Principal Investigator: Prof. Dr. Thomas Kaiser

Achieved results and methods

This comprehensive summary synthesizes our research efforts aimed at enhancing indoor localization accuracy through the 2^nd phase of MARIE. In [3], the study presents an innovative approach for optimizing Three-Dimensional (3D) coverage in indoor environments by the placement of a minimal set of chipless RFID tags. The research introduces a novel 3D localization planning algorithm, validated through numerical evaluations and 3D ray-tracing simulations. The algorithm achieves high coverage with fewer tags, emphasizing its practical applicability. The work in [4] and [5] discusses enhancing the Radar Cross Section (RCS) of tags using retro-directive devices and resonating structures. This research demonstrates the capability of achieving high RCS levels over broad bandwidths and wide incident angles, essential for wide-angle response in RFID systems. In [6], a Photonic Crystal (PhC) with unique spectral signatures in the specular direction, is introduced to RFID tags, enabling them to sense the Angle of Arrival (AoA) of incoming waves. This advancement is critical for improving angular resolution and mitigating clutter return, a common issue in environments with reflecting surfaces [7]. The work in [8] explores the theoretical limits of localization accuracy in 3D self-localization using chipless RFID systems in the THz band. The study derives the Cramer Rao Lower Bound (CRLB) and Position Error Bound (PEB), highlighting the challenges and potential for achieving sub-mm accuracy. The work in [9] addresses the challenges of wireless connectivity in the THz band by proposing the use of RIS. This innovative solution aims to maintain virtual (via one RIS reflection) Line-of-Sight (LoS) connectivity, with the study focusing on developing path loss models for RIS-assisted RFID communications. In [10], the impact of transceiver impairments on localization accuracy in scenarios involving RIS is considered. The study employs a Maximum Likelihood Estimator (MLE) and introduces the Bayesian Fisher Information Matrix (FIM) and Bayesian CRLB as benchmarks for accuracy assessment. The works in [11,12] integrates SAR with an indoor localization system using passive chipless frequency-coded tags, demonstrating high-resolution indoor mapping and the potential for applications in emergency response and surveillance.

Localization infrastructure optimization In [3], we explore the optimization of RFID systems as infrastructure focusing on achieving efficient 3D coverage. This method guarantees extensive coverage in indoor environments using a minimal number of chipless RFID tags, which is essential for precise localization of objects, particularly when dealing with moving robots or UAVs. This algorithm takes into account various constraints, such as room dimensions, tag range, antenna patterns, and link budgets. Our methodology formulates the tag placement as a binary integer programming problem, aiming for minimal tag use while considering system limitations and propagation constraints. We address potential coverage gaps due to optimization discretization by proposing an iterative method to ensure comprehensive 3D coverage. The proposed algorithm was validated through numerical evaluations and 3D ray-tracing simulations in THz band, considering various environmental topologies. Specifically, for a system configuration described in [3] with a threshold received power of -60 dBm, Fig.1 demonstrates that our algorithm could achieve 99.9% 3D coverage with a minimal number of tags when the received power equals -60 dBm and greater, confirming their practical applicability.

In [4], retro-directive devices such as lenses and corner reflectors are combined with various resonating structures in order to augment the RCS of the tag and achieve a wide-angle response. Fig. 2(A) depicts the integration of dielectric resonators with a dihedral corner reflector, resulting in high RCS levels across a broad bandwidth and wide incident angles of 70°. Additionally, Fig. 2(B) illustrates a homogeneous lens paired with a curved One-Dimensional (1D) PhC resonator along the focal line as proposed in [5]. In Fig. 2(E), an experimental demonstration of a single notch shows an achieved wide-angle response of 80° with a stable appearance of the notch maintained over the entire angle range.

In [6], an element of intelligence is introduced to the tag by enabling the capability to sense the AoA of incoming waves. This is achieved by joining a flat metal plate with a 1D PhC resonator at an angle of 90°, see Fig. 2(C). Based on the signatures shown in Fig. 2(F), high angular resolution is achieved in the angular range of -35° to 35°. Tags mounted close to a reflecting wall may experience significant clutter return, which may overshadow tag signatures. Two approaches are proposed to mitigate clutter: twisting polarization [7] and Orbital Angular Momentum (OAM) mode conversion [13]. As illustrated in Fig. 2 (D1), polarization twisting is achieved by employing two DRs tilted by ±45° and spaced by λ/4. In Fig. 2 (D2), OAM mode conversion is achieved by implementing a helically arranged DR array to convert the incoming OAM mode order from 0 to order 1.

