Precise synchronization is essential in various technical disciplines, being especially challenging in mobile scenarios. Unfortunately, state-of-the-art global navigation satellite system (GNSS) disciplined oscillators (GNSSDOs) are designed and optimized for stationary operation. We present a novel solution that is optimized for mobile use from the ground up. The centerpiece is a precise oven-controlled crystal oscillator (OCXO) that is optimized for low sensitivity to dynamic accelerations. A state-of-the-art GNSS timing module is used to discipline it. We evaluate the system by comparing it with state-of-the-art test equipment in a real-world test drive through diverse environments. After compensating for the stationary offset, the state-of-the-art devices deviated by up to 2315 ns, while with our devices, the deviation never exceeded 22.6 ns. It is evident that the devices designed for laboratory use perform inadequately in mobile operation and that our novel solution enables a significant leap in accuracy.

Integrated sensing and communication (ISAC) is a key enabler of 6G, supporting environment-aware services. A fundamental sensing task in this setting is reliable multi-target detection and tracking. This paper proposes a temporal graph neural network (TGNN)-based tracking method that exploits delay and Doppler information from the wireless channel. The delay-Doppler map is modeled as a sequence of graphs, and tracking is formulated as a temporal node classification problem, enabling joint clustering and data association of dynamic targets. Using ray-tracing-based channel outputs as ground truth, the method is evaluated across multiple scenes with varying target positions, velocities, and trajectories and is compared with a Kalman filter baseline. Results demonstrate reduced normalized mean squared error (NMSE) in delay and Doppler, leading to more accurate multi-target tracking.

Parameter estimation for multiple-input-multiple-output (MIMO) channel sounding data aims at accurately describing channel measurements with physically realistic and interpretable parameters. The performance of model-based approaches, e.g., maximum likelihood, is determined by the accuracy of the imposed signal model. For channel sounding data, it has turned out to be beneficial to use two distinct concepts for the description of the propagation process. The specular components account for the dominant propagation paths of plane waves, whereas diffuse components model the weaker but more diverse propagation processes by means of a colored noise process. In order to improve the accuracy of the model for the diffuse components, we propose a simple but still flexible parametric covariance model that allows us to account for a smooth power angle profile that describes the correlation in the spatial domain. Moreover, the model for the deterministic part of the signal is usually contaminated by calibration errors, which in turn deteriorate the reliability of the specular path estimates. This is most prominently visible by the estimation of so-called ghost paths. To mitigate this, we introduce a new model order selection scheme based on the so-called misspecified Cram & eacute;r-Rao bound (MCRB) that accounts for the unavoidable modeling errors. In addition, to avoid the fitting of ghost paths caused by the faulty modeling of strong specular components, we locally decrease the estimated signal-to-noise ratio (SNR) in the time domain around already estimated ones. Furthermore, as these changes to the signal model require more computational resources compared with existing algorithms, we also showcase how necessary quantities like likelihoods, score functions, and Fisher information matrices can still be computed efficiently. We implement our proposed extensions within the Richter maximization approach (RIMAX) framework. We also showcase that they improve the reliability of the produced estimates compared with plain vanilla RIMAX on real measurement data.

Over-the-air (OTA) tests have become an essential tool to assess the performance of a wireless device under controllable and repeatable conditions. Such benefits provide a deep insight into the device under test (DUT) performance; nevertheless, the realism achieved by OTA tests is still moderate in comparison to open-field tests. This contribution takes the OTA testing method for wireless devices and combines it with the wave field synthesis (WFS) technique to increase the efficiency, reliability and especially the realism of the tests. The main focus of this contribution is to use the aforementioned method to emulate realistic GNSS scenarios inside an anechoic chamber, where multiple virtual satellite signals are electronically generated in the far field along with their individual trajectories. For the validation and verification of the method, a commercial GNSS antenna and receiver are placed as the DUT inside the chamber. The WFS calibration methods implemented in an OTA testbed are described in detail along with the performance analysis obtained by each method. This study strives to establish the foundation for a forthcoming standardisation of the proposed methodology, applicable not only to GNSS devices but also to any variety of wireless devices.

A majority of modern positioning solutions are based on global navigation satellite systems (GNSS). These systems provide an exact, reliable, flexible and cheap solution for the related tasks, like sea navigation, aviation or the automotive sector. However, they are vulnerable to deliberate or collateral disturbance by means of jammer or spoofer signals. Hardening the systems in an environment of ever increasing demand for reliability is therefore one of the big challenges in the field. This work presents a solution focused on an analog beamforming solution, due to its cost effectiveness. A prototype of a four antenna receiver is shown with a measured jammer suppression of above 30 dB . The measurement results were obtained in a realistic over the air scenario. The presented work functions as a proof of concept for an integrated version to be implemented in future.

