Principal Investigator: Prof. Dr. Thomas Kaiser, UDE

Achieved results and methods

Besides the strong collaborative publications of M01 with numerous MARIE partners (see M01.3.2), another long-lasting achievement of the 2^nd phase is the establishment of the PreSyse Lab (see Fig. 1, for details see "Difficulties"), which has been in use since December 2023. It is housed in a 9 m x 6 m x 3 m lab room at the UDE and is equipped with advanced equipment essential for dynamic measurements. At the core of the PreSyse lab are two six-axis arm robots, each capable of carrying loads of up to 120 kg, allowing the mounting of equipment such as VNA, TDS, extenders, but also even more heavy objects such as a human THz phantom. The robots are spaced approximately 4 m apart with a work envelope of 2.5 m. Since they can touch each other, especially with mounted gauges, special care must be taken when programming. The robots offer a position repeatability of 130 μm and a single point precision of 100 μm, which is not sufficient for the high precision requirements of MARIE (see e.g. Chapter 1, LP1). Therefore, two additional 3D laser tracking systems with accuracies of less than 5 μm per meter in all 3D directions and less than 1 μm per meter in a single direction are installed in the room to ensure high-precision tracking of THz transceivers and object motion for 3D MARIE scenarios. An additional 0.3 m linear axis is also available with down to 2 μm precision, which can carry a VNA and can also jointly be attached to the robot to maximize its precision, e.g. for a highly linear trajectory for SAR processing, even though it is in full 3D space. In addition, a 4-port VNA base equipment up to 67 GHz, extenders covering different frequency bands up to 1.5 THz with bandwidths up to 400 GHz and a commercial THz-TDS setup with up to 6 THz carrier frequency are available. The PreSyse Lab is also equipped with portable panels coated with RF absorbing or reflecting materials.

Regarding Image in highly demanding NLoS scenarios: previous studies [1]-[5] highlight the significant challenge posed by high penetration losses in THz frequencies, indicating a substantial dependence on LoS links for both indoor and outdoor applications. To validate or challenge this claim, numerous look-through measurements were conducted [6]-[9]. In a LoS experiment [8] aimed at sensing, imaging, identifying objects and precisely localize them, a THz SAR VNA-based testbed is implemented. The results are depicted in Fig. 2.

More recently, we achieved initial results for NLoS radar sensing via wall reflections in an indoor scenario [10], showcasing our Integrated Sensing And Communications (ISAC) approach (cf. Fig. 3). Our findings suggest that even drywall can effectively serve as a reflector, enabling reliable NLoS wireless communications while supporting sensing and imaging capabilities. Interestingly, due to the diffuse scattering from rough drywall, it was even possible to image the wall itself rather accurately.

Our investigations in the 1^st phase were limited to homogeneous, single-layer, isotropic, and Gaussian rough surfaces. During the 2^nd phase, we conducted measurement-based studies using statistically controlled 3D-printed random rough surfaces to explore non-Gaussianity and lateral roughness effects [11]. This novel approach allowed us to differentiate between slightly rough indoor materials of less than 50 microns and assort them from most to least rough based on the Rayleigh roughness factor [6], [12].

Regarding the Model beam-based multipath channels including the recently discovered multiple ping pong effect: we tried to eliminate the undesired ping pong distortions [13] by self-developed highly broadband THz absorbers. Additionally, we investigated the impact of water in transmission mode on five hygroscopic indoor material groups using the VNA-based SWISSto12 material characterization kit (MCK) THz transmission waveguide measurement system in the 0.75–1.1 THz frequency range [14]. The high-gain antennas like THz horns possess distinctive features, including enhanced directivity, and a divergent beam spot size [15], [16] enabling coverage of a larger area with minimal misalignment issues [17] but causing ping pong reflections [18] in channel transfer functions. Past studies have utilized foam absorbers to minimize this phenomenon [19]-[22], yet complete elimination was unachievable as these absorbers are designed for frequencies below 100 GHz. In our approach [23] we employed gel absorbers in combination with foam absorbers. This combined method successfully eliminated ping pong reflections for all THz-VNA measurements employing extenders (see Fig. 4).

Regarding human motions: we investigated e.g. the LoS probability (see Fig. 5) of THz-transmission by ray tracing simulation in a room when a human is present and walks in different ways. In general, the human body shadowing leads to a certain amount of blocked THz-waves due to absorption. An optimal THz transmitter tilting is elaborated in Fig. 5 to study the influence of variable THz receiver heights on LoS-probability. At a height of 0.75 m the LoS-probability almost jumps to 100 % (see Fig. 5 (b)).

Regarding the human THz phantom (see later WP3): the in vivo wireless propagation study in [24] uses four commercially available tissue phantom samples. The samples have a thickness of 1 mm and are designed to mimic different body regions and have physiological properties such as mechanical stiffness, thickness, and penetration forces that correspond to those of living human tissue. They are composed of salt, water and plant fibers to simulate the physical conditions of human tissue. We first investigated transmission through such skin samples in the frequency range from 110 GHz to 170 GHz (D-Band), as shown in Fig. 6.

