In this paper, a geometrically-based single-bounce multiple-input-multiple-output (GBSB- MIMO) channel model is proposed for the long term evolution-metro (LTE-M) system in tunnel scenarios. The periodic slots of leaky coaxial cable (LCX) are equivalent to a series of magnetic dipoles. Based on the electric field distribution of the LCX, the theoretical model for line-of-sight (LOS) and non-line-of-sight (NLOS) propagation paths in the tunnel environment are derived. The measurement and simulation results are compared in terms of condition number (CN), channel capacity, and channel correlation function (CF), which verifies the feasibility and rationality of the new proposed model. The simulation results demonstrate, under such circumstances when the Ricean K-factor is smaller, the LCX spacing has a little influence on the channel capacity and CN due to a large number of the NLOS propagation paths in the tunnel, while as Ricean K-factor increases, the LCX spacing has more influence on the channel capacity and CN. By comparison of the measurement and simulation results, it was found that the capacity and CN of the LCX-MIMO is better than the dipole MIMO with different Ricean K-factor values, and under such conditions when the spacing between LCXs and the dipole antennas is increased. Therefore, this paper delivers an optimal system design for channel capacity based on the LCX deployment in the tunnel environment.

This letter presents a 2 × 2 multiple-input-multiple-output (MIMO) measurement campaign using leaky coaxial cables (LCXs) in tunnel environment at 1.8 GHz according to the LTE-M standard. Emphasis is given to understand the effect of polarization and LCX spacing on the condition number and capacity of the radio MIMO channel in the tunnel. The analysis of the measured results reveals two important findings: the MIMO system using LCXs in the tunnel has a favorable performance, especially in the case of LCX cross polarization; the MIMO performance is not highly dependent on the LCX spacing. Therefore, it is believed that these results provide a prospective guidance for employing MIMO technology in tunnels.

The use of fifth-generation (5G) communication technology by high-speed trains (HSTs) has a lot of potential to satisfy current needs for high data rates. Therefore, accurate modeling of the HST wireless channels is crucial for the design and performance assessment of the 5G systems. This paper proposes a general three-dimensional (3D) non-stationary small-scale fading model for 5G HST millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) channels, which captures the wireless channel characteristics in different HST operating scenarios. The proposed channel model has two characteristics. Firstly, it incorporates the distribution of overhead line poles along the railway to characterize the periodic scattering components from the poles. Secondly, it represents complex HST operating scenarios as combinations of five types of scattering clusters, namely hills, trees, lakes, buildings, and concrete. Furthermore, the impact of environmental complexity (EC) on channel statistical properties in the 5G HST massive MIMO scenarios is investigated. Afterwards, based on the birth-death process of scattering clusters, the proposed channel model can characterize the channel non-stationarity in the space-time-frequency domain. Simulation results demonstrate that the proposed model effectively captures the channel non-stationarity, the scattering characteristics of overhead line poles, and the impact of different ECs on system performance. The accuracy and practicality of the proposed model are validated through a comparison with measurement results.

In recent years, the use of massive multiple-input multiple-output (MIMO) systems and higher frequency bands for next-generation urban rail transportation systems has emerged as an intriguing research topic due to its potential to significantly increase network capacity by utilizing available narrowband and broadband spectrums. In metro and mining applications, the high-reliability wireless sensor network (WSN) plays a vital role in providing personal safety, channel optimization, and improving operational performance. Through the duration of 1921–2023, this paper provides the survey on the progress of fifth-generation (5G) and beyond-fifth-generation (B5G) wireless communication systems in underground environments such as tunnels and mines, the evolution of the earliest technologies, development in channel modeling for vehicle-to-vehicle (V2V) communications, and realization of different wireless propagation channels in high-speed train (HST) environments. In addition, the most recent advanced channel modeling methods are examined, including the development of new algorithms and their use in intelligent transportation systems (ITS); mathematical, analytical, and experimental techniques for propagation design; and the significance of the radiation characteristics, antenna placing, and physical environment effect on wireless communications. Leaky coaxial cable (LCX) and distributed antenna system (DAS) designs are introduced in the demonstrated systems for improving the channel capacity of narrowband and wideband channels as well as the spatial characteristics of various MIMO systems. The review article concludes by figuring out open research directions for future technologies.

An important and typical scenario of radio propagation in a railway or subway tunnel environment is the cascaded straight and curved tunnel. In this paper, we propose a joint path loss model for cascaded tunnels at 3.5 GHz and 5.6 GHz frequency bands. By combining the waveguide mode theory and the method of shooting and bouncing ray (SBR), it is found that the curvature of tunnels introduces an extra loss in the far-field region, which can be modeled as a linear function of the propagation distance of the signal in the curved tunnel. The channel of the cascaded straight and curved tunnel is thus characterized using the extra loss coefficient (ELC). Based on the ray-tracing (RT) method, an empirical formula between ELC and the radius of the curvature is provided for 3.5 GHz and 5.6 GHz, respectively. Finally, the accuracy of the proposed model is verified by measurement and simulation results. It is shown that the proposed model can predict path loss in cascaded tunnels with desirable accuracy and low complexity.

This letter investigates the channel propagation characterization in a short tunnel through a time-domain channel measurement at 28-GHz frequency band. The statistic parameters of the channel, such as root-mean-square delay spread (RMS-DS), K-factor, and shadowing are studied, and the results reveal that the parameters align better to the normal distribution. RMS-DS is positively correlated with the shadowing but negatively correlated with the K-factor. We also deduced that the K-factor is negatively correlated with the shadowing. From the angular measurements, it is inferred that the signal propagation in the tunnel is around 0° and 180° directions, and the maximum number of reflections at 30 and 75 m are 6 and 8, respectively. Furthermore, it is found that the angular spread pursues a decreasing function of transmitter-receiver distance.