Future connected cooperative automated mobility can benefit from high-data-rate wireless communication links between vehicles to exchange LIDAR and/or RADAR data. The millimeter wave (mmWave) frequency range offers large bandwidth for rapid sensor data exchange but suffers from higher signal attenuation compared to the centimeter wave (cmWave) band. In this paper we obtain cmWave and mmWave multipath component (MPC) parameters by combining a single omnidirectional antenna that is used as virtual array with the CLEAN algorithm. The cmWave MPC information enables low-overhead mmWave beamforming by means of an antenna array. Empirical multi-band measurement data ($3.2 \text{GHz}, 34.3 \text{GHz}$, and 62.35 GHz) shows that considering the 5 strongest MPCs, a signal-to-noise ratio of up to 25 dB for a V2I scenario can be achieved.

Future vehicular communication systems will integrate millimeter wave (mmWave) technology to enhance data transmission rates. To investigate the propagation effects and small-scale fading differences between mmWave and conventional centimeter wave (cmWave) bands, multi-band channel measurements have to be conducted. One key parameter to characterize small-scale fading is the Rician $K$-factor. In this paper, we analyze the time-varying $K$-factor of vehicle-to-infrastructure (V2I) channels across multiple frequency bands, measured in an urban street environment. Specifically, we investigate three frequency bands with center frequencies of 3.2 GHz, 34.3 GHz and 62.35 GHz using measurement data with 155.5 MHz band-width and a sounding repetition rate of 31.25 μs. Furthermore, we analyze the relationship between K-factor and root-mean-square (RMS) delay spread. We show that the Ricean $K$-factor is similar at different frequency bands and that is correlated with the RMS delay spread.

Since the design of wireless MIMO systems requires knowledge of the double-directional (i.e., directionally resolved at both link ends) channel characteristics, and 5G/6G use higher frequency bands, there is the need for double-directional measurements in the mmWave spectrum, along with channel sounders that can accurately perform such measurements. This paper introduces a novel channel sounding approach based on a redirecting rotating mirror arrangement (ReRoMA). The method is low-cost and flexible as it requires only a single radio frequency chain at each link end and performs mechanical beamsteering. However, in contrast to existing rotating-horn systems, it physically separates the signal generation/transmission and the beam steering components, resulting in orders-of-magnitude faster measurements. The paper outlines the fundamental concept, describes details of the implementation, and demonstrates its application and accuracy using a 60GHz prototype for measurements in static reference scenarios, as well as dynamic measurements. We illustrate the detected propagation paths using dynamic angular and delay power spectra and correlate these findings with the surrounding environmental structure. Locations of environmental objects are detected within the Fourier resolution determined by bandwidth and horn width, with no noticeable degradation due to the faster measurements.

Vehicle-to-everything (V2X) communications is an important part of future driver assistance and traffic control systems that will reduce accidents and congestion. The millimeter-wave (mmWave) band shows great promise to enable the high-data-rate links that are required or at least beneficial for such systems. To design such systems, we first need a detailed understanding of the vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2X) propagation channels. This paper provides a systematic account of a series of measurement campaigns for such channels, conducted by the four research institutions of the authors over the past year. After a description of the similarities and differences of the channel sounders used in the campaigns, a description of the measurements in two European and one American city is given, and the scenarios of convoy, opposite-lane passing, and overtaking, are described. This is then followed by key results, presenting both sample results of power delay profiles and delay Doppler (or angular) spectra, as well as the statistical description such as delay spread and size of stationarity region. We also discuss the availability of spatial diversity in V2I connections and the correlation of the channels between different frequency bands.

The coordination of vehicles is a crucial element of autonomous driving, as it enhances the efficiency, convenience, and safety of road traffic. In order to fully exploit the capabilities of such coordination, communication with high data rate and low latency is required. It can be reasonably argued that millimeter-wave (mm-wave) vehicle-to-vehicle (V2V) systems are capable of fulfilling the aforementioned requirements. Nevertheless, in order to develop a system that can be deployed in real-world scenarios and to gain an understanding of the various effects of mm-wave propagation, it is necessary to perform radio propagation measurements and to derive radio channel models from them across a range of scenarios and environments. To this end, we have conducted measurement campaigns at 60\,GHz in a variety of situations, including driving in a convoy, driving in opposite direction on a six-lane road, and overtaking. These measurements employ a channel sounder based on ReRoMA, a recently introduced concept that enables the real-time measurement of dynamic double-directional radio channels. The evaluations presented herein encompass key channel parameters, including the path loss (path loss coefficient of approximately 1.9), the root mean square (RMS) delay spread (within a range of 5\,ns to 110\,ns), the angular spreads (in a range of 0.05 to 0.4), the power distribution among multipath components, and the channel stationarity time (multiple seconds).

