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This paper presents an outdoor long-range (from 315 m up to 5.3 km) fixed channel campaign for both ultra high frequency and super high frequency bands with co-polarized horizontal and vertical antenna configurations. It investigates the channel characteristics of device to device communication scenarios underlaying the 5th generation networks by providing detailed research. Both line of sight and non-line of sight measurements in 1.3 GHz and 5.8 GHz frequency bands with bandwidth up to 600 MHz were conducted. The path loss, root mean square delay spread, coherence bandwidth, and channel frequency response variation are characterized. We observed that the variation is negligible in microcell line of sight environment for both above mentioned frequencies, whereas it significantly increases with frequency in different macrocell non-line of sight environments. The distance dependency of path loss was also derived. It was observed that the root mean square delay spread decreases with frequency for both line of sight microcell and non-line of sight macrocell measurements. A dependency between the root mean square delay spread and transmitter-receiver distance in non-line of sight environments was also captured. The relation between the coherence bandwidth and the root mean square delay spread was depicted. It demonstrates an exponential function in all considered channel combinations.

It is essential to capture joint channel characteristics in cooperative multi-link systems in order to make realistic performance assessments and channel modeling. This paper investigates the joint statistical properties of large-scale parameters (LSPs) in high-speed railway (HSR) multi-link propagation scenarios. A dedicated long-term evolution (LTE) network-based multi-link channel sounding system is established, which breaks the constraints in HSR measurements and supports simultaneous time–frequency–space measurements for multiple links. To guarantee the effectiveness of multi-link measurements, a time-delay-window-based partition method is used to identify channel impulse responses from different links. Based on the processed multi-link data, the statistics of LSPs, involving shadow fading, K-factor, delay spread, and azimuth spread, is estimated and compared for the two links in a plain viaduct scenario on HSR. Then, auto-correlations and two kinds of cross correlations, such as intra-link correlation and inter-link correlation in the measured scenario, are calculated and analyzed. In addition, a simulation model of joint statistical characteristics for LSPs is proposed and validated according to the obtained measurement results. These results can be used to assess the actual performance of cooperative multi-link systems on HSR, and the proposed model can provide realistic inputs for channel models in HSR multi-link propagation scenarios.

The main objective of this paper to determine multipath and time-varying channel behaviour of short-terrestrial millimetre-wave point-to-point radio links. In an attempt to invigorate the impact of rain attenuation on mm-wave channel parameters such as the RMS delay spread, path loss received power strength and Rician distribution with a K factor. A brief analysis of rain fading was presented based on the simultaneous measurement of one-minute rain rate and its effects on a short experimental link of 38 GHz. Rain fade average is observed as high as 16 dB for 300 m path at about 125 mm/hr rain intensity. The statistical spatial channel mode (SSCM) simulation software was utilized for an operating frequency of 38 GHz. To generate of power delay profile (PDP). For both omnidirectional and directional antenna. The RMS delay spread and path loss has been estimated using the environmental parameters of Kuala Lumpur city which illustrates the theoretical performances of 5G in Malaysia. It is observed that RMS delay spread, path loss received power strength and K factor effected dramatically by rain fade. (SSCM) simulation software has to be modified to consider rain fade dynamic characteristics to achieve ultra-reliability requirements of outdoor applications in the tropical regions. This study is important for understanding signal propagation phenomena in short distance and enabling the utilization of the millimetre wave band for an urban micro-cellular environment for 5G communication system.

In this paper, the intra-wagon channels at 60 and 300 GHz bands are characterized through measurement-validated ray-tracing (RT) simulations. To begin with, an in-house-developed three-dimensional RT simulator is calibrated and validated by a series of millimeter-wave and Terahertz channel measurements inside a high-speed train wagon. Then, the validated RT simulator is used to conduct extensive simulations with different transmitter (Tx) and receiver deployments. At low frequencies, the channel is strongly influenced by the line of sight (LOS), and therefore, is usually classified into LOS and non-LOS (NLOS) regions. However, the simulation results at 60 and 300 GHz bands show that the first-order reflection also imposes a significant impact on the channel characteristics. This motivates us to further classify the NLOS region into light-NLOS (L-NLOS) and deep-NLOS (D-NLOS) according to the existence of the first-order reflection. Through analyzing the area ratios of LOS, L-NLOS, and D-NLOS regions, we evaluate the Tx deployment strategies and suggest the optimum one. Based on RT simulation results, totally 12 cases (three propagation regions with two Tx deployments at two frequencies) are characterized in terms of path loss, shadow fading, root-mean-square delay spread, Rician $K$ -factor, azimuth/elevation angular spread of arrival/departure, cross-polarization ratio, and their cross correlations. All these parameters are fed into the 3GPP-like quasi-deterministic radio channel generator (QuaDRiGa). The good agreement between QuaDRiGa and RT proves that the 13 tables provided in this paper effectively parameterize the intra-wagon scenario for the standard channel model family. These results provide valuable insights into the system design and evaluation for intra-wagon communications.

Traditional orthogonal frequency-division multiplexing (OFDM) systems mitigate multipath interferences with the help of a cyclic prefix (CP) appended between two adjacent OFDM blocks. The length of the CP must be made longer than channel delay spread to avoid intersymbol interference (ISI). The use of CPs degrades spectrum efficiency significantly, which is a deal price paid in all existing OFDM systems. Motivated to deal with this issue, this paper proposes a CP-free OFDM scheme with successive multipath interference cancellation (SMIC), which does not require CPs and removes ISI before fast Fourier transform (FFT) at a receiver using stored feedback equalization (SFE). The SFE operation leaves a CP gap between adjacent OFDM blocks, and the CP is regenerated with estimated signals. In this way, we convert linearly shifted OFDM blocks induced by multipath propagation back to cyclically shifted OFDM blocks for successful FFT operation. SMIC-OFDM achieves a much higher spectrum efficiency than traditional OFDM owing to the save of CPs in transmission signals. The performance of SMIC-OFDM in terms of the bit error rate, capacity, and computational complexity is compared to that of traditional OFDM to verify the effectiveness of the proposed SMIC-OFDM scheme.

