Drones are expected to be promising aerial platforms in air-to-ground (A2G) integrated communication networks, where the A2G propagation channel is fundamental for reliable communication links. This paper proposes a hybrid parameter generation framework combining the deterministic and statistical methods for a cluster-based A2G channel model. In this framework, the map-based deterministic method is used to generate delay and angle parameters, which can achieve great scenario consistency. However, it is difficult for users to provide precise material information of scatterers, which would cause deviation of power parameters. To tackle this issue, a measurement-driven power generation method is proposed. Firstly, a bandwidth-dependent clustering method is developed to group the rays into clusters. Then, the cluster power is generated by measurement-driven statistical models with a power-decomposition idea. It decomposes the power parameter into several parts that are less dependent on the scenario. Moreover, it can avoid massive measurement campaigns and is more robust when applied in unmeasured scenarios. Finally, a new channel measurement campaign in a street canyon scenario is performed for validations. The proposed method is also compared with a ray-tracing (RT) method and the 3rd Generation Partnership Project (3GPP) channel model. It is shown that the proposed framework and power generation method are great alternatives for accurate and robust modeling requirements under specific A2G communication scenarios.

Unmanned aerial vehicle (UAV) air-to-air (A2A) communications are emerging as a vital component of future low-attitude wireless networks. This paper introduces a novel A2A channel model and evaluates its statistical properties through analysis and experimental validation. The proposed model adopts a quasi-deterministic approach that incorporates rooftop specular reflection (RSR), distinguishing it from traditional ground reflection. Airframe occlusion (AO) is represented using a diffraction-based segmented function to accurately characterize its impact on the line-of-sight path. Key statistical indicators are derived based on the proposed model, and numerical results show that the presence of RSR reduces both the level crossing rate and average fade duration. Enhanced Rician factor improves spatial-temporal correlation, while higher transmission power and lower UAV mobility reduce outage probability. Compared to standardized models, the proposed model demonstrates improved capability in capturing the effects of RSR and AO while maintaining compatibility. Additionally, an A2A channel measurement platform leveraging the high autocorrelation properties of Zadoff-Chu sequences is developed to extract channel impulse response. Experimental measurements of channel capacity, outage probability, and root mean square delay spread closely align with simulated results, validating the model’s accuracy and reliability.

For autonomous aerial vehicles (AAV)-assisted integrated sensing and communication (ISAC) systems, a semantic-based channel state information (CSI) feedback scheme is proposed in this article. Unlike traditional full CSI feedback, the proposed scheme minimizes feedback burden by utilizing predefined semantic databases at both the transmitter and receiver. First, a deep-learning-based clustering method is developed to construct the semantic database from measured CSI samples. Then, an incremental clustering-based identification method is proposed, enabling dynamic updates and adjustments to semantic databases as new CSI is continuously acquired. Finally, the proposed CSI feedback scheme is validated through scenario identification, and extensive channel measurements are conducted in three typical campus scenarios: 1) playground; 2) lake; and 3) buildings. The results show that the accuracy of the semantic feedback-based scenario identification reaches 97.5%, which is 0.6% higher than the accuracy of the full-CSI feedback-based scenario identification. Specifically, the CSI is fed back through semantic database labels, requiring only a few bytes. This significantly reduces feedback burden while maintaining high accuracy of ISAC tasks. Furthermore, the proposed feedback scheme can also be extended to other AAV-assisted applications, such as the Internet of Things and emergency response.

In the increasingly critical field of aerial communication, unmanned aerial vehicles (UAVs) have gained significant attention as prominent representatives, and accurate air-to-air (A2A) channel modeling plays a pivotal role in the design and evaluation of reliable communication systems. This paper presents a A2A channel model for UAV communications. It introduces a quasi-deterministic approach to address limitations in existing modeling frameworks. The proposed model uses a truncated ellipsoid to capture the distribution of scatterers in A2A scenarios and, for the first time, incorporates rooftop specular reflection (RSR). Power correction factors, based on the UAV’s airframe structure, position, and posture, are introduced to provide a comprehensive and realistic depiction of the A2A communication channel. The performance of proposed model is assessed by simulating key statistical channel characteristics and comparing with other alternatives. The simulations illustrate how channel behavior is influenced by factors such as flight level, flight trajectory, and UAV posture. The results show that RSR leads to the channel hardening effect, while airframe occlusion causes the received signal power to vary gradually with changes in UAV’s position and posture. The validity of the model is confirmed through comparison with measurement data and ray-tracing results, proving its accuracy and practical application.