Studies have shown that the benefits of fully auto-mated traffic, namely increased safety and efficiency, are being negated in mixed traffic scenarios already by a low percentage of human drivers. Besides human driving and perception errors, this effect is caused by the lack or coarseness of communication between the automated intersection control system and the human drivers, typically only through classical traffic lights. In this work, we propose to extend the interaction capabilities between human-driven vehicles (HDVs) and an automated intersection with warnings and maneuver recommendations, such as lane-change or velocity recommendations (comp. green light optimal speed advisory (GLOSA)) or safety warnings. These so-called soft inputs enable HDVs to travel more efficiently and safely by exploiting 5G communication, collective perception, and state-of-the-art intersection control concepts. The human compliance behavior to such inputs is modeled, calibrated, and simulated based on expert knowledge and naturalistic driving data. A suitable maneuver recommendation formulation for a conflicting unprotected left-turn scenario is outlined and tested in simulation studies in a typical urban intersection scenario. Analyzing relevant key performance indicators shows the high achievable performance and its trade-off characteristics with respect to HDV penetration rate and compliance behavior.

Cooperative connected automated mobility depends on sensing and wireless communication functions. With increasing carrier frequency both functions can be realized with the same hardware, however, the attenuation of radio signals increases quadratically with the carrier frequency. Hence, link setup becomes challenging in vehicular scenarios due to the required beam finding process. In this paper we investigate the multipath components of the vehicle-to-infrastructure (V2I) radio channel in three frequency bands with center frequencies of 3.2 GHz, 34.3 GHz and 62.35 GHz using measurement data with 155.5 MHz bandwidth and a sounding repetition rate of $31.25 \mu \mathrm{~s}$. The channel impulse responses are collected simultaneously at all three carrier frequencies. Using the high temporal sampling rate we apply the CLEAN algorithm, enabling the estimation of the weight, delay and Doppler frequency of multipath components. By analyzing the collinearity of the Doppler normalized scattering function between the frequency bands we found that the collinearity between the 3.2 GHz and 34.3 GHz band as well as between the 3.2 GHz and 62.35 GHz is smaller in the non-line of sight (NLOS) region but increases for the line-of-sight (LOS).

With the move towards 6G and associated technology deployment in higher frequency bands, measurements of directionally-resolved channels and sounders capable of performing such measurements are a necessity. In this paper, we present a new concept of channel sounding based on a Redirecting Rotating Mirror Arrangement (ReRoMA), capable of performing double-directional channel measurements at millimeter wave frequencies by mechanical beam steering orders of magnitude faster than existing rotating-horn arrangements. We present this new concept, describe a prototype operating at 60 GHz, and use it to perform, as proof-of-principle, a dynamic cart-to-cart channel measurements at a T-intersection scenario. We show that this sounding principle works and allows the directional evaluation of the channel. We visualize the different resolvable propagation paths in terms of dynamic angular and delay power spectrum, and relate them to the environmental geometry.

The development of communication systems for in-telligent transportation systems (ITS) relies on their performance in high-mobility scenarios. Such scenarios introduce rapid fluctuations in wireless channel properties. As a promising solution for vehicle-to-everything (V2X) communication, the orthogonal time frequency space (OTFS) approach has emerged. Nevertheless, the performance of OTFS systems is closely tied to time- and frequency diversity of the wireless propagation channel. However, there is a lack of understanding of the stationarity of the wireless channels, especially in the millimeter wave (mmWave) frequency bands. In this paper, we address this research gap by conducting a comprehensive stationarity analysis of measured sub-6 GHz and mmWave high-speed wireless channels. We evaluate the spatial stationarity of a scenario, where the transmitter is moving at high velocity. Furthermore, we investigate the influence of the transmit antenna orientation on the channel spatial stationarity. We could show that the spatial stationarity is proportional to the wavelength.

The role of wireless communications in various domains of intelligent transportation systems is significant; it is evident that dependable message exchange between nodes (cars, bikes, pedestrians, infrastructure, etc.) has to be guaranteed to fulfill the stringent requirements for future transportation systems. A precise site-specific digital twin is seen as a key enabler for the cost-effective development and validation of future vehicular communication systems. Furthermore, achieving a realistic digital twin for dependable wireless communications requires accurate measurement, modeling, and emulation of wireless communication channels. However, contemporary approaches in these domains are not efficient enough to satisfy the foreseen needs. In this position paper, we overview the current solutions, indicate their limitations, and discuss the most prospective paths for future investigation.