In two earlier papers Pazsit et al. (2016), Nagy et al. (2018) we investigated the possibility of extracting the traditional multiplicity count rates from the cumulants of fission chambers signals in the current mode. The first three cumulants for up to three fission chambers were derived in an extended stochastic model of the detector signals, introduced earlier for simpler problems Pal et al. (2014), Pal and Pazsit (2015). It was shown that if all neutrons emitted from the sample simultaneously are also detected simultaneously, the multiplicity rates can be retrieved from the cumulants of the detector current, but the method breaks down if the detections of neutrons of common origin take place with a time delay spread wider than the pulse shape. It was seen that even if the pulses overlap partially, the retrieval of the multiplicity rates depends on the time delay distribution, which is usually not known. To remedy these shortcomings, in this work we extended the theory from the previous one-point (in time) treatment, where all moments (cumulants) refer to the same time instant, to two- and three-point distributions (correlations). It was found that the integrals of suitably chosen two- and three-point moments with respect to the time differences become independent of the probability density of the time delays of detections. With this procedure, within practical limits, the multiplicity rates can be retrieved from the detector current(s) for arbitrary time delay distributions, and hence also with thermalised neutrons. The underlying theory, the outline of the derivations and the full results are given in the paper.

In this paper, a general preamble-based time domain channel estimation model for multiple-input multiple-output (MIMO)-filter bank multicarrier (FBMC) system based on offset quadrature amplitude modulation (OQAM) is derived. In contrast to the conventional frequency domain interference approximation method (IAM)-based channel estimation model, the derived time domain model does not require symbol time to be sufficiently longer than the maximum channel delay spread, viz. the derived time domain model does not require the channel to be frequency flat. Based on the time domain model, the weighted least squares (WLS) and minimum mean square error (MMSE) channel estimators for MIMO-FBMC/OQAM systems are investigated. The Cramer–Rao lower bound for the WLS, MMSE and IAM estimators are derived. It is shown numerically that all the estimators achieve their respective bounds, and the proposed WLS and MMSE estimators significantly outperform the existing IAM channel estimator. The comparison of the bit error rate performance of the aforementioned estimators also validates our work.

Nowadays, rail traffic is expected to evolve into a new era of `smart rail mobility', where trains, infrastructure, travellers and goods will be increasingly interconnected. Railway communications are required to support various high-data-rate applications, the communication system should be carefully designed, which makes railway scenario becomes a more and more important communication scenario. In this study, the channel characteristics are analysed for rural railway at 28 GHz. Four different deployments are considered via calibrated ray-tracing simulations in the target environment. The large-scale parameters of the mmWave channel, including the path loss, root-mean-square delay spread, Rician K -factor, angular spreads and cross-polarisation ratio are explored. The statistical properties, decorrelation distance and cross-correlations are analysed as well. The studied channel characteristics can be used to support the link level and system level design of the communication system in the similar environment.

Background/ Objectives: Recently, air-to-ground wireless communication systems have been used in various equipment such as unmanned aerial vehicle (UAV). In this paper, we analyze the air-to-ground channel in 400 MHz band with experiment using light aircraft. Methods/ Statistical analysis: The characteristics of the air-to-ground channel are analyzed as follows. We generate and store a pilot signal in commercial vector signal generator and an aircraft equipped with the signal generator takes off. As a pilot signal, Zadoff-Chu sequence is used for its good autocorrelation characteristics. In the ground station, the received waveform is stored by using spectrum analyzer. When an airplane approaches near the ground station, airplane transmits the pilot signal and the ground station begins to store the received signals. After completing the signal storing, air-to-ground channel analysis is performed. Findings: From the experiments, we obtained several hundreds of received waveforms, and those waveforms are used for statistical air-to-ground channel analysis. According to the analysis results, most of the measurement channels are single path or line of sight channels. However, in a certain measurement, multiple paths or non-line of sight channels are observed. Those results confirm that multipath environments exist at certain air-to-ground condition. The maximum number of multipath components is 4 and the maximum delay spread is 0.4 μsec. Therefore, when designing a wireless communication system considering air-to-ground environments, the design should consider 4-path fading channels and maximum delay spread over 0.4 μsec. When designing air-to-ground wireless communication systems such as aircraft and drone systems, the experimental and analyzed channels in this paper might be useful. Improvements/ Applications: In this paper, we analyze the air-to-ground channel characteristics of a mountainous area in Korea with field experiment using light aircraft. The analyzed channel characteristics are helpful to those who want to design wireless communication systems between aircraft to ground or UAV to ground. In addition, the analyzed channel can be used for verification of compatibility for the existing communication systems to air-to-ground applications.

In this paper, we measure, simulate, and characterize the train-to-infrastructure (T2I) inside-station channel at the terahertz (THz) band for the first time. To begin with, a series of channel measurements is performed in a train test center at 304.2 GHz with 8 GHz bandwidth. Rician $K$ -factor and root-mean-square (RMS) delay spread are extracted from the measured power-delay profile. With the aid of an in-house-developed ray-tracing (RT) simulator, the multipath constitution is physically interpreted. This provides the first hand information of how the communicating train itself and the other train on site influence the channel. Using this measurement-validated RT simulator, we extend the measurement campaign to more realistic T2I inside-station channel through extensive simulations with various combinations of transmitter deployments and train conditions. Based on RT results, all cases of the target channel are characterized in terms of path loss, shadow fading, RMS delay spread, Rician $K$ -factor, azimuth/elevation angular spread of arrival/departure, cross-polarization ratio, and their cross correlations. All parameters are fed into and verified by the 3GPP-like quasi-deterministic radio channel generator. This can provide the foundation for future work that aims to add the T2I inside-station scenario into the standard channel model families, and furthermore, provides a baseline for system design and evaluation of THz communications.