The upcoming 3GPP global mobile communication standard 6G strives to push the technological limits of radio frequency (RF) communication even further than its predecessors: Sum data rates beyond 100 Gbit/s, RF bandwidths above 1 GHz per link, and sub-millisecond latency necessitate very high performance development tools. We propose a new SDR firmware and software architecture designed explicitly to meet these challenging requirements. It relies on Ethernet and commercial off-the-shelf network and server components to maximize flexibility and to reduce costs. We analyze state-of-the-art solutions (USRP X440 and other RFSoC-based systems), derive architectural design goals, explain resulting design decision in detail, and exemplify our architecture’s implementation on the XCZU48DR RFSoC. Finally, we validate its performance via measurements and outline how the architecture surpasses the state of the art with respect to sustained RF recording, while maintaining high Ethernet bandwidth efficiency. Building a 6G integrated sensing and communication (ISAC) example, we demonstrate its real-time and rapid application development capabilities.

We present our new scalable multi-channel and multi-node sounder, the ILMSound G3 . It is configurable in terms of the number of switched Tx and parallel Rx nodes. The basic structures of the Tx and Rx nodes are given with consideration of the measurement system requirements.The ILMSound G3 is validated via a proof of concept measurement at 2.53 GHz in an urban environment. The system was configured as a sounder with two Tx nodes and one Rx node, which records two MIMO links simultaneously. Each node uses an antenna array. The transmitters were moved by cars whereas the receiver was elevated 20 m above the ground by a lifting platform acting as a base station. The used system configuration results in a snapshot rate of approx. 0.5 kHz, which covers the expected time variance of the chosen environment.The measurement results were analyzed using HRPE, providing a parametric description of the specular propagation paths of the radio channel per measurement link. Each estimated path is described by its directions of departure and arrival, delay, Doppler shift, and fully polarimetric complex path weights.

This paper deals with the evolution of downlink codebook based multiple-input multiple-output (MIMO) within the third generation partnership project (3GPP) through release 15 to 17. There exist already several tutorials on this topic. However, to the authors’ knowledge, the comparison in terms of complexity performance trade-off between practical precoder selection strategies in frequency or delay domain has not been tackled so far in the literature. This paper describes with matrix formalism the two main codebook types specified within 3GPP, i.e., Type-I codebook (low resolution) and Type-II codebook (high resolution). The Rel. 17 port selection is also detailed as well as the multi-user MIMO (MU-MIMO) precoding strategy to be applied relying on Type-II codebook feedback. For the enhanced Type-II codebook, two main practical precoder selection strategies are detailed at the UE side (i) based on singular value decomposition per sub-band in the frequency domain, (ii) based on wideband singular decomposition in the delay domain. Monte Carlo simulations demonstrate that the delay domain strategy performance may suffer from spatial interference for single user MIMO high rank transmissions when the channel is both spatially correlated and frequency selective. On the other hand, the frequency domain strategy complexity increases linearly with the number of sub-bands while it is not the case for the delay domain selection strategy. As a result, the delay domain selection strategy is particularly relevant for Rel. 17 Type-II codebook port selection or for a low frequency selective channel with few significant consecutive delays.

To efficiently extract estimates about the propagation behavior of electromagnetic waves in a radio environment it is common to invoke the narrowband-assumption. It essentially states that the relative bandwidth of the measurement system is so low that the frequency response of a single propagation path only depends on it Time-of-Flight and the response of the measurement device can be calibrated independently of the measured channel. Recent advances into higher relative bandwidths and antenna arrays with larger spatial aperture render this assumption less likely to be satisfied, which leads to a model mismatch during estimation. In this case estimates are inherently biased and have a special statistical behavior. This behavior can be captured by the so-called Misspecified Cramér-Rao Bound, which formulates a lower bound for the variance of estimates that are biased due to model mismatch. We analyze this bound in contrast to the traditional Cramér-Rao Bound and show the shortcomings in the setting of joint ToF-DoA estimation in the mmWave spectrum. The conducted numerical studies also show that planar array geometries inherently suffer from violation of the narrowband assumption irrespective of the individual elements' frequency response, whereas circular structures show it to a lesser degree.

