This demo presents a hardware-in-the-loop (HiL) framework for validating and verifying wireless communication hardware under controlled and repeatable laboratory conditions. The setup integrates modems as transmitters and receivers with a real-time channel emulator. To simplify the hardware interfaces we exchange multi-path component parameters between the channel model and the radio channel emulator, following the proposed structure in the ongoing IEEE P1944 standardization for site-specific radio channel representations. The demo features an urban vehicular communication scenario, demonstrating the capability to replicate realistic propagation conditions with non-stationary propagation conditions. Our HiL test system allows dynamic motion changes of the vehicles based on the received data during the emulation process. Site-specific channel emulation enables the lab-based validation of realistic vehicular applications, 5G and 6G physical layer technologies, and the training and testing of AI/ML based receiver architectures.

Next-generation wireless communication systems will incorporate millimeter wave (mmWave) as one of the key technologies to increase data rates. These forthcoming mmWave systems will integrate multiple-input multiple-output (MIMO) technology to ensure sufficient link margin and are expected to be deployed in conjunction with sub-6 GHz systems. Configuring a MIMO communication link usually relies on estimating channel state information (CSI), which is difficult to acquire at mmWave frequencies due to the low pre-beamforming signal-to-noise ratio (SNR). In this paper, we propose a novel approach to estimate mmWave MIMO channels by leveraging out-of-band information from a sub-6GHz band. Utilizing this approach, we develop three channel estimation methods and compare these methods with one using only in-band information. We investigate the influence of theK-factor, the SNR and the number of antennas on the performance of the proposed channel estimation methods through simulations. Additionally, we compare these methods in terms of their computational complexity. Finally, we validate the performance of the proposed methods through channel measurements conducted in an outdoor environment. The results demonstrate that the proposed methods outperform in-band mmWave channel estimation in terms of spectral efficiency, particularly in scenarios of low SNR and high K-factor.

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.

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.

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).

Reconfigurable intelligent surfaces (RISs) can provide a reliable and low-latency millimeter wave (mmWave) communication link in cases of a blocked line-of-sight (LoS) between the base station (BS) and the user equipment (UE). In such cases, the RIS mounted on a wall or ceiling can act as a bypass for the radio communication link. We present an active RIS with 127 patch antenna elements arranged in a hexagonal grid. The RIS operates at a center frequency of 23.8 GHz and each RIS element uses an orthogonal polarization transformation to enable amplification using a field-effect transistor (FET). The source and drain voltages of each FET are controlled using two bits. We consider that the RIS control unit is aware of the UE coordinates within the measurement area, relevant to the industrial control scenarios. We measure the received power on a 2D grid of $60 \mathrm{~cm} \times 90 \mathrm{~cm}$ using an $x y$-positioning table, with the RIS working in reflective and active mode. The results demonstrate the ability of the RIS to effectively focus the radio signal on the desired target points. We characterize the halfpower beam width in azimuth and radial directions with respect to the RIS position, enabling us to obtain a practical RIS configuration update criterion for a mobile UE. These results clearly show that RISs are prominent solutions for enabling reliable wireless communication in indoor industrial scenarios.

Reconfigurable intelligent surfaces (RISs) will play a key role to establish reliable low-latency millimeter wave (mmWave) communication links for indoor automation and control applications. In case of a blocked line-of-sight between the base station (BS) and the user equipment (UE), a RIS mounted on a wall or on a ceiling enables a bypass for the radio communication link. In this work, we present an active RIS for the mmWave frequency band. Each RIS element uses a field effect transistor (FET) to amplify the reflected signal and an orthogonal polarization transformation to increase the isolation between impinging and reflected radio wave. By switching the bias voltage at gate and drain of the FET we can establish four states for each RIS element: two reflection states with different phase shifts, an active amplification and an off state. We present results of the active RIS with 37 patch antenna elements arranged in a hexagonal grid for a center frequency of 25.8 GHz. The RIS field patterns obtained by numerical simulations and by empirical measurements in an anechoic chamber are compared. They show a good match and the received power is improved by 12 dB in the active mode of the RIS compared to the reflective mode.