Unmanned aerial vehicles (UAVs) have been applied to various promising applications because of their features of high-mobility, portability, rapid deployment, and modest power consumption. Small UAVs fly at low altitudes and typically short distances from the ground controller. It is of interest and value to model the propagation channel between low-altitude UAVs and ground stations. This article presents results of channel measurements at 3.9 GHz carried out in a suburban environment with a small-size UAV flying at low altitudes, up to 40 m. The authors also present comparative results from ray-tracing simulations. Height-dependent models for path loss, root mean square (RMS) delay spread, and the number of multipath components are provided. The results illustrate that although multipath effects exist at low altitudes in this environment, the two-ray path loss model works reasonably well in describing the channel behaviour. Initial ray tracing simulations show reasonable agreement with measurements. In addition, RMS delay spread results indicate that distant scatterers have an appreciable effect on the UAV air-ground (AG) propagation channels, which can be different from some terrestrial channels. Authors' results should be of use in the modelling of low-altitude AG propagation channels and in the performance analysis of UAV-enabled AG communication systems.

The unmanned aerial vehicles (UAVs) have been widely applied in various fields due to their advantages like high mobility and low cost. Reliable communication is the premise to ensure the connectivity between UAV nodes. To provide reasonable references for the design, deployment, and operation of UAV communication systems, the precise prediction of radio channel parameters are required. In this study, the authors propose prediction methods for path loss and delay spread in air-to-ground millimetre-wave channels based on machine learning. Random forest and K-nearest-neighbours are the algorithms employed in the methods. Then, a feature selection scheme is proposed to further improve the prediction accuracy and generalisation performance of the machine-learning-based methods. Generally, machine learning algorithms require massive data for training purpose. However, measuring data is time-consuming and costly, especially when the scenario or frequency changes. Therefore, transfer learning methods are introduced to predict path loss with limited data. The proposed methods for path loss prediction are compared to Okumura-Hata and COST-231 Hata models. The lognormal distribution is the contrast model in delay spread prediction. Based on the data generated by ray-tracing software, the new methods have a smaller root mean square errors than contrast models.

We propose a multiple-access (MA) method in the uplink of a orthogonal time frequency space modulation-based wireless communication system where the channel has high Doppler and delay spread. Each user terminal (UT) is allocated delay-Doppler (DD) resource blocks which are spaced at equal intervals in the DD domain. This limits the corresponding time-frequency (TF) transmit signal to a sub-domain of the entire TF domain. By allocating non-overlapping portions to the TF transmit signal of different UTs, multi-user interference (MUI) is avoided. The proposed MA method is analytically shown to be MUI free and therefore has a significantly higher sum spectral efficiency when compared to other methods proposed in literature which utilize guard bands in the DD domain to reduce MUI.

Hybrid, mixed analogue/digital beam-forming architectures are a central research topic for the development of mm-wave communication for fifth generation. Most approaches are based on phase shifters in the analogue part, which are inherently narrowband. In this study, the authors evaluate the effects and performance of narrowband beam-steering on wideband signals using two different modelling approaches: spatial-narrowband model, in which the inter-antenna element delays are neglected, and spatial-wideband model, in which the inter-antenna element delays are considered. They show that neglecting the spatial-wideband effects on channel models overestimates the focusing of beam-forming, the expected delay spread reduction, and achievable data-rate with an increasing number of antennas and bandwidth. These effects are quantised by simulations using a realistic ray-tracing scenario at 60 GHz.

This paper presents an experimental and analytical investigation of ultra-wideband (3–8 GHz) body-to-body (B-to-B) channel characterization. The channel parameters obtained are compared with off-body (off-B) and no-body (no-B) links. Measurements have been carried out considering different body positions and orientation of the Tx–Rx pair. The type of channel link and position of the wearable antenna lead to significant changes in the radio link channel. Path loss (PL) computed shows the increase in magnitude for non line of sight (NLOS) in comparison to line of sight (LOS), though the variation over a distance of 5 m is more significant for LOS (10 dB) than NLOS links (4–5 dB). For total NLOS links, B-to-B channels have slightly higher PL and delay spread parameters in comparison to off-B channel. The B-to-B LOS channels have least PL (44–56 dB), rms delay spread (0.5–4 ns), and multipath components (3–20) which can be attributed to the directional radiation pattern due to the presence of the human body for both Tx and Rx nodes. For back-to-back antenna orientation of the B-to-B channel, maximum signal strength degradation and multipath are observed. It can be concluded that for certain links, the B-to-B channel shows a significant change in channel behavior in comparison to off-B/no-B scenario.

Elaborate channel modeling is an essential prerequisite for design, testing, and improvement of the massive multiple-input–multiple-output (MIMO) system. In this letter, dense multipath component (DMC) modeling for massive MIMO channels is presented based on measurement campaigns in three different indoor scenarios. The parameters for DMC are extracted by applying a RiMAX-based estimator. The distance-dependence and the variance behaviors of the DMC parameters across the large-scale array are analyzed and modeled. Moreover, the model validation is performed by using an empirical cluster-based massive MIMO channel model. Entropy capacity, condition number, channel ellipticity, and root-mean-square delay spread, which reflect the spatial structure and multipath richness of the channel, are used as the validation metrics. The validation shows that the incorporation of DMC can significantly improve the accuracy of channel modeling.

Multi-link transmission is one of the promising communication techniques capable of improving system capacity and cell-edge user spectral efficiency in railway communication networks. The performance of multi-link transmission heavily depends on the propagation characteristics of radio channels, especially the correlation property between multiple radio channels. Thus, it is important to characterize the multi-link channel in railway communication networks. In this paper, extensive wideband measurements in a viaduct railway environment at 460 MHz with two base stations and one mobile station are performed. Large-scale parameters (LSPs), including large-scale fading, Ricean $K$ -factor, delay spread, and angle spread, are extracted from the measurement data. Based on the measurements, auto-correlation and cross-correlation properties of each LSP are investigated. A Markov-based multi-link tapped-delay-line model for railway communications is established, where the Markov chains are introduced to model the birth and death state of multipath components in multi-link scenarios. Using the relationship between the correlation coefficients of complex random variables (RVs), the amplitude and the phase of taps with different delays are modeled as correlated RVs. The proposed channel model is implemented and validated with measurements.

