1. Fundamental of propagation mechanism and channel modeling
1) Empirical and semi-deterministic channel modeling,
Semi-deterministic modeling with low data resolution requirement and low computation time is always of interest. By conjunctively utilizing the extended Hata model and the Deygout model, this lecture introduces a hybrid model for viaduct and cutting scenarios of high-speed railway. The introduced model achieves higher accuracy than empirical and statistical models, but uses totally free sources. It can be easily implemented for the network planning, and therefore, it meets the demand for fast development
of high-speed railway.
2) Deterministic channel model,
In a realistic high-speed railway environment, the track, terrain, vegetation, cuttings, barriers, pylons, buildings, and crossing bridges are the main sources of reflection, diffraction, and scattering. Moreover, the radiation pattern and the polarization of the transmitting and receiving antennas considerably influence the propagation. This lecture introduces a deterministic modeling approach covering all the effects in a realistic high-speed railway environment for the first time. The antenna influence and the mechanisms of transmission, scattering, and reflection are evaluated by developing a 3D ray-optical tool. The diffraction loss is obtained by the multi-edge diffraction models using raster databases. This approach compensates the limitation of the existent empirical and stochastic models used for the high-speed railway, and promotes the deterministic modeling towards to the realistic environment. Therefore, it allows a detailed and realistic evaluation and verification of the train control communications systems.
3) Complete propagation modeling in confined space,
The wave propagation in tunnels does not only depend on the carrier frequency and properties of the tunnel, but also differs along with the distance between transmitter (Tx) and receiver (Rx). This lecture introduces a complete structure and model for propagation in tunnels. Compared to existing models, the complete model unifies different academic viewpoints, reveals all possible propagation mechanisms, and localizes all the dividing points between every two adjacent propagation mechanism zones. When the user is close to the Tx, the propagation is dominated by the free-space mechanism, then the multimode mechanism is established. Afterwards, when the high-order modes have been largely attenuated, the fundamental-mode guided propagation is dominant. Finally, when the user is extremely far away, the waveguide effect vanishes because of attenuation of reflected rays. Measurements and simulations validate the model. This complete structure and model can be essential to establish a comprehensive understanding of the propagation in tunnels and can be applied for network planning and interference analysis of advanced communication systems.
2. Key technology and platform for realistic channel modeling
1) High-performance ray-tracing technique, platform and practice,
The application scenarios and requirements are more diverse in the fifth-generation (5G) era than before. In order to successfully support the system design and deployment, accurate channel modeling is important. Ray-tracing (RT) based deterministic modeling approach is accurate with detailed angular information and is a suitable candidate for predicting time-varying channel and multiple-input multiple-output (MIMO) channel for various frequency bands. However, the computational complexity and the utility of RT are the main concerns of users. Aiming at 5G and beyond wireless communications, this lecture introduces a comprehensive tutorial on the design of RT and the applications. The role of RT and the state-of-the-art RT techniques are reviewed. The features of academic and commercial RT based simulators are summarized and compared. The requirements, challenges, and developing trends of RT to enable the visions are discussed. The practices of the design of high-performance RT simulation platform for 5G and beyond communications are introduced, with the publicly available high-performance cloud-based RT simulation platform as the main reference. The hardware structure, networking, workflow, data flow and fundamental functions of a flexible high-performance RT platform are discussed. The applications of high-performance RT are presented based on two 5G scenarios, i.e., a 3.5 GHz Beijing vehicle-to-infrastructure scenario and a 28 GHz Manhattan outdoor scenario. The questions on how to calibrate and validate RT based on measurements, how to apply RT for mobile communications in moving scenarios, and how to evaluate MIMO beamforming technologies are answered. This tutorial will be especially useful for researchers who work on RT algorithms development and channel modeling to meet the evaluation requirements of 5G and beyond technologies.
2) Realistic channel modeling for crossing bridges,
Bridges that cross a railway’s right-of-way are one of the most common obstacles for wave propagation along a high-speed railway. They can lead to poor coverage or handover failure but have been rarely investigated before. To describe the influence of this non-negligible structure on propagation, measurements have been taken at 930 MHz along a real high-speed railway in China. Based on different mechanisms, the entire propagation process is introduced by four zones in the case of an independent crossing bridge (ICB) and two zones in the case of groups of crossing bridges. First, all the propagation characteristics, including extra propagation loss, shadow fading, small-scale fading, and fading depth, have been measured and extracted. The results are shown in a complete table for accurate statistical modeling. Then, two empirical models, i.e., ICB and crossing bridges group (CBG), are first established to describe the extra loss owing to the crossing bridges. The introduced models improve on the state-of-the-art models for this problem, achieving a root mean square error (RMSE) of 3.0 and 3.7 dB, respectively.