Long-range wide-area network (LoRaWAN) is a widely used technology in the Internet of Things (IoT), which provides long-range (LoRa) communication with low power consumption. In LoRaWAN, an accurate path loss (PL) model is essential to realize link budget and network coverage planning. In this article, we present an artificial intelligence (AI)-enhanced generalizable scheme for PL prediction in LoRaWAN. We propose a network that performs corrective adjustments to improve the PL estimates of empirical models. The network termed STransRadio benefits from the self-attention computation in Swin Transformer to model the LoRa correlation about propagation for enhancing the adjustment prediction accuracy. To generalize our scheme to new scenarios, an multiscenario deep transfer learning (MDTL) algorithm is proposed, which finetunes the pretrained STransRadio network with limited data. We conduct simulations and measurements in the 868-MHz bands to assess the performance of the scheme in terms of prediction accuracy and generalization ability. The effectiveness of the proposed scheme has been verified with both simulations and measurements. Moreover, the STransRadio network in the scheme outperforms the convolutional neural network (CNN) and deep vision transformer (DeepViT). With the MDTL algorithm, our scheme can achieve excellent prediction performances when it is applied in a new scenario with limited training data. Furthermore, we verify that the scheme utilized in the simulated scenario can be transferred to both the new simulated scenario and the realistic scenario. With only 100 samples, the scheme achieves root mean square error (RMSE) values of 7.27 and 5.96 dB between the predicted and actual PL, respectively.

The new generation of capsules that circulate through vacuum tubes at speeds up to 1200 km/h that is being developed, demands communication systems that can operate at these speeds with high capacity and quality of service. Currently the two technologies available are the new generation of 802.11ax networks and 5G NR. Using these technologies at such high speeds in a confined environment requires a careful study and design of the configuration of the network, and optimization of the physical interface. This paper describes the requirements for critical and business communications, proposing a WLAN and 5G network design based on the analysis of the propagation characteristics and constrains of vacuum tubes and using propagation measurements and simulations made in similar environments at frequencies of 2.5/5.7/24 GHz. These measurements and simulations show that path losses in this environment are observed with difference between 10 and 15 dB, as a consequence of the guided propagation, so that the use of bands is preferred. Finally, considering the propagation constrains and requirements of a Hyperloop system, a complete wireless communication system is proposed using two networks with 802.11 and 5G technology.

Unmanned aerial vehicles (UAVs) have aroused great attention for future wireless communications since the drone can be used as an aerial base station and also as a cellular user to perform on-demand tasks. Because the reliable and robust connection between UAV and ground station (GS) plays an essential role in the UAV-based air-to-ground (AG) communications, the accurate radio channel model is needed. In this paper, we conducted AG wideband radio channel measurements in a hilly environment at 6.5 GHz with a bandwidth of 500 MHz, which is one of the high bands for ultra-wideband (UWB) communications in IEEE standards. The large-scale characteristics, including path loss and shadow fading, are represented by the close-in (CI) model. The small-scale characteristics that illustrate the fading and the multipath effect are analyzed with three critical parameters, including the small-scale fading, Rician K-factor and root-mean-square (RMS) delay spread. Afterward, the tapped delay line (TDL) model is built according to the detected multipath in the power delay profile (PDP), and the channel impulse response (CIR) is discussed for wideband channel modeling. The results can be used for the design and analysis of the AG systems.

Powered by the Internet of Things (IoT), the cellular, vehicular, and other emerging networks, such as space-air-ground integrated networks, are expected to comprehensively evolve into the new era of the Internet of Everything (IoE) in which the propagation links between the IoT devices or sensors become diversified. Thus, a general propagation channel model is required. The line-of-sight (LOS) probability plays an essential role in the channel modeling, especially for built-up environments where the blockages from buildings are unfavorable to the signal transmission. The 4-D LOS probability model with considering the 3-D environment and frequency is comprehensively investigated in this article. Using the geometry-based stochastic method, the LOS probability is derived for arbitrary sizes, heights, and orientations of buildings in finely defined urban scenarios. The major contribution is the universality, simplicity, and good extension of the proposed model with comprehensive considerations of the influence of frequency, the type of urban, and the height of transceiver. The simulation results show that the propagation with higher frequency has a higher LOS probability. Moreover, the size, height, and density of building in urban environments are closely related to the LOS probability. The good agreements of the comparisons with the ray-tracing (RT) model and standard models show the accuracy of our proposed model. These results will be useful in the modeling of various channels and the design of IoT wireless communications systems.