Outdoor to indoor penetration loss is one of the crucial challenges faced at millimeter-wave frequencies. This paper presents the results from 28-GHz channel sounding campaigns performed to investigate the impact of this phenomenon on the wireless propagation channel characteristics in small cell and fixed wireless access scenarios. The measurements are performed with a real-time channel sounder equipped with phased array antennas that allow beam-forming and electronic beam steering for directionally resolved measurements. Thanks to the short measurement time and the excellent phase stability of the system, we obtain both directional and omnidirectional channel power delay profiles without any delay uncertainty. We compare the measured path loss, delay spread, and angular spread for indoor and outdoor receiver locations for two different types of buildings. We find that the penetration loss strongly depends on the angle of incidence, and the scatterers on the outside of the building strongly impact how much power is coupled into the building. Based on the results, we provide statistical models for path loss, delay spread, and angular spread.

Autonomous driving is delightedly an innovative and revolutionary paradigm for future intelligent transport systems. To be fully functional and efficient, vehicles will use hundreds of sensors and generate terabytes of data that will be used and shared for safety, infotainment and allied services. Communication among vehicles or between vehicle and infrastructure thus requires data rate, latency and reliability far beyond what the legacy dedicated short-range communication (DSRC) and long-term evolution-advanced (LTE-A) systems can support. In this work, the authors motivate the use of millimetre-wave (mmWave) massive multiple-input multiple-output (MIMO) technology to facilitate gigabits-per-second (Gbps) communication for cellular vehicle-to-infrastructure scenarios. As a fundamental component, the authors characterise the mmWave massive MIMO vehicular channel using metrics such as path loss, root-mean-square delay spread, Rician K -factor, cluster and ray distribution, power delay profile, channel rank and condition number as well as data rate. They compare the mmWave performance with the DSRC and LTE-A capabilities, and offer useful insights on vehicular channels. The results show that mmWave massive MIMO can deliver Gbps data rates for next-generation vehicular networks.

Powered by the Internet of Things, the vehicular ad-hoc networks are expected to evolve into the Internet of Intelligent Vehicles in which each vehicle can be much more efficient in various vehicular and transportation applications. In order to realize this vision, a seamless low-latency and ultrareliable vehicle-to-vehicle (V2V) communication network is required. Thus, it is of importance to characterize the V2V channels in various realistic environments, especially when the line-of-sight between transmitter (Tx) and receiver (Rx) is obstructed. In this paper, we characterize obstructed V2V channels in the 5-GHz band through measurement-calibrated ray-tracing (RT) simulations. To begin, the main objects in the real world are divided into two groups: 1) the small-scale structures (e.g., lampposts, traffic signs, etc.) and 2) the large-scale structures (such as buildings and ground). Then, we integrate the radar cross sections of the small-scale structures into our RT simulator through a framework based on high frequency prediction techniques. For the large-scale structures, we calibrate the electromagnetic and scattering parameters of the large-scale structures through V2V channel measurements. After such integration and calibration, extensive RT simulations for V2V channels with Tx and Rx located on vehicles traveling in the opposite or same direction are realized with various antenna deployments in urban and open space environments, with and without sloped terrain. Based on the RT results, we characterize the path loss, shadow fading, and delay spread of the channel for each case, and show agreement with measured results in the literature for all these channel characteristics.

We explore the potential of mathematically modelling the ultra-wideband (3.1-6.3 GHz) off-body radio channel using the same statistical distribution for corridor environments. Radio channel descriptive parameters of received signal strength, mean excess delay ( τ mean ) and root mean square (RMS) delay spread ( τ RMS ) are presented, modelled and compared for five indoor corridor environments. Statistical distributions for these corridors exhibit strong similarities despite each possessing starkly different geometrical and physical characteristics, and test arrangements. Each of the corridor environments was best modelled by the Rician distribution for the line-of-sight (LOS) received power and the Lognormal distribution for the non-LOS (NLOS) received power. Distribution of tmean was best modelled by the normal distribution for both LOS and NLOS scenarios. The distribution of τ RMS was likewise best modelled by the normal distribution for both LOS and NLOS arrangements. For corridors 1 and 2 (considered to exhibit the greatest differences), a finite element study was conducted to substantiate that the results cannot be dismissed as statistical anomalies created by the averaging effect over long journey paths. This work is relevant for the mathematical modelling of UWB off-body channel sounding in interior corridors and may reduce efforts required to model future indoor corridor variants.

Vehicular communication is characterized by a dynamic environment and high mobility. In this paper we present a shadow fading model targeting system simulations based on real measurements performed in underground gallery. As first results, we present in this paper the delay spread statistics for each investigated environment. We also study the large-scale, small-scale fading and extract some time channel parameters such as root-mean-square (RMS) delay for a realistic underground propagation environment at 5.9 GHz. Since there are so far few published results for these confined environments, the results obtained can be useful for the deployment of vehicular-to-vehicular (V2V) and vehicular-to-infrastructure (V2I) communication systems inside underground mines galleries.

Industrial control applications require real-time communication systems with time-critical and safety-critical features, as well as the ability to operate in harsh propagation environments. This means that data communications need to be bounded both in time and reliability domains, while dealing with severe interference coming from other wireless communication systems, including jammers, and with signal degradation phenomena such as delay spread, deep fading or Doppler spread. The present work proposes a spectrum handoff (i.e. channel switching) mechanism for Industrial Wireless Sensor and Actuator Networks (IWSAN) which is capable of detecting interferences and hopping to another channel within a given time bound. This is possible because the spectrum handoff strategy is capable of detecting interference in the MAC layer with the high requirements demanded by real-time IWSAN applications in terms of time and reliability. A theoretical analysis has first been provided using Network Calculus, which determines the delay bounds, and simulations in OPNET have been done afterwards in order to validate the theoretical results, taking into account several industrial environments (e.g. lognormal shadowing, Rice and Rayleigh fading and interference). Furthermore, the proposed communication system, based on the IEEE 802.11a/g physical layer, has been implemented in a FPGA, and simulations in OPNET network simulator and measurements on real hardware have been carried out in order to characterize its performance. The obtained results prove the capability of the proposed spectrum handoff mechanism to guarantee bounded delays and reliable operation in harsh propagation environments with interference, what makes it a suitable candidate for replacing wired communication systems in industrial environments.