Localization accuracy limits In [8], we explore the theoretical accuracy bounds of 3D self-localization based on chipless RFID systems at THz band. The methodology comprises a system model with a reader of unknown location and multiple known-locations chipless tags. Signal processing is executed using backscattered signals from tags, employing MLE to estimate the Return Time of Flight (RToF). This work derives the CRLB and PEB. CRLB, a statistical measure, assesses the theoretical limits of localization accuracy, considering aspects such as sampling period, Signal-to-Noise Ratio (SNR), and bandwidth. PEB, extracted from the FIM, quantifies the reader's location estimation accuracy. Simulations validate these theoretical findings, highlighting that enhanced SNR and bandwidth improve accuracy. This study concludes that sub-mm and even sub-100µm accuracy in THz band chipless RFID-based localization systems is feasible but facing challenges due to the requisite high sampling rates and complex signal processing.

RIS-assisted RFID wireless communications Owing to the inherent challenges of wireless connectivity in the THz band, which include severe path loss, vulnerability to blockages, and limited coverage, we propose in [9] the utilization of RIS as an innovative solution. This approach is designed to maintain virtual LoS connectivity between the reader and the tag in the event of direct LoS obstruction, thereby effectively manipulating signal propagation to mitigate these issues. The work in [9] focuses on the development of comprehensive free-space path loss models tailored for RIS-assisted RFID wireless communications, incorporating various physical parameters such as tag radar cross-section and the inherent properties of RIS. The proposed models considers formulation of path loss in both radiative near-field and far-field scenarios. The study also delves into the design considerations of RIS unit-cells, emphasizing their role in signal direction. To validate the theoretical models, we conducted extensive experimental measurements using fabricated RIS provided from S03, as shown in Fig.3 (A) and compared the results with numerical simulations. These experiments utilized two array RISs of differing sizes, enabling a comprehensive analysis of path loss in both specular and non-specular reflection scenarios. Fig. 3 (B) shows the measured path loss is compared with the developed path loss formulas for the non-specular reflection scenario. The measured path loss is greater by approximately 3 dB compared to the numerical curves. This increase in loss is expected, given that the reflected path splits evenly between the two aforementioned paths. Accordingly, the measurements are in good agreement with the proposed path loss models.

Hardware impairments In [10], a joint localization-synchronization process is considered for scenarios where the reader is impacted by the presence of a RIS, and no longer operates ideally. This process employs a MLE, which incorporates various transceiver impairments such as power amplifier non-linearity, amplitude/phase imbalance in I/Q mixers, phase noise in LoS, sampling jitter, and finite-resolution quantization in analog-to-digital converters. The mobile reader is designed to estimate its position and achieve synchronization by using an MLE with a cost function that accounts for these hardware impairments. This methodology is further extended to tracking, utilizing a Kalman Filter based Tracker (KFT) that processes the initial localization-synchronization results. The KFT specifically addresses the impact of hardware impairments on localization accuracy. Moreover, the study introduces the Bayesian FIM and the Bayesian CRLB as benchmarks for assessing the accuracy of the tracking process in the RIS/V-aided system under the influence of hardware impairments.

Localization and tracking system evaluation To ensure the precise generation of SAR images at high frequencies, it is imperative to accurately localize and track the path of the UAV carrying the SAR transceivers. In [12], we introduce a highly accurate millimeter-wave indoor self-tracking system utilizing chipless tags infrastructure to enhance SAR sensing capabilities. Illustrated in Fig. 4(A), three tags are utilized, incorporating Frequency Selective Surface (FSS) (C09) and Dielectric Resonator (DR) (S04). The FSS array and DR array are positioned in front of a trihedral corner reflector. SAR imaging is executed by S05 using Vector Network Analyzer (VNA) for various objects affixed to a Styrofoam column. The experimental results presented in Fig. 4(B) affirm the viability of the localization system, demonstrating a remarkable accuracy in the path direction, with an absolute average error of 1.8 mm. The system will undergo further assessment through additional experiments conducted in the PreSyse lab at a higher frequency range (220 GHz – 330 GHz). Specifically, FSS-based tags utilizing cross-dipole unit cells from C09 will be fabricated to operate within this specified range, ensuring a more stable angle response. These tags will undergo evaluation in realistic scenarios, with measurements conducted on readout range, tag sensitivity, and tag angular retro-directivity in both 2D and 3D. To further assess the performance of the proposed localization and tracking algorithms, the reader movement will be simulated using different trajectories, facilitated by the 6-axis robot arm installed in the PreSyse lab.