Multidimensional channel sounding measures the geometrical structure of mobile radio propagation. The parameters of a multipath data model in terms of directions, time-of-flight and Doppler shift are estimated from observations in frequency, time and space. A maximum likelihood estimation framework allows joint high-resolution in all dimensions. The prerequisite for this is an appropriate parametric data model that represents the multipath propagation correctly. At the same time, a device data model is necessary that typically results from calibration measurements. The used model should be as simple as possible since its structure has a considerable effect on the estimation effort. For instance, the inherent effort in parameter search is reduced if the influence of the parameters is kept orthogonal. Therefore, the data model is characterized by several approximations. The most important is the “narrowband assumption” which assumes a low relative bandwidth and also avoids considering any frequency response in magnitude and phase. We extend the well-known multidimensional \gls{rimax} parameter estimation framework by including proper frequency responses. The advantage reveals most clearly with high bandwidth in the mmWave and sub-THz range. It allows for a more realistic modeling of antenna arrays. It breaks with the usual narrowband model and allows a better modeling of mutual coupling and time delay effects. If the interacting object extends over several delay bins (hence an extended target in radar terminology) we propose a model that assigns a short delay spread, respectively a frequency response to the propagation path that associates itto the respective object. We verify the validity of the device model by numerical experiments on simulated and measured antenna data and compare it to a state-of-the-art method. Additionally, we use synthetic data based on raytracing results and measurements both ranging from \SI{27}{\giga\hertz} up to \SI{33}{\giga\hertz} with known ground truth information and show that the proposed estimator not based on the narrowband assumption delivers better performance for higher relative bandwidths than the conventional \gls{rimax} implementation.

Wireless devices supporting global navigation satellite systems (GNSS) services have become an essential tool in different areas of technology such as agriculture, construction, automotive, etc. Therefore the performance and reliability of such devices are important aspects that need to be addressed in the testing stage during the development of the units. The integration of the Over-the-Air (OTA) testing method with the 3D Wave Field Synthesis (3DWFS) technique offer not only the benefit of having tests under controllable and repeatable conditions but also the ability to recreate complex and realistic scenarios in a controlled environment with full polarimetric support for the testing of wireless devices. This contribution applies this technology to emulate a GNSS scenario within an anechoic chamber. For the results validation, a realistic GNSS outdoor scenario was recorded and compared with the emulated scenario where 3DWFS was applied for each individual satellite. This represents a significant step for the GNSS community and also for the future development and testing of wireless devices.

In satellite communications, it is becoming challenging to provide the tracking performance which is required for Non-Geostationary Orbit (NGSO) constellations with the traditional Satellite Communications (SatCom) On The Move (SOTM) terminal structure which employs bulky parabolic antennas. On the other hand, in terrestrial networks, the single omnidirectional communication with User Equipment (UE) does not provide enough throughput to fulfill the need for higher speed connections. As a consequence, manufacturers started to invest in developing new terminals which use phased array antennas to enable beamforming to increase the directivity and null the interference in terrestrial networks and to provide rapid tracking performance as well as seamless handovers in SOTM. However, this generates new challenge as these antennas change beam patterns depending on the beam steering angle. It is not trivial to evaluate the performance of beamforming antennas since the measurement of the high number of beam patterns that the phased array can form in all directions is time consuming. In this paper, we propose a methodology to measure a large number of beam patterns of a phased array antenna in a more time efficient approach compared to traditional antenna measurement methods. The measured patterns can be used to evaluate the antenna performance and capabilities in different conditions and verify the terminal ability to fulfill the requirements specified by the standards.

This article focuses on potential enhancements of current 5G New Radio (NR) codebooks (CBs) for user equipment (UE) mobility scenarios. An overview of the incremental evolution of the 5G NR Type-II CBs is presented, and its inherent drawbacks in mobility scenarios are discussed. The current Type-II CBs fail to deliver adequate performance when used by moving UEs at medium to high speeds, mainly due to the current channel coherence time-based CSI measurement and reporting. Measured channel data from a number of real-world scenarios are analyzed by means of Doppler power spectrum and delay-Doppler power spectrum to verify the so-called channel stationarity time, which is several-fold higher than the channel coherence time. Based on the presented analysis, conclusions are drawn that for stationarity time-based CSI measurements and reporting, Doppler frequency components in addition to space and delay components can be exploited for Type-II CB enhancements. The enhanced Type-II CB achieves a drastic feedback reduction due to the sparse nature of the channel in the delay-Doppler domain and improved performance compared to the 5G NR Type-II CB.