This paper deals with the statistical modeling of key features of power line communication (PLC) channels that are necessary for designing data communication systems that operate over theses channels. The key features are average channel attenuation, root mean squared delay spread, coherence bandwidth and coherence time. All these features were estimated from in-home PLC channels measured in seven distinct and typical Brazilian residences. Assuming that each feature is a random variable, four criteria (i.e., maximum likelihood estimate and three different information criteria) are used to select the statistical distribution that offers the best fits for each attribute. During this process, the symmetry and asymmetry of the histogram associated with each feature is pointed out. The reported results focus on three frequency bands, namely: from 1.7 MHz up to 30 MHz, from 1.7 MHz up to 50 MHz and from 1.7 MHz up to 100 MHz, which are useful for with Europe, North America and Brazil. The findings report similarities and discrepancies with previous statistical models related to US and Europe and enables one to better design practical PLC devices.

This paper focuses on the analysis of propagation characteristics for train-ground communication (TGC) systems in tunnel scenarios at both low frequency and millimeter-wave (mmWave) bands, based on ray-tracing (RT) simulation. The material parameters in the RT simulation are calibrated by measurement data collected in realistic tunnel environments. A practical three dimension (3D) tunnel TGC environment considering the existence of train cars is established, which is further divided into three kinds of scenarios, involving the direct coverage, relay coverage, and in-train coverage scenarios. Both large-scale and small-scale propagation characteristics, such as path loss and root mean square delay spread, are analyzed for the three tunnel TGC scenarios. The obtained results can provide useful information for the design of future fifth-generation (5G) tunnel TGC systems.

Vehicle-to-vehicle (V2V) channels exhibit unique properties due to the highly dynamic environment and low elevation of the antennas at both ends of the link. Of particular importance for the behavior of V2V channels, and consequent reliability of the communication link, is the severity and dynamics of blockage of both the line-of-sight and other multipath components (MPCs). The characteristics of blockage become more important as the carrier frequency increases, and the ability of the signal to penetrate through objects diminishes. To characterize the effects of vehicle blockage, we performed V2V channel measurements in four different frequency bands (6.75, 30, 60, and 73 GHz) in urban and highway scenarios. We analyzed the impact of the blocker size and position on the received power and fast fading parameters, as well as the frequency dependence of these parameters under blockage. Our results show that there is a strong influence of the size of the blocking vehicle on the blockage loss and the angular/delay spread. The position of the blocker relative to the transmitter and receiver also plays an important role. On the other hand, the frequency dependence is quite limited, with the blockage loss increasing slightly and the number of scattered MPCs reducing slightly as frequency increases. The main conclusion of this paper is that V2V communication will be possible in high (millimeter-wave) frequencies, even in the case of blockage by other vehicles.

The paradigm shift towards high data rate demands of mobile users in IMT-2020 commonly known as 5G, led to the possibility of using Aerial Base Stations (ABS) to fulfill such requirements. However, for implementation of ABS, an appropriate air-to-ground channel model is needed. It is an important factor to incorporate the understanding of the channel fading behavior before designing the system. In this article, we present novel channel propagation results obtained from ray tracing simulations for different environments, such as Suburban, Urban and Urban-High-Rise, according to ITU Radio-communication parameters. The details of different channel characteristics such as Spatial Correlation and Cumulative Distribution Function for Small Scale Parameters as Delay Spread and Angle-of-Arrival are presented for different ABS heights. We also focus on various channel modeling approaches and frameworks for 3D channel models.

Overtaking case is a common traffic situation, but it is likely to pose a threat to traffic safety. To ensure reliable communication, it is necessary to study the characteristics of wireless channel under overtaking cases. In this paper, we focus on Vehicle-to-Vehicle (V2V) radio channel characteristics based on measurements at 5.9GHz. Through research and analysis, we can present empirical results of propagation channels in four scenarios. Our main concern is the difference in channel characteristics between overtaking and non-overtaking case. We propose a method to divide overtaking cases and non-overtaking cases points based on small-scale fading distribution, and results show that the dividing point is about 11 meters between the measured vehicles. Judging by Akaike Information Criterion(AIC), Weibull distribution is best under overtaking cases, while it follows Ricean distribution under non-overtaking. Then, we prove that the result of the AIC decision is valid by the Kolmogorov-Smirnov test. Therefore, we further compare Weibull $\beta $ -factor and Ricean K-factor under two cases. The values of root-mean-square (RMS) delay spreads and average fade duration under overtaking cases are higher than those under non-overtaking, but RMS Doppler spreads and the level crossing rate are smaller than those under non-overtaking. The cross-correlation between RMS delay spread and Ricean K-factor is also calculated. The correlation value in OT case is higher than that in NOT case. The value of the correlation in the case of overtaking is higher than the case of non-overtaking. By comparing the above methods, it is proved that the dividing point between overtaking cases and non-overtaking cases is effective.

Automatic gain control (AGC) is widely adopted in orthogonal frequency division multiplexing (OFDM) systems to compensate the significant variation of the received signal. In OFDM systems, the duration of the cyclic prefix (CP) is usually equal to the delay spread of the channel to minimize the system overhead. The inter-symbol interference (ISI) between consecutive OFDM symbols spreads over a large portion of the CP. The CP contaminated by the ISI should not be used to estimate the received power. If the duration of the remaining CP is shorter than the settling time of the AGC, then the gain adjustment will be carried out within the useful portion of the OFDM symbol. The subcarrier orthogonality is lost and the system performance is degraded by the resulting inter-carrier interference (ICI). In this paper, we propose a simple and efficient scheme to mitigate the ICI. The proposed scheme can be easily implemented in actual systems because of its low computational complexity. The simulation results show that the proposed scheme can increase the signal-to-interference-plus-noise ratio significantly compared to the traditional scheme.