Passenger trains and especially metro trains have been identified as one of the key scenarios for 5G deployments. The wireless channel inside a train car is reported in the frequency range between 26.5 GHz and 40 GHz. These bands have received a lot of interest for high‐density scenarios with a high‐traffic demand, two of the most relevant aspects of a 5G network. In this paper we provide a full description of the wideband channel estimating Power‐Delay Profiles (PDP), Saleh‐Valenzuela model parameters, time‐of‐arrival (TOA) ranging, and path‐loss results. Moreover, the performance of an automatic clustering algorithm is evaluated. The results show a remarkable degree of coherence and general conclusions are obtained.

The next generation of mobile communications, 5G, will provide a wideband network based on microwave and millimeter‐wave (mmW) communication radio links with the goal of fulfilling the strict and severe requirements of the future test cases. In particular, this paper research is focused on mmW bands in metropolitan railway tunnels. For that purpose, a propagation measurement campaign was performed at 24 GHz band in a passenger train on a realist subway environment, and these results were combined with simulations ad hoc for tunnels and a theoretical modal propagation model. A narrowband and a wideband study have been conducted with the aim of obtaining the path loss, fading, power‐delay profile, and angle of arrival, taking into consideration horizontal and vertical polarization in the receiving and transmitting antennas. This validation can be used to design and deploy wideband mobile communication networks at mmW bands in railway scenarios.

New wave of urbanization, ever more stringent emission standards, and high pressure on improving efficiency of private and public transport have made the development of more sustainable transportation systems one of the fundamental societal challenges of the next decade. Connected vehicles have been envisioned to provide enabling key technologies to enhance transportation efficiency, reduce incidents, improve safety, and mitigate the impacts of traffic congestion. The seamless integration and convergence of vehicular communication networks, information and transportation systems, and mobile devices and networks will face a number of technical, economic, and regulatory challenges. It is of paramount importance to (i) design vehicular communication systems that enable road users and other actors to exchange information in real time with high reliability, (ii) enable pervasive sensing to monitor the status of vehicles and the surroundings, (iii) develop data analytics tools for processing large amounts of data generated by the connected vehicles, and (iv) develop middleware platforms for data management and sharing.

Railway, subway, airplane, and other transportation systems have drawn an increasing interest on the use of wireless communications for critical and noncritical services to improve performance, reliability, and passengers experience. Smart transportation systems require the use of critical communications for operation and control, and wideband services can be provided using noncritical communications. High speed train (HST) is one of the best test cases for the analysis of communication links and specification of the general requirements for train control and supervision, passenger communications, and onboard and infrastructure wireless sensors. In this paper, we analyze in detail critical and noncritical networks mainly using the HST as a test case. First, the different types of links for smart rail transportation are described, specifying the main requirements of the transportation systems, communications, and their applications for different services. Then, we propose a network architecture and requirements of the communication technologies for critical and noncritical data. Finally, an analysis is made for the future technologies, including the fifth-generation (5G) communications, millimeter wave (mmWave), terahertz (THz), and satellites for critical and high-capacity communications in transportation.

Small- and medium-sized unmanned aerial vehicles (UAVs) can fly for a short distance (<2 km) from a control station in a nonsegregated air space (altitudes < 100 m). It is of great interest to model the propagation channel under such condition, where there is an important influence from the environment. This paper presents multiple measurements carried out in low altitudes with a medium-sized UAV flying over a semiurban environment. Path loss exponent is given based on the measurements done at different altitudes and a height-dependent Rician K factor model is proposed. The results clearly reveal the existence of two propagation zones with very distinct channel characteristics. The breakpoint indicates the height where the condition of the channel changes rapidly. At low altitudes, the obstacles generate a large amount of multipath and the propagation is greatly affected, while at higher altitudes the influence mitigates. Our results are useful for the modeling of low altitude air-to-ground (AG) propagation channels and the performance analysis of UAV-enabling AG communication systems, such as the channel capacity and the throughput.

Due to the extra loss resulting from tunnel curve, most of the theoretical models cannot be directly used for propagation inside curved tunnels. In this paper,extensive simulations are made for propagation at different frequencies in different types of curved tunnels, by using a ray-tracing simulator that is validated by numerous measurements. From the simulated received power, the extra losses of tunnel curve in various cases are extracted, analyzed, and stochastically modeled by normal distributions. Thus, with the parameters presented in this paper, the total propagation loss inside curved tunnels can be predicted by adding the stochastically generated extra loss to the propagation loss in the straight tunnel. This provides a fairly simple and effective way to extend the theoretical propagation models to fulfill network planning and system design for communication systems in real curved tunnels.