Wireless vehicular communication will increase the safety of road users. The reliability of vehicular communication links is of high importance as links with low reliability may diminish the advantage of having situational traffic information. The goal of our investigation is to obtain a reliable coverage area for non-stationary vehicular scenarios. Therefore we propose a deep neural network (DNN) for predicting the expected frame error rate (FER). The DNN is trained in a supervised fashion, where a time-limited sequence of channel frequency responses has been labeled with its corresponding FER values assuming an underlying wireless communication system, i.e. IEEE 802.11p. For generating the training dataset we use a geometry-based stochastic channel model (GSCM). We obtain the ground truth FER by emulating the time-varying frequency responses using a hardware-in-the-loop setup. Our GSCM provides the propagation path parameters which we use to fix the statistics of the fading process at one point in space for an arbitrary amount of time, enabling accurate FER estimation. Using this dataset we achieve an accuracy of 85% of the DNN. We use the trained model to predict the FER for measured time-varying channel transfer functions obtained during a measurement campaign. We compare the predicted output of the DNN to the measured FER on the road and obtain a prediction accuracy of 78%.

Future vehicular communication systems will extend deployed frequency bands from sub-6 GHz to millimeter wave (mmWave). To investigate different propagation effects between sub-6 GHz and mmWave bands in high-mobility scenarios, we proposed a suitable testbed setup to compare these two bands in a fair manner. Experiments conducted using the proposed testbed provide realistic results, but they are only usable if they can be faithfully reproduced. To quantify the reproducibility of the proposed testbed, we perform channel measurements at center frequencies of 2.55 GHz and 25.5 GHz at a velocity of 50 km/h. We investigate the influence of antenna pattern, time between measurements, signal-to-interference-and-noise ratio (SINR) and signal bandwidth on the reproducibility in terms of the channel correlation.

One of the key research directions to increase the capacity of new radio (NR) vehicle-to-everything (V2X) communication systems is extension of employed frequency bands from sub-6 GHz to millimeter wave (mmWave) range. To investigate different propagation effects between sub-6 GHz and mmWave bands in high-mobility scenarios, one needs to conduct channel measurements in both frequency bands. Using a suitable testbed setup to compare these two bands in a fair manner, we perform channel measurements at center frequencies of 2.55 GHz and 25.5 GHz, velocities of 50 km/h and 100 km/h, and at 126 different spatial positions. Furthermore, we conduct a comparative study of the multi-band propagation based on measurement results. We estimate the power delay profile (PDP) and the Doppler power spectral density (DSD) from a large set of measurements collected in a measurement campaign. Finally, we compare measured wireless channels at the two employed frequency bands in terms of root-mean-square (RMS) delay spread and RMS Doppler spread.

In this paper we present a hardware-in-the-loop (HiL) framework for testing wireless vehicle-to-everything (V2X) communication hardware, i.e., modems under realistic channel conditions. The framework includes a wireless channel emulator, which is capable of emulating non-stationary wireless channels in real-time. We validate the HiL framework by comparing the frame error rate (FER) obtained via emulation with data obtained during a V2X measurement campaign using the same IEEE 802.11p based modems. To do this we acquire measured time-variant channel transfer function and FER measurements simultaneously. The results show that our HiL approach is feasible and that we can obtain FER measurements in the laboratory that closely match the measurement results obtained on the road, giving the maximal distance of 0.099 between their cumulative distribution functions.

In this paper we present dual-band massive multiple-input multiple-output (MIMO) channel measurements for a railway station scenario. The massive MIMO link shall provide an ultra-reliable low-latency communication link from the control center to the locomotive. In the measurement campaign the locomotive moves from line-of-sight (LOS) to non line-of-sight. We present dual frequency band measurements, where the massive MIMO array at the base station has 24 receive antenna elements at 1890 MHz and 8 receive antenna elements at 748 MHz. The measurement bandwidth is 20 MHz and we use a repetition rate of 1 ms to acquire the time-variant channel frequency response from the locomotive to all 32 antenna elements in parallel. We provide a first analysis of the root mean square (RMS) delay spread, the path loss coefficients and the channel hardening in both frequency bands. The measurements show that the RMS delay spread at lower frequencies is slightly smaller compared to higher frequencies, due to a stronger LOS component at lower frequencies. Furthermore, the results indicate that increasing the number of receive antennas in vertical domain, does not allow for better channel hardening and that lower frequencies allow for better channel hardening with the same amount of antennas.