This paper devises a robust receiver for OFDM systems in the presence of residual timing offsets and unknown channel prior information. The proposed receiver constructs typical receiver models and resorts to the model selection technique to choose the best-matched receiver model to improve the channel estimation and signal detection. The typical receiver models are classified by considering the channel delay spread and the level of timing offset. Based on the receiver model selected by the Bayesian model selection algorithm, the channel length and timing offset parameters in the receiver model can provide the effective channel statistical information to make the channel estimator adapt to the altered circumstances and thus more accurate. Furthermore, the effective interference variance parameters in the selected receiver model are used to enhance the channel estimation and refine the soft symbol detection. The simulation results show that the proposed receiver achieves a significant performance gain compared to the existing methods. It is also shown that the proposed scheme makes the receiver robust to the diverse channel conditions and the timing offset states at a cost of the only a moderate increase in complexity.

In recent years, ambient backscatter communications have gained a lot of interests as a promising enabling technology for the Internet-of-Things and green communications. In ambient backscatter communication systems, ultra-low power devices are able to transmit information by backscattering ambient radio-frequency signals generated by legacy communication systems such as Wi-Fi and cellular networks. This paper is concerned with ambient backscatter communications over legacy orthogonal frequency division multiplexing (OFDM) signals. We propose a backscatter modulation scheme that allows backscattering devices to take advantage of the spectrum structure of ambient OFDM symbols to transmit information. The proposed modulation scheme allows both binary and higher-order modulation using noncoherent energy detection. We investigate the detector design and analyze the error performance of the proposed scheme. We provide an exact expression for the error probability for the binary case, whereas accurate approximate expressions for the error probability are derived for the $M$ -ary case. We corroborate our analysis using Monte–Carlo simulation and investigate the effects of varying the OFDM symbol size, maximum channel delay spread, and the number of receive antennas on the error performance. Our numerical results show that the proposed technique outperforms other techniques available in this paper for backscatter communication over ambient OFDM signals in different scenarios.

This paper presents the analysis of short-term fading behavior in high-speed railway (HSR) communication networks, based on passive channel measurements. A long-term evolution (LTE)-based channel sounding system is used to measure the propagation characteristics in a dedicated HSR LTE network with multiple scenarios, such as rural, station, and suburban, and with the echo channel effect (ECE) caused by the same signaling from different remote radio units. The measurement data are partitioned into four cases, involving rural, rural with the ECE, station, and suburban, aiming to investigate the impact of two factors, e.g., the scenario and the ECE, on the propagation characteristics. Based on the processed data, the short-term fading behavior is characterized in terms of the fading severity and the time-frequency-space dispersion in the four cases. The distance-dependent models and statistical models of Ricean K-factor, delay spread, Doppler spread, and angular spread are proposed and compared with previous paper. The presented results will be useful in time-frequency-space channel modeling for future HSR mobile communication systems.

We introduce an approach of representing the dynamics of delays in railway systems by Eulerian aggregation of individual train delays onto the railway’s network segments. The approach lends itself naturally to the analysis of multi-scale spatio-temporal evolution of delays — colloquially known as the spread of “oil-stain” on the network. To illustrate the potential of this representation, we apply it to describe the dynamics on a national (also referred to as the macro-) scale of the Dutch railways: specifically we perform a principal component analysis of the Euler-aggregated global delay data, identify two key principal components to define a reduced phase-space, and discuss the delay evolution for the system as a moving point in this phase-space on a few example days.

This paper introduces a measurement campaign to investigate the characteristics of massive multiple-input multiple-output (MIMO) channels in both line-of-sight (LoS) and obstructed LoS suburban outdoor environments. The measurements are conducted at the center frequency of 3.5 GHz with a bandwidth of 160 MHz. The transmitter is equipped with a 4 × 8 planar patch antenna array and the receiver is equipped with a 4 × 8 cylindrical patch antenna array. Each patch consists of two antenna ports which have main polarizations of +45° and -45°, respectively, resulting in a total of 64 × 64 sub-channels. We examined the influence of different polarization combinations on channel characteristics such as power delay profile (PDP), K-factor and delay spread for both LoS and obstructed LoS scenarios. Results show that the characteristics are more dependent on the Rx antennas rather than the Tx antennas. When the Rx antennas are facing the Tx antenna array, the received power is higher, the K-factor is lower and the delay spread is lower in cross-polarization cases with respect to the co-polarization ones in general. When the Rx antennas are back-facing the Tx antenna array, lower powers, higher K-factors and higher delay spreads can be observed in cross-polarization cases with respect to the co-polarization cases. In conclusion, the polarimetric influence on the channel characteristics depends on the antenna structure as well as the environment.

This paper presents a measurement-based analysis of wideband 39 GHz millimeter wave (mm-wave) dual-polarized propagation channel under the impact of foliage presence between a transmitter (Tx) and a receiver (Rx). The measurements were conducted in a rich-vegetation area, and the so-called direction-scan-sounding (DSS) method which rotates a horn antenna in angular domains was applied, aiming at investigating the direction-of-arrival (DoA)-dependent characteristics of polarimetric channels. Four Tx-to-Rx polarization configurations were considered, including co-polarization scenarios with vertical Tx-polarization to vertical Rx-polarization (VV) and horizontal to horizontal (HH), as well as cross-polarization with vertical to horizontal (VH) and horizontal to vertical (HV), which allow scrutinizing the differences in delay-direction dispersion for usually-encountered scenarios. A foliage loss model for various vegetation depths in VV polarization configuration, was also presented in this paper. The results show that the foliage-loss DoA spectra for VH and HV are similar, while the spectra exhibit less penetration loss in most directions for VV than for the HH. Furthermore, the presence of vegetation between the Tx and the Rx leads to larger dispersion in delay compared to the clear line-of-sight (LoS) scenario, particularly for vertical polarization in the Tx side, and additionally, the foliage presence also results in evident DoA dispersion, specially in the HV scenario. Selectivity in directions caused by foliage is more significant in vertically-polarized Tx scenarios than in the horizontally-polarized Tx scenarios. A statistical model is established summarizing these comparison details.