To ensure the stability of radio signal transmission in high-speed railway station, a detailed channel modeling for this special environment is essential and valuable. In this work, an evaluation of the performance of radio propagation in a real high speed railway station scenario is conducted. The corresponding measurements in the actual environment are carried out with the assistance of hardware testbed at 950 MHz and 2150 MHz, respectively. The propagation in both Line of Sight (LOS) and None-Line of Sight (NLOS) conditions are considered and tested. The measurement scenarios and configurations are presented in detail. Furthermore, the key parameters, such as power delay profile, reverberation time are extracted from the broadband measurement results and compared at both frequencies to achieve a rational solution for broadband communication system applying in the development of next generation high-speed railway.

Wave propagation characteristics in curved tunnels are of importance for designing reliable communications in subway systems. This paper presents the extensive propagation measurements conducted in two typical types of subway tunnels—traditional arched “Type I” tunnel and modern arched “Type II” tunnel—with 300- and 500-m radii of curvature with different configurations—horizontal and vertical polarizations at 920, 2400, and 5705 MHz, respectively. Based on the measurements, statistical metrics of propagation loss and shadow fading (path-loss exponent, shadow fading distribution, autocorrelation, and crosscorrelation) in all the measurement cases are extracted. Then, the large-scale fading characteristics in the curved subway tunnels are compared with the cases of road and railway tunnels, the other main rail traffic scenarios, and some “typical” scenarios to give a comprehensive insight into the propagation in various scenarios where the intelligent transportation systems are deployed. Moreover, for each of the large-scale fading parameters, extensive analysis and discussions are made to reflect the physical laws behind the observations. The quantitative results and findings are useful to realize intelligent transportation systems in the subway system.

High-speed railway (HSR) brings convenience to peoples' lives and is generally considered as one of the most sustainable developments for ground transportation. One of the important parts of HSR construction is the signaling system, which is also called the “operation control system,” where wireless communications play a key role in the transmission of train control data. We discuss in detail the main differences in scientific research for wireless communications between the HSR operation scenarios and the conventional public land mobile scenarios. The latest research progress in wireless channel modeling in viaducts, cuttings, and tunnels scenarios are discussed. The characteristics of nonstationary channel and the line-of-sight (LOS) sparse and LOS multiple-input-multiple-output channels, which are the typical channels in HSR scenarios, are analyzed. Some novel concepts such as composite transportation and key challenging techniques such as train-to-train communication, vacuum maglev train techniques, the security for HSR, and the fifth-generation wireless communications related techniques for future HSR development for safer, more comfortable, and more secure HSR operation are also discussed.

The propagation situations inside tunnels vary with the distance between transmitter and receiver, as well as the relative size of the users. This paper presents a novel hybrid propagation model covering three propagation mechanisms with two types of users. In the first case, the users are relatively small in size compared with the tunnel. The entire process is interpreted by combining the free space propagation model and the limited multi-mode propagation model. In the second case, for relatively large users , the free space propagation can be replaced by the near shadowing effect to some extent or totally. Hence, the whole procedure is described by coupling the statistical model in the near shadowing zone, the free space model, and the limited multi-mode model. Two groups of measurements are employed to validate the model. The results show good agreement and, therefore, the model supports an effective way to predict the propagation situation inside tunnels for different types of users.

Communications Based Train Control Systems require high quality radio data communications for train signaling and control. Actually most of these systems use 2.4GHz band with proprietary radio transceivers and leaky feeder as distribution system. All them demand a high QoS radio network to improve the efficiency of railway networks. We present narrow band, broad band and data correlated measurements taken in Madrid underground with a transmission system at 2.4 GHz in a test network of 2 km length in subway tunnels. The architecture proposed has a strong overlap in between cells to improve reliability and QoS. The radio planning of the network is carefully described and modeled with narrow band and broadband measurements and statistics. The result is a network with 99.7% of packets transmitted correctly and average propagation delay of 20ms. These results fulfill the specifications QoS of CBTC systems.

There is no unanimous consensus yet on the propagation mechanism before the break point inside tunnels. Some deem that the propagation mechanism follows the free space model, others argue that it should be described by the multimode waveguide model. Firstly, this paper analyzes the propagation loss in two mechanisms. Then, by conjunctively using the propagation theory and the three-dimensional solid geometry, a generic analytical model for the boundary between the free space mechanism and the multi-mode waveguide mechanism inside tunnels has been presented. Three measurement campaigns validate the model in different tunnels at different frequencies. Furthermore, the condition of the validity of the free space model used in tunnel environment has been discussed in some specific situations. Finally, through mathematical derivation, the seemingly conflicting viewpoints on the free space mechanism and the multi-mode waveguide mechanism have been unified in some specific situations by the presented generic model. The results in this paper can be helpful to gain deeper insight and better understanding of the propagation mechanism inside tunnels.