Our first study focuses on single-layered phantoms, leading to multiple internal reflections and interference phenomena at the air-sample interfaces, which result in the observed fluctuating behavior in all cases. The SWISSto12 MCK design enables efficient guidance of EM wave propagation to the material under test within the low-loss measurement setup shown in Fig. 6 (left). Generally, penetration losses tend to increase with higher frequencies and thicknesses, particularly in the case of the 1 mm thick skin phantom, which shows a decreasing trend in the D-band and is illustrated in Fig. 6 (right). This observation highlights the difficulty in detecting penetration losses for thin tissue phantoms, as they display nearly constant losses when it comes to artificial skin. These results align with the relatively flat slope of water molecule absorption in the D-Band.

Regarding THz plant measurements: we investigated SAR-based THz plant status monitoring [25]. The measurements setup focuses on plant monitoring where two leaf samples of the plant Geranium are considered as shown in Fig. 7. One of the leaves shown in Fig. 7 belongs to a plant that is infested with aphids, which are sap-sucking insects. With time, the aphids grow and leave their skin on the leaf surface as slightly visible in Fig. 7 as white dots. A high-resolution image of the leaves is obtained and the intensity scale is normalized with respect to the maximum magnitude of reflection from the mapped environment. For the infected leaf, the complete leaf surface is observable. However, the skeleton of a healthy leaf is not flat compared to the infected leaf. The curved side portion of the healthy leaf resulted in weak reflection. It is assumed that the energy received from the curved portion is below the noise level of the employed system. Hence, the curved portion is not visible in the resulting image.

Regarding the slow material burning: such experiments have been achieved in UDE's fire detection laboratory and are shown in chapter 1 and in the M05 project. Therefore, the strong collaboration between M01 and M05 will be continued in the 3^rd phase.

Selected project-related publications

[1] F. Sheikh, Y. Zantah, I. Mabrouk, M. Alissa, J. Barowski, I. Rolfes, and T. Kaiser, "Scattering and Roughness Analysis of Indoor Materials at Frequencies from 750 GHz to 1.1 THz," IEEE Transactions on Antennas and Propagation, vol. 69, no. 11, pp. 7820-7829, Nov. 2021, [doi: 10.1109/TAP.2021.3076577].

[2] F. Sheikh, Y. Zantah, N. Zarifeh, and T. Kaiser, "Channel Measurements of 0.9-1.1 THz Wireless Links using VNA Extenders," 2021 46th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Chengdu, China, pp. 1-2, 2021, doi: 10.1109/IRMMW-THz50926.2021.9566918

[3] A. Batra, F. Sheikh, M. Khaliel, M. Wiemeler, D. Göhringer, and T. Kaiser, "Object Recognition in High-Resolution Indoor THz SAR Mapped Environment“, Sensors 2022, 22, 3762, doi: 10.3390/s22103762

[4] M. Alissa, B. Friederich, F. Sheikh, A. Czylwik, and T. Kaiser, "Experimental Investigation of Terahertz Scattering: A Study of Non-Gaussianity and Lateral Roughness Influence," IEEE Access, vol. 8, pp. 170672-170680, 2020, doi: 10.1109/ACCESS.2020.3025361

[5] F. Sheikh, Y. Gao, and T. Kaiser, "A Study of Diffuse Scattering in Massive MIMO Channels at Terahertz Frequencies," in IEEE Transactions on Antennas and Propagation, vol. 68, no. 2, pp. 997-1008, Feb. 2020, doi: 10.1109/TAP.2019.2944536.

[6] F. Sheikh, A. Batra, A. Prokscha, D. Lessy and T. Kaiser, "Look Through Hygroscopic Indoor Materials at Frequencies from 750 GHz to 1.1 THz," 2022 Antenna Measurement Techniques Association Symposium (AMTA), Denver, CO, USA, pp. 1-6, 2022, [doi: 10.23919/AMTA55213.2022.9955003].

[7] F. Sheikh, A. Prokscha, J. M. Eckhardt, T. Doeker, N. A. Abbasi, J. Gomez-Ponce, B. Sievert, J. T. Svejda, A. Rennings, J. Barowski, C. Schulz, I. Rolfes, D. Erni, A. F. Molisch, T. Kürner and T. Kaiser, "THz Measurements, Antennas, and Simulations: From the Past to the Future," IEEE Journal of Microwaves, vol. 3, no. 1, pp. 289-304, Jan. 2023, [doi: 10.1109/JMW.2022.3216210].

[8] A. Prokscha, F. Sheikh, M. Jalali, Y. Zantah, B. Sievert, M. Al-Hasan, D. Erni, and T. Kaiser, "A Look Through Artificial Human Tissues at Ka-Band and D-Band," 2023 Sixth International Workshop on Mobile Terahertz Systems (IWMTS), Bonn, Germany, 2023, pp. 1-5, [doi:1109/IWMTS58186.2023.10207780].

[9] F. Sheikh, N. Zarifeh, and T. Kaiser, “Terahertz Band: Channel Modelling for Short range Wireless Communications in the Spectral Windows”, IET Microwaves, Ant. & Propagation, vol. 10, no. 13, pp. 1435-1444, Oct. 2016, doi: 10.1049/iet-map.2016.0022

[10] J. Balzer, C. J. Saraceno, M. Koch, P. Kaurav, U. Pfeiffer, W. Withayachumnankul, T. Kürner, A. Stöhr, M. El-Absi, A. Abbas, T. Kaiser, A. Czylwik, "THz Systems Exploiting Photonics and Communications Technologies," IEEE Journal of Microwaves, vol. 3, no. 1, pp. 268-288, Jan. 2023, [doi: 10.1109/JMW.2022.3228118].