This paper presents the radio propagation measurement, simulation, and analytical results in the 41-GHz millimeter-wave band for non-line-of-sight scenarios in a confined in-building corridor environment. High gain directional antennas are used at both the transmitter and receiver to realize long-distance transmission. Directional channel characteristics of the 41-GHz band, including the path loss model, the root-mean-square delay spread, the multipath statistics, the small scale fading characteristics, the power delay profile, and the power levels received from different antenna directions at different locations, are analyzed in detail. Moreover, in the investigation of the path loss models, we employ ray tracing simulations to extend the path loss model to a more general form so that the structural characteristics of the corridor, such as the length of the line-of-sight section and the corridor corner angle, are considered as modeling parameters. The effects of different transmit/receive antenna locations on the path loss model are also investigated. The combined effects of the highly directional antennas and the confined corridor environment on the wireless channel characteristics are analyzed in detail.

Due to the high-mobility of unmanned aerial vehicles (UAVs), actual UAV channel measurements show that low-altitude air-to-ground (A2G) channels in UAV communications illustrate non-stationary properties. This fact motivates us to develop a non-stationary channel model for UAV-to-ground links. In this paper, we propose a three-dimensional (3D) wideband non-stationary A2G vehicle-to-vehicle (V2V) geometry-based stochastic channel model (GBSM). In the proposed model, both UAVs and ground terminals can be moving, which makes the model more general. In order to mimic the non-stationary channel characteristics, parameters like the number of clusters, power, time delays, angels of departure (AoDs), and angles of arrival (AoAs) are all time-variant. The proposed model combining a line-of-sight (LoS) component, a ground reflection component, a cylinder model, and multiple confocal truncated ellipsoid models has the ability to investigate the impact of UAV heights and transceivers' movements on channel characteristics in diverse environments. Statistical properties like temporal autocorrelation function (ACF), spatial cross-correlation function (CCF), Doppler power spectral density (PSD), and stationary interval of A2G channels are derived and analyzed in detail. In addition, derived root mean square delay spread (RMS-DS), temporal ACF and spatial CCF are validated against channel measurement results. Furthermore, by adjusting channel parameters, the proposed GBSM is sufficiently generic and adaptable to model various UAV-to-ground communication scenarios.

TeraHertz (THz) communications are envisioned as a promising technology, owing to its unprecedented multi-GHz bandwidth. One fundamental challenge when moving to new spectrum is to understand the science of radio propagation and develop an accurate channel model. In this paper, a wideband channel measurement campaign between 130 GHz and 143 GHz is conducted in an office room. The channel measurement campaign in the office room scenario consists of three cases: 1) LoS case in the office area 2) Line-of-Sight (LoS) case in the hallway and 3) NLoS case. Directional antennas are utilized and rotated for resolving the multi-path components (MPCs) in the angular domain. With careful system calibration that eliminates system errors and antenna effects, realistic power delay angular profiles (PDAP) are developed. In light of the measurement results, channel parameters and insights in the THz indoor channel are comprehensively analyzed, including the path loss, delay spread, angular, and correlations among the channel parameters. Furthermore, the propagation of NLoS multipath components (MPCs) in the office room scenario and its impact on the THz channel are discussed and compared with the channel measurement in the meeting at 140 GHz. Our analysis shows that the MPCs that reflect from the partitions dominate in NLoS propagation in office room.

An iterative optimization method of transmit/receive frequency domain equalization (FDE) is proposed for single carrier transmission systems, where both transmit and receive FDE weights are iteratively determined with a recursive algorithm so as to minimize the error signal at a virtual receiver. The computer simulation results show that SC systems using the proposed transmit/receive equalization method achieves better BER performance than those using the conventional receive FDE. BER performance of SC systems using the proposed method was improved by about 2.7 dB at BER=10-3 compared to case of those using conventional receive FDE in attenuated 6-path quasistatic Rayleigh fading with normalized delay spread value of tau/T = 0.769. In addition, when decision feedback equalizer (DFE) with sufficient number of feedback taps is adopted in both systems, the proposed system achieves better BER performance than the conventional system in a low Eb/N0 region and BER performance of the proposed system becomes close to that of the conventional one as Et/No increases in the above channel condition.

The radio wave is the carrier that transmits information in wireless communications. It propagates across space through a complex and dynamic mechanism. However, the radio waves are invisible, which makes it hard to design wireless communication systems. This demonstration presents a system architecture and implementation via virtual reality (VR) that can make people open the "eyes" to see the radio waves, and provide a novel method of using the radio waves designing and optimizing wireless communication systems. Users can see how the radio waves propagate in a 3D view. Furthermore, channel impulse responses (CIRs) and channel fading properties, e.g., path loss and delay spread, can be derived and visualized through the virtual interface. Also, this demo enables users to investigate the performance of base station (BS) deployment and hybrid beamforming (HBF) algorithms via VR. In a nutshell, this demo is helpful to feel, understand, and use the radio waves to improve the efficiency of wireless communication technologies and systems.

The vehicle-to-vehicle (V2V) radio channel is non-stationary due to the rapid movement of vehicles. However, the stationarity of the V2V channels is an important indicator of the V2V channel characteristics. Therefore, we analyzed the non-stationarity of V2V radio channels using the local region of stationarity (LRS). We selected seven scenarios, including three directions of travel, i.e., in the same, vertical, and opposite directions, and different speeds and environments in a similar driving direction. The power delay profile (PDP) and LRS were estimated from the measured channel impulse responses. The results show that the most important influences on the stationary times are the direction and the speed of the vehicles. The average stationary times for driving in the same direction range from 0.3207 to 1.9419 s, the average stationary times for driving in the vertical direction are 0.0359–0.1348 s, and those for driving in the opposite direction are 0.0041–0.0103 s. These results are meaningful for the analysis of the statistical characteristics of the V2V channel, such as the delay spread and Doppler spread. Small-scale fading based on the stationary times affects the quality of signals transmitted in the V2V channel, including the information transmission rate and the information error code rate.