This paper discusses a freely available and open dataset containing vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I) and vehicle-to-pedestrian (V2P) OFDM-based wireless channel measurement data including synchronised sensor data such as radar, LiDAR and high precision GPS. The wireless channel measurement is conducted at the carrier frequencies of 3.2 GHz and 5.81 GHz which are the most promising frequency bands in which future V2X communication systems will operate. The dataset contains the wireless channel measurement data of various V2X scenarios along with synchronized sensor information from a vehicle. In addition to the wireless channel measurement data, the dataset also includes frame error rate measurements from a IEEE 802.11p based communication system, synchronized to the other measurement data.

In this paper we present and discuss results of a wireless vehicle-to-vehicle (V2V) dualband channel measurement campaign at center frequencies of 3.2 GHz and 34.3 GHz in an urban street scenario. The measurement is conducted using a bandwidth of 155.5 MHz and a sounding repetition rate of 62.5 $\mu \mathrm{s}$ for both bands. At the transmitter side we use omni-directional antennas and at the receiver side directional antennas with 17° opening angles. With this setup, we present the first comparison of simultaneous and dynamic multiband V2V measurements using the time-variant power delay profile (PDP) and the Doppler spectral density (DSD). We find close similarities for the line-of-sight, specular as well as diffuse reflections in both frequency bands, enabling future work for out-of-band beam finding in vehicular mmWave systems.

In this paper we present a software defined radio (SDR) based measurement framework that allows for simultaneous measurements of wireless communication channels over multiple bands. We present and discuss results of a wireless vehicular-to-infrastructure multiband channel measurement campaign at center frequencies of 3.2 GHz, 34.3 GHz and 62.35 GHz in a street crossing scenario. The measurement is conducted using a bandwidth of 155.5 MHz and a sounding repetition rate of 31.25 μs. We compare the measurements using the time-variant power delay profile (PDP) and the Doppler spectral density (DSD).

In this paper we adopt and modify a well-known locality-aware hashing scheme to the problem of active stochastic scatterer selection in vehicular non-stationary geometry-based stochastic channel models (GSCM). We show, how under relaxed assumptions on the query set an efficient selection of active stochastic scatterers during simulation is computationally feasible. The proposed approach enables real-time simulation and emulation of large-scale GSCMs by restricting the active stochastic scatterer set to meet given resource constraints. We showcase our approach by introducing a GSCM that is boot-strapped via OpenStreetMap data. The stochastic scatterers are placed automatically along buildings, traffic signs and vegetation. We validate and investigate the impact of the proposed approach on the accuracy of a GSCM by means of second order statistics of the time- and frequency-varying fading process. For validation and performance evaluation we parameterize our GSCM using a vehicular wireless channel measurement campaign conducted in the inner city of Vienna. The impact of selecting only a subset of scatterers is then evaluated using the calibrated GSCM.

We present a strategy to overcome channel aging and to enable massive multiple-input multiple-output (MIMO) in highly time-varying scenarios. Given the self-contained 5G slot structure, we introduce channel prediction methods for the down-link utilizing up-link reference symbols and investigate the prediction quality for synthetic and empirical high mobility channel measurement data. Further, we combine channel prediction with orthogonal precoding to decrease the bit error rate (BER) and allow longer prediction horizons. We show through link-level simulations that this framework exhibits a performance drop of only 1 dB compared to perfectly known channel state information for a velocity of 160 km/h at 3.5 GHz. We conclude the paper with a parametric BER study in dependence of the prediction/precoding horizon length.

In this paper we introduce the AIT ray tracer (RT) that is able to accurately simulate propagation effects of wireless communication channels. It covers the classical propagation mechanisms such as specular reflections, transmission and edge-diffraction. The novelty of our RT is that it allows to obtain correlated diffuse scattering and, thus, provides a time-variant Doppler spectral density. This is achieved by introducing a lattice-based surface tiling, which does not alter the position of the scattering points during simulation but provides high resolution diffuse scattering. The selection of the basis vectors for the finite lattice ensures that each ray falls into a separate delay bin (at least for first-order scattering). We implement our RT using the NVIDIA OptiX ray tracing engine, this enables us to trace a vast amount of rays in a single pass. We compare our RT simulations to a measurement conducted in a non-stationary urban intersection scenario achieving a near perfect match in terms of second order statistics.