The delay caused by the reflected ray in broadband communication has a great influence on the communications in subway tunnel. This paper presents measurements taken in subway tunnels at 2.4 GHz, with 5 MHz bandwidth. According to propagation characteristics of tunnel, the measurements were carried out with a frequency domain channel sounding technique, in three typical scenarios: line of sight (LOS), Non-line-of-sight (NLOS) and far line of sight (FLOS), which lead to different delay distributions. Firstly IFFT was chosen to get channel impulse response (CIR) h(t) from measured three-dimensional transfer functions. Power delay profile (PDP) was investigated to give an overview of broadband channel model. Thereafter, a long delay caused by the obturation of tunnel is observed and investigated in all the scenarios. The measurements show that the reflection can be greatly remained by the tunnel, which leads to long delay cluster where the reflection, but direct ray, makes the main contribution for radio wave propagation. Four important parameters: distribution of whole PDP power, first peak arriving time, reflection cluster duration and PDP power distribution of reflection cluster were studied to give a detailed description of long delay characteristic in tunnel. This can be used to ensure high capacity communication in tunnels.

A high performance wireless network is essential for for the railway communication and control systems. Research on the fading characteristics in railway environment is of great importance for the design of the railway wireless network. In this paper, measurements are taken in railway terrain cuttings area using track side base stations of the GSM-R network. The fitted path loss model, shadow fading, and dynamaic range of the small scale fading are obtained and compared to the results of viaduct scenario. The propagation environment of the terrain cuttings turns out to be worse than the viaduct area. The path loss exponent is found to be 4.3. The shadow loss can be reasonably described by a log-normal distribution. It is also found that the bridges over the cuttings can cause extra loss of about 5 dB. The dynamaic range of the small scale fading is from 27 dB to 40 dB with a mean value of about 33 dB.

This paper focuses on three vital aspects in propagation characteristics inside tunnels: break point; near zone and far zone in a practical environment. Firstly, all the distinguishing features of radio channel in the areas before and after break point at different angles such as modal theory, ray-tracing theory, statistics and propagation model have been summarized. By collectively analyzing the research perspectives above, a panoramic view on the propagation characteristics of break point; far zone and near zone has been proposed. Based on a measurement carried out in Madrid operational subway environment at 2.4 GHz, some new radio channel characters of the break point; the regions before and after break point at higher frequency have been discussed and analyzed.

Transmission interference is a key performance indicator of railway digital mobile communication system(RDMCS), whose performance will become worse in high-speed railway. The factors that influence the transmission interference of RDMCS are analyzed. Based on the relationship between speed and transmission interference, the scheme based on distributed antennas system (DAS) is proposed to improve transmission interference of RDMCS. The proposed scheme can reduce the collision probability between data transmission and handover with the improved performance of transmission interference. Effectiveness of the proposed scheme is verified by simulations and theoretical analysis.

In this paper, outage probability caused by multiple interferers in Nakagami-m fading channels is studied. A novel method is proposed to derive the exact and closed form expressions of outage probability in the presence of multiple Nakagami independent cochannel interference. Unlike some previous conclusions, the method proposed in this paper is not only usable with various integer Nakagami fading parameters or average powers but also presents exact derivation of outage probability. To circumvent the difficulties, proper iteration functions are adopted by studying and integrating the definitions without numerical integration and residue calculation. The method generally deals with the cases that with or without minimum level constraint at the receiver for satisfactory reception. Finally, the exact expressions are compared with previous proposed approximated expressions and provide the understanding of the nature of interference.

The paper analyzes the special characteristics of GSM mobile communications in systems where the mobile stations move at speeds up to 500 km/h. One of the principal propagation problems of the train track's special environment is the speed of the MS. Another problem is the short loss of communications during the handover process. The propagation environment of the train is very special. Typically, a high speed track is full of cuttings, tunnels, bridges, etc; for this reason, the planning of the coverage must be done very carefully. The facilities and capabilities of GSM technology would be very interesting for railway applications, but GSM-R technology has not been commercially proved for high speed applications, so that a great effort is required to adapt GSM technology to this new applications. Although the changes in BTS and terminal are very small, the planning and network design is completely different from that of commercial networks.