One of the main challenges in millimetre wave (mm-wave) communication for fifth-generation (5G) and beyond systems is the shadowing of received signals. A mobile device in proximity to a human body can result in shadowing of the received mm-wave signal. By investigating different antenna heights, it is possible to model shadowing based on which, the channel capacity and the resulting average attenuation in the shadow region of the human body is derived. It is proven that the attenuation in the body’s shadow for a mm-wave signal at 26GHz can be reduced when the transmit and receive antenna heights are adjusted. Forwards and backwards movement of the body causes positive and negative Doppler Shift, respectively, producing frequency dispersion, in addition to the channel delay-dispersion. It is also demonstrated that a steerable beam reduces the signal attenuation in the shadow region significantly, as the delay-dispersion of the channel reduces.

Ultraviolet (UV) wireless is very suitable for local area Non-line of sight(NLOS) dedicated covert communication. Overcoming the path loss and delay spread from NLOS channel is a critical mission for the UV NLOS transmitter. In the first part of this paper, the main recent short distance line of sight (LOS) and long distance NLOS UV communication transmitters are reviewed. Next, “UV NLOS acquisition tracking pointing (ATP)” is introduced. Then, “UV high speed modulation technology”, “high-power UV source” and etc. are analyzed and summarized. The prospect of “High repetition rate high power UV pulse modulation” and “High power sub-millisecond UV pulse OFDM modulation” is put forward in the end.

This paper investigates the prediction model based on deep learning for the wireless channel characteristics of massive MIMO systems in high-speed railway (HSR) scenarios. Based on the propagation graph theory, we simulate the massive MIMO channel in a HSR cutting scenario. The datasets of spatial-temporal channel characteristics, involving channel state information, Ricean K-factor, delay spread, and angle spread, are generated for the model training and testing, and two kinds of prediction problem formulations, such as single-step and multi-steps, are designed. By considering both the spatial and temporal correlation properties in HSR massive MIMO channels, a novel channel prediction model that combines the convolutional long short-term memory (CLSTM) and convolutional neural network (CNN) is proposed and called as Conv-CLSTM. The hyperparameters of Conv-CLSTM are determined by comparative experiments and autocorrelation and similarity analysis. According to the performance evaluation, it is showed that the proposed Conv-CLSTM outperforms the other deep learning and machine learning models.

In this paper, we propose a novel anti-Doppler spread technique in IRS-aided predictable high-mobility vehicular communication. The predictable information of vehicle (e.g., speed, trajectory, and timetable) can be used to design a suboptimal phase shift set by solving a multi-objective problem, which can make IRS capable of maximizing instantaneous signal-to-noise ratio (SNR), minimizing Doppler spread, and keeping delay spread to a relatively low range simultaneously. The technique we proposed in this paper is more cost-effective, more practical, and of higher energy efficiency since the phase shift set by every element of the IRS for one vehicle pass can be designed and stored in advance, instead of processing in real-time. Simulation results show the effectiveness of the proposed phase shift set as compared to benchmark schemes.

Environmental modeling errors have significant influence on the accuracy of ray tracing (RT) based channel modeling method. The aim of this paper is to provide reference for determining the appropriate accuracy of indoor databases to achieve prediction results within tolerance. Therefore in this paper, we discuss the influence of modeling errors on the RT-based channel modeling accuracy for millimeter-wave frequency bands. Different from the previous work, our discussion mainly foucus on the indoor environments and try to analyze the relationship between the channel parameters including received power, delay spread (DS), angle spreads (AS) and the types of scatterers. Simulation results show the RT results are more sensitive to the geometric errors caused by the room boundary in the line-of-sight (LOS) case while indoor scatterers have a greater impact in the non-LOS (NLOS) case. Additionally, AS is more sensitive to geometric errors of environment than DS. These results have guiding significance for channel modeling and simulation work using RT.

An application of drone-based wireless networking in agricultural scenarios needs to analyze the radio link budget because of the uncertain propagation from the surrounding. This paper presents a link budget analysis between drone as a drone small cell (DSC) and Internet of Things (IoT) as a ground sensor. The measurement results show the received signal strength indicator (RSSI), path loss, and delay spread at 2.4 GHz frequency when considering the numbers of ground sensors to 15 points and use a single drone enabled with WiFi portable link. It can be found that drone DSC can compensate for the limited power of ground sensors and link budget analysis can optimally evaluate the communication channel in this scenario.

Next generation wireless communication systems are considering orthogonal time frequency space (OTFS) waveform as an alternative to the existing orthogonal frequency division multiplexing (OFDM) waveform. It is more robust to high Doppler and high carrier frequencies. Multiple recent papers have evaluated OTFS at higher Doppler with long term evolution (LTE) system, to show its performance benefits over OFDM. In this paper, we provide the performance evaluation of OTFS waveform at mmWave frequencies using the third generation partnership project (3GPP) 5G new radio (NR) transmit-receive chain for different quadrature amplitude modulation (QAM) schemes. We compare the OTFS performance with that of OFDM, for 3GPP defined tapped delay line (TDL) and cluster delay line (CDL) channel models at multiple mobility conditions. We analyze the performance difference between OTFS and OFDM using both minimum mean-squared error (MMSE) and decision feedback equalizers (DFE) at the receiver, and show the need for DFE at higher modulation order, to compensate the inter symbol interference. We show that the OTFS has 10 dB gain in signal to noise ratio (SNR) over OFDM, for TDL-C channel with delay spread of 300 ns, at high speed train mobility of 500 kmph for 64-QAM, when evaluated with DFE. We further present the computational complexity of OTFS system, in comparison to OFDM.