# SDR-Based 28 GHz mmWave Channel Modeling of Railway Marshaling Yard
### Related Works
Hotspots, such as depots and shunting yards, are also important railway scenarios. The work in [16] presented a comprehensive study on millimeter-wave-based mobile hotspot network (MHN) systems for high-speed train communications, including system design, field trial, channel modeling based on measurement campaign, simulation, and validation. In [17], mmWave channel measurements are reported for a railway depot environment using a wideband channel sounder operating at 60 GHz; path loss, RMS delay, as well as K-factors were extracted. The vacuum tube ultra-high-speed train (UHST) is a hot topic worldwide, and is a potential development direction of future transportation. In [18], a three-dimensional non-stationary mmWave geometry-based stochastic model (GBSM) is proposed to investigate the channel characteristics of UHST channels in vacuum tube scenarios. Concerning the frequency bands, 26 GHz, 28 GHz, 30 GHz, 38 GHz, 40 GHz, 60 GHz, 73 GHz, 90 GHz, 93.2 GHz, and up to 300 GHz have all been studied by scholars, among which, 28 GHz and 60 GHz seem to have a higher level of attention. In [19], the authors conducted a 28 GHz-band 5G experimental trial on the actual Shinkansen that runs at a maximum speed of 283 km/h, and the experimental trial could achieve throughput exceeding 1.0 Gbps and consecutive handovers among the three BSs. In [20], field experiments of a 28 GHz-band 5G system at indoor train station platforms were carried out, and through the actual measurements, the results confirmed that the prototype 5G system could achieve mobile broadband capacity (more than 1 Gbps), even when the UE was located anywhere at the indoor train station platform. Ref [4] focuses on channel characteristics in rural railway environments at 28 GHz, and the mmWave channel characteristics of rural railway scenarios are studied via a calibrated ray-tracing simulator. In [5], influence analysis of typical objects in rural railway environments at 28 GHz is performed. In [21], millimeter-wave (mmWave) high-speed train measurements were conducted at 28 GHz with a speed up to 170 km/h in two different HST scenarios, viaduct and tunnel, in which large- and small-scale fading characteristics were extracted. Moving on to research methodology, measure campaigns using measurement results published in the open literature, ray-tracing simulations, ultrawideband (UWB) channel sounding, deep learning, deep reinforcement learning (DRL), machine learning(ML), the space alternating generalized expectation-maximization (SAGE) algorithm, simulation platforms such as MATLAB, NYUSIM, mathematical methods such as non-convex problem with a near-optimal solution, coalition game, as well as the Bayesian approach, have all been used by scholars. Among all these methodologies, measure campaigns are basically very costly and time-intensive [22]; thus, ray-tracing simulations calibrated with measured data are commonly used, which means only a relatively small number of channel samples can be obtained, and then measurements are thus used to quantify the accuracy of a ray tracer. In [9], propagation measurements are conducted in the mmWave band for the 12 most common railway materials, the corresponding electromagnetic parameters are obtained, and a 3D ray-tracing (RT) simulator is calibrated. In [23], a series of horizontal directional scan-sounding measurements are performed inside a real high-speed train wagon at 60 GHz and 300 GHz frequency bands, and the channel characteristics are extracted, based on which, a self-developed RT simulator is validated through the reconstruction of the three-dimensional wagon model and the calibration of the electromagnetic properties of the main materials. In [5], based on the channel measurement conducted at 28 GHz with train-to-infrastructure deployment in the rural railway environment, a ray tracing (RT)simulator is calibrated in terms of environment modeling and electromagnetic calculation. In [11], ray tracer (RT) is employed to simulate the propagation in a high-speed railway viaduct scenario at a frequency of 93.2 GHz.