship and full time opportunities for students, particularly targeting women and URM students.