Selected project-related publications

1. El-Absi, A. Al-Haj Abbas and T. Kaiser, "Chipless RFID Tags Placement Optimization as Infrastructure for Maximal Localization Coverage," in IEEE Journal of Radio Frequency Identification, vol. 6, pp. 368-380, 2022, [DOI: 10.1109/JRFID.2022.3189555]
2. Alhaj Abbas, M. El-Absi, A. Abuelhaija, K. Solbach and T. Kaiser, "Corner reflector tag with RCS frequency coding by dielectric resonators" in IET Microw. Antennas Propag, 15: 560-570, 2021, [DOI: 10.1049/mia2.12067]
3. Alhaj Abbas, Y. Zantah, A. Abuelhaija and T. Kaiser, "Millimeter-Wave Retro-Directive Frequency Coded Lens by Curved One-Dimensional Photonic Crystal Resonator," in IEEE Access, vol. 10, pp. 132988-133000, 2022, [DOI: 10.1109/ACCESS.2022.3226124]
4. Alhaj Abbas, Y. Zhao, J. Balzer, K. Solbach and T. Kaiser, "Photonic-Crystal-Resonator-Based Corner Reflector With Angle-of-Arrival Sensing," in IEEE Journal of Radio Frequency Identification, vol. 7, pp. 451-462, 2023, [DOI: 10.1109/JRFID.2023.3302009]
5. Alhaj Abbas, M. Hassan, A. Abuelhaija, D. Erni, K. Solbach and T. Kaiser, "Retrodirective Dielectric Resonator Tag with Polarization Twist Signature for Clutter Suppression in Self-Localization System", in IEEE Transactions on Microwave Theory and Techniques, Dec. 2021, [DOI: 10.1109/TMTT.2021.3108151]
6. El-Absi, A. Al-Haj Abbas, D. Tubail, F. Ilgac, A. Abuelhaija, Y. Zantah, S. Ikki, A. Sezgin and T. Kaiser, "Path Loss Modeling of RFID Backscatter Channels With Reconfigurable Intelligent Surface: Experimental Validation," in IEEE Access, vol. 11, pp. 108532-108543, 2023, [DOI: 10.1109/ACCESS.2023.3317892]
7. Tubail, M. El-Absi, S. Ikki and T. Kaiser, “Hardware-Aware Joint Localization-Synchronization and Tracking in 5G and Beyond," in IEEE Open Journal of the Communications Society, vol. 5, pp. 1899-1915, 2024, [DOI: 10.1109/OJCOMS.2024.3377720]
8. Batra, M. El-Absi, M. Wiemeler, D. Göhringer and T. Kaiser, "Indoor THz SAR Trajectory Deviations Effects and Compensation with Passive Sub-mm Localization System," in IEEE Access, vol. 8, pp. 177519-177533, 2020, [DOI: 10.1109/ACCESS.2020.3026884]
9. Batra, A. Alhaj Abbas; J. Sánchez-Pastor, M. El-Absi, A. Jiménez-Sáez; M. Khaliel; J. Barowski, D. Göhringer, I. Rolfes, R. Jakoby and T. Kaiser, "Millimeter Wave Indoor SAR Sensing Assisted with Chipless Tags-Based Self-Localization System: Experimental Evaluation," in IEEE Sensors Journal, vol. 24, no. 1, pp. 844-857, 1 Jan.1, 2024, [DOI: 10.1109/JSEN.2023.3332431]
10. Balzer, C. Saraceno, M. Koch, P. Kauray, U. Pfeiffer, W. Withayachumnankul, T. Kürner, A. Stöhr, M. El-Absi, A. Al-Haj Abbas, T. Kaiser, and A. Czylwik, "THz Systems Exploiting Photonics and Communications Technologies," in IEEE Journal of Microwaves, vol. 3, no. 1, pp. 268-288, Jan. 2023, [DOI: 10.1109/JMW.2022.3228118]