Repeatable system-level test methods for vehicle-to-everything communication (V2X) are crucial in improving road safety and supporting connected autonomous driving. They rely on geometrical channel models that provide realistic signal propagation delays, Doppler shifts and path loss. In this work we focus on the validation of an OpenStreetMap (OSM) geometry based stochastic channel model (GSCM) that has a high degree of automation and low computational complexity. In an urban intersection scenario we compare the time-variant power delay profile (PDP) and Doppler spectral density (DSD) obtained from a measurement campaign to the ones obtained by the OSM-GSCM. Furthermore, we measure packet error rates (PER) of commercial modem hardware by connecting a transmitter and a receiver to the AIT channel emulator, i.e. using a hardware-in-the-loop (HiL) setup. We compare the PER obtained when the channel emulator uses the OSM-GSCM as input and when it uses the measured impulse responses. The OSM-GSCM path loss shows minor differences to the measurements due to the 2D structure of the OSM-GSCM, while the PER, PDP and DSD exhibit a good match between OSM-GSCM and empirical measurement data.

The advantages of measuring multiple wireless links simultaneously has been gaining attention due to the growing complexity of wireless communication systems. Analyzing vehicular communication systems presents a particular challenge due to their rapid time-varying nature. Therefore multi-node channel sounding is crucial for such endeavors. In this paper, we present the architecture and practical implementation of a scalable mobile multi-node channel sounder, optimized for use in vehicular scenarios. We perform a measurement campaign with three moving nodes, which includes a line of sight (LoS) connection on two links and non LoS(NLoS) conditions on the third link. We present the results on the obtained channel delay and Doppler characteristics, followed by the assessment of the degree of correlation of the analyzed channels and time-variant channel rates, hence investigating the suitability of the channel's physical attributes for relaying. The results show low cross-correlation between the transfer functions of the direct and the relaying link, while a higher rate is calculated for the relaying link.

This paper presents results obtained from a vehicle- to-vehicle channel measurement campaign carried out in the millimeter-wave band around a 60 GHz center frequency and with 8 GHz of bandwidth. We characterize a situation of two oncoming cars on a two-lane road in the campus of the Brno University of Technology. For several vehicle passes we evaluate: (1) observed root mean square (RMS) delay spreads as a function of the received power, (2) temporal decorrelation of the channel impulse response and (3) a dependency of the Pearson correlation coefficient on the received power. For the measurement campaign, a correlative time-domain channel sounder was used.

The performance of wireless communication systems is fundamentally determined by the properties of the underlying wireless communication channel. Vehicular communication channels exhibit time-variant multi-path propagation with non-stationary channel statistics. Thus, channel emulation tools for the reproducible test of wireless communication systems are urgently needed to enable the development of ultra-reliable low-latency communication links. In this paper we validate the vehicle-in-the-loop (ViL) test of vehicle-to-everything (V2X) communication links by means of time-variant channel emulation. The validation is performed by comparing the received signal strength indicator (RSSI) and packet error rate (PER) of measurements on a proving ground with the RSSI and PER obtained from ViL tests. For the ViL tests, the wireless communication channel is emulated using a geometry-based stochastic channel model, which is updated in real-time, dependent on the position and velocity of the vehicles. We collect results of different scenarios on the proving ground and from ViL tests. The results show an qualitative match between ViL test and measurement on the proving ground. An exact quantitative match can be obtained with the calibration parameters from the measurements.

This paper reports on a power-delay profile measurement campaign emulating a mobile vehicle-to-infrastructure urban-highway environment. The measured location is in the city of Brno, Czech Republic. Utilizing a correlative 50 GS/s time-domain channel sounder with a center frequency of 59.6 GHz and 8 GHz bandwidth and with open-ended WR15 waveguide antennas, we characterize the representative millimeter wave radio channels in terms of the RMS delay spread and its variability caused by mobile scatterers (i.e., vehicles). The RMS delay spread exhibits notable heteroscedasticity as its standard deviation can decrease by 40%. The measured high resolution power-delay profiles exhibit clustering behavior, where a typical number of reflected multipath components is four to five.

