This letter studies the performance of a multiple-input-multiple-output (MIMO) transmission method using a single leaky coaxial cable (LCX) where two signals are fed at the dual ports of the LCX simultaneously. The performance of this scheme is evaluated in terms of channel matrix rank, channel correlation, and channel capacity. First, a keyhole effect in the slots of LCX is studied analytically, and a closed-form expression of the channel correlation is presented. Then, we concentrate on the influence of the slot period on the correlation, which provides a reference for the LCX design. Finally, both the channel correlation and the channel capacity are simulated by the Monte-Carlo method to verify the theoretical derivation. The results indicate that a 2 × 2 MIMO system using a single LCX has a good performance and also show the relationship between the slot period and the correlation.

A 28 GHz millimeter-wave (mm-wave) propagation measurement campaign was performed to predict the multiple-input multiple-output (MIMO) channel performance in a tunnel environment. Two kinds of polarization configurations were studied in this paper. In order to overcome the serious path loss (PL) of mm-wave propagation, a high gain directional horn antenna was used in the measurements of tunnel environment. The measurement results and simulation results at same specific locations are compared. The result of comparison indicates that the simulation model matches the measurement well. Then the MIMO capacity at other locations of the tunnel can be predicted by the simulation model. Further-more, it is inferred that the antenna array elements of horizontally polarized configuration have higher capacity than the vertically polarized under the constant signal-to-noise ratio (SNR).

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.

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.

This study studies the multiple‐input–multiple‐output (MIMO) performance in a tunnel scenario for the millimetre‐wave (mm‐wave) band. We simulated and measured channel impulse response at 28 GHz for grids of transmitters and receivers to study the feasibility of mm‐wave MIMO system for metro communication in tunnel environment. The ray‐tracing method was utilised to obtain the MIMO channel matrices. The performance of the channel is studied in terms of 2 × 2 and 4 × 4 MIMO, and the influences of antenna spacing on the channel capacity are investigated. The results provide a reference for the fifth‐generation wireless network in the future communication‐based train control system applications.