3) Realistic channel modeling for train stations,
Train stations are one of the largest and most unavoidable obstructions for electromagnetic wave propagation on a high-speed railway. They can bring about severe extra propagation loss, and therefore, lead to poor coverage or handover failure. However, their influence has been rarely investigated before. Based on rich experimental results of 930MHzmeasurements conducted on train stations of high-speed railway in China, this lecture introduces two empirical models for the extra propagation loss owing to train stations for the first time. The extra loss depends on four conditions: the distance between the transmitter (Tx) and the train station, the type of the train station, the track carrying the train, and the propagation mechanism zones. Hence, the models are established for every case of all the combinations of these four conditions. The validation shows that the introduced models accurately predict the extra propagation loss and support an effective way to involve the influence of the train station in the simulation and design of the signaling and train control communications systems.
3. Realistic channel generation for 5G/B5G communications
1) New paradigm of realistic channel modeling at high frequency with high mobility,
The upcoming fifth-generation (5G) mobile communication system is expected to support high mobility up to 500 km/h, which is envisioned in particular for high-speed trains. Millimeter wave (mmWave) spectrum is considered as a key enabler for offering the “best experience” to highly mobile users. Despite that channel characterization is necessary for the mmWave system design and validation, it is still not feasible to directly do extensive mmWave mobile Channel measurements on moving high-speed trains (HST) at a speed up to 500 km/h in the present. Thus, rather than conducting mmWave HST channel sounding directly with high mobility, this lecture introduces a viable paradigm for realizing the realistic HST channels at the 5G mmWave band. We first propose the whole paradigm. Then, we define the scenario of interest and select the main objects and materials. Afterwards, the electromagnetic and scattering parameters of the materials are measured and estimated between 26.5 GHz and 40 GHz. With this information, the most influential materials are determined through significance analysis. Correspondingly, we reconstruct the three-dimensional mmWave outdoor HST and tunnel scenario models. Through extensive ray-tracing simulations, we determine the main propagation mechanisms in these two scenarios, the channel models based on that are validated by measurements.
2) THz channel sounding, simulation and modeling,
Terahertz (THz) communications are envisioned as a key technology for the sixth-generation wireless communication system (6G). However, it is not practical to perform large-scale channel measurements with high degrees of freedom at THz frequency band. This makes empirical or stochastic modeling approaches relying on measurements no longer stand. In order to break through the bottleneck of scarce full-dimensional channel sounding measurements, this lecture introduces a novel paradigm for THz channel modeling towards 6G. With the core of high-performance ray tracing (RT), the presented paradigm requires merely quite limited channel sounding to calibrate the geometry and material electromagnetic (EM) properties of the three-dimensional (3D) environment model in the target scenarios. Then, through extensive RT simulations, the parameters extracted from RT simulations can be fed into either ray-based novel stochastic channel models or cluster-based standard channel model
families. Verified by RT simulations, these models can generate realistic channels that are valuable for the design and evaluation of THz systems. Representing two ends of 6G THz use cases from microscopy to macroscopy, case studies are made for close-proximity communications, wireless connections on a desktop, and smart rail mobility, respectively. Last but not least, new concerns on channel modeling resulting from distinguishing features of THz wave are discussed regarding propagation, antenna array, and device aspects, respectively.
4) Discussion and summary
Bio:
Ke Guan (IEEE S’10-M’13-SM’19) received B.E. degree and Ph.D. degree from Beijing Jiaotong University in 2006 and 2014, respectively. He is a Professor in State Key Laboratory of Rail Traffic Control and Safety & School of Electronic and Information Engineering, Beijing Jiaotong University. In 2015, he has been awarded a Humboldt Research Fellowship for Postdoctoral Researchers. In 2009, he was a Visiting Scholar with Universidad Politecnica de Madrid, Spain. From 2011 to 2013, he was a Research Scholar with the Institut fuer Nachrichtentechnik (IfN) at Technische Universitaet Braunschweig, Germany. From September 2013 to January 2014, he was invited to conduct joint research in Universidad Politecnica de Madrid, Spain. He has authored/coauthored two books and one book chapter, more than 200 journal and conference papers, and four patents. His current research interests include measurement and modeling of wireless propagation channels, high-speed railway communications, vehicle-to-x channel characterization, and indoor channel characterization for high-speed short-range systems including future terahertz communication systems.