We present a maximum-likelihood estimation algorithm for radio channel measurements exhibiting a mixture of independent Dense Multipath Components. The novelty of our approach is in the algorithms initialization using a deep learning architecture. Currently, available approaches can only deal with scenarios where a single mode is present. However, in measurements, two or more modes are often observed. This much more challenging multi-modal setting bears two important questions: How many modes are there, and how can we estimate those? To this end, we propose a Neural Net-architecture that can reliably estimate the number of modes present in the data and also provide an initial assessment of their shape. These predictions are used to initialize for gradient- and model-based optimization algorithm to further refine the estimates. We demonstrate numerically how the presented architecture performs on measurement data and analytically study its influence on the estimation of specular paths in a setting where the single-modal approach fails.

With the fast development of wireless devices, over-the-air (OTA) testing is becoming the preferred method among developers and manufacturers of wireless equipment. The ability to recreate a scenario under controllable and repeatable conditions keeps the method under constant development, providing new features that increase the realism during the tests. A recent proof of that is the integration of 3D wave field synthesis (3DWFS) to OTA testing, which becomes a significant step to accurately emulate wireless scenarios within a controlled environment.In this context, this contribution improves the OTA system calibration for 3DWFS; efficiently increasing the emulation quality of electromagnetic plane waves impinging from any angular position within an anechoic chamber. In fact, this enhancement implicitly delivers a new method for accurate estimation of the antenna radiation pattern in 3D. This is not only a highly demanded application among antenna manufacturers but in this case also proves the validity of the results and consolidates the integration of 3DWFS to OTA testing.

While traditional testing methods as conducted and open field tests have their limitations in terms of realism and repeatability of the scenario's conditions, Over-the-Air (OTA) testing enhances the way that GNSS and mobile communications systems with integrated antennas are tested, since it provides the freedom to accurately emulate an impinging wave with arbitrary polarization and direction radiated from any source/reflection such as GNSS satellites and terrestrial base stations. Therefore, real world scenarios can be precisely reproduced with total control of the environment's conditions. In this context, this paper describes the process to implement full polarimetric 3D Wave Field Synthesis (WFS) in an OTA testbed, from the principle to characterize the electromagnetic (EM) field in three dimensions to the system calibration and correspondent verification measurements inside the anechoic chamber, which will provide a deeper insight on the quality and reliability of the EM field for testing purposes.

The bottleneck in OTA testing electrically large equipment is the large number of sources required for sufficient angular density. That is, if the sources are active. In that case, high-bandwidth, high-frequency equipment with excellent stability, processing power, and configurability is needed per antenna element. The costs of scaling up the current MIMO OTA configuration to higher frequencies and/or large equipment is prohibitive. In case passive radiators are used, the maximum control speed is determined by the maximum expected Doppler frequency. Such performance is order of magnitudes cheaper, even for large numbers of elements. In this contribution, we will present a new OTA concept that uses passively radiating reconfigurable structures and is inherently bi-directional. The concept that we call Projection-OTA, accommodates test objects in FR1 that are electrically large to very large and shows good prospects for emulating beam dynamics for testing 5G NR in FR2. Whether such passive structures will be made up by reflect arrays, meta-surfaces, or even intelligent surfaces is to be determined. We will discuss the prospects and limitations of this concept.

The potential of broadband THz communication system is not limited to extremely high data rate transmissions up to several TBit/s. Rather, the true application potential of THz systems will be created by the combination of the communication and radar/sensor functionality under the use of the enormous available bandwidth with partially more than 10 GHz. This enables to develop new applications in the areas of 3D imaging radar systems, sensor systems, car-to-car communication in combination with car radar, and security application like private alarm system via Wi-Fi router. The reuse of future standard 3GPP and IEEE802 THz technology will allow the implementation of low-cost THz communication systems, which enable the joint use of THz systems as a high-resolution sensor systems, which optimize themselves by the use of artificial intelligence to provide never-before-achieved resolutions for radar and sensor applications additional to high-speed Tbit/s data transmissions.

Industry 4.0 is the scenario in which the 5G and beyond networks are expected to show all their potential. However, while propagation at sub-6 GHz has been widely investigated in industry environments, mm-waves propagation and channel modelling in those scenarios is still under early research. Therefore, we introduce novel simultaneous multi-band ultra-wideband measurements at 6.75 GHz and 30 GHz in LOS and NLOS with RX below and above clutter level. This unique set-up allows a direct comparison between the sub-6 GHz and mm-wave channel. Results have shown larger specular to dense multi-path components power ratio and shorter large-scale parameters at mm-waves.