Ultra-reliable wireless communication links require the utilization of all diversity sources of a wireless communication channel. Hadani et al. propose a two dimensional discrete symplectic Fourier transform (DSFT) as orthogonal pre-coder for a time-frequency modulation scheme. In this paper we explore general orthogonal precoding (OP) and its performance in time- and frequency-selective channels. We show that iterative parallel interference cancellation (PIC) and iterative channel estimation methods can be used for the detection of OP. A scalar signal model for OP transmission is obtained by PIC. Based on this signal model, we can prove that all constant modulus sequences, e.g. the DSFT basis functions or Walsh-Hadamard sequences, lead to the same performance for OP. We validate our receiver structure by numerical link level simulations of a vehicle-to-vehicle communication link with a relative velocity of 0... 200 km/h, We demonstrate that OP achieves a gain of about 4.8 dB if compared to orthogonal frequency division multiplexing at a bit error rate of 10-4. Our performance results for coded OP are the best results for a fully documented receiver architecture, published so far.

The performance of wireless communication systems is fundamentally determined by wireless communication channel properties. Wireless vehicular communication channels exhibit multipath propagation and non-stationary channel statistics. Methods and tools for the repeatable test of wireless communication systems and signal processing algorithms in such environments are urgently needed to enable the development of reliable communication links with low-latency. In this paper we present the measurement and validation of a real time channel emulation method for non-stationary vehicular scenarios based on a geometry-based stochastic channel model.

Multi-Gb/s visible-light communication is demonstrated using a commercial off-the-shelf resonant-cavity light emitting diode (LED), which is originally rated for 150 Mb/s. By applying analog frequency response equalization and multi-carrier modulation schemes, a transmission capacity of up to 4.3 Gb/s is obtained over a single wavelength in a close-proximity scenario. Nyquist-shaped multi-band modulation and orthogonal-frequency-division multiplexing are applied with high spectral sub-carrier efficiencies of up to 8 b/symbol. The transmission rate is experimentally investigated as a function of the loss budget and further related to link reach based on free-space measurements under clear weather conditions. Analog signal transmission is also validated using real-time signal (de-)modulation with a high-definition video payload. We further demonstrate that on/off keying in combination with simpler baseband modulation can be facilitated for data rates of up to 750 Mb/s. This proves that commercially available LEDs can serve as a versatile low-cost transmitter. Finally, the joint transmission of energy and data has been validated. A power feed with an irradiance of 240 W/m2is experimentally shown to enable a remotely supplied optical burst receiver for periodic access to Gb/s data rates.

Sub-band divided ray tracing (SDRT) is one technique that has been extensively used to obtain the channel characteristics for ultra-wideband (UWB) radio wave propagation in realistic indoor environments. However, the computational complexity of SDRT scales directly with the number of sub-bands. Although we have proposed a low-complexity SDRT algorithm for one terminal position [1], the computational complexity is still extremely high when involving multiple mobile terminal positions. Moreover, some indoor positioning techniques require for high positioning accuracy data from measurements/simulations with a very fine spatial resolution. To cope with this, we propose an algorithm to reduce the computational complexity of SDRT for multiple mobile terminal positions. The algorithm uses a projection of all propagation paths on a subspace spanned by two-dimensional discrete prolate spheroidal (DPS) sequences at each sub-band. It is important to note that, since the geometrical information of the propagation paths is the same in all sub-bands, the subspace dimension and basis coefficients in frequency dimension do not need to be recalculated at different sub-bands. We justify the simplifications of the proposed method by numerical simulations. Furthermore, we evaluate the effect of antenna characteristics on the proposed algorithm. Our proposed algorithm reduces the computational complexity by more than one order of magnitude for indoor scenarios.

Intelligent transport systems (ITS) require low-latency dependable wireless communication links in between vehicles as well as between vehicles and the infrastructure. In vehicular communication scenarios communication channels are time- and frequency (doubly) dispersive and the channel statistics are non-stationary, i.e., they change over time. Hence, the design of appropriate channel estimators is challenging. Recently an adaptive reduced-rank channel estimation technique for non-stationary time-variant channel estimation was introduced by Zemen and Molisch, 2012. This technique uses a hypothesis test to obtain an estimate of the current channel statistics on a per frame basis. The optimum number of hypotheses is not known. In this paper we present new empirical insights on the optimum choice of the number of hypotheses for the hypothesis test for non-stationary time-variant channel estimation. With these considerations the complexity of an adaptive reduced-rank channel estimator can be reduced and its performance improved.