No abstract

To suppress the deep fading of radio waves caused by the tunnel waveguide effect, this paper presents an optimized scheme for the spatial and polarization diversities of tunnel antennas. Through the correlation coefficient analysis of path loss curves obtained by antennas placed at different positions, the two antenna positions that generate path loss curves with the lowest correlation coefficient are found, and these two antennas are defined as a diversity antenna pair. Using this scheme, the spatial diversity properties of the transmitting antenna and receiving antenna, as well as the spatial-polarization combined diversity property of the transmitting antenna, are obtained. Furthermore, the impact of antenna polarization on the spatial diversity property is investigated. The performance of the proposed scheme for spatial and polarization diversities is evaluated in terms of the intensity and uniformity of the path loss. The simulation results illustrate that the proposed diversity optimization scheme can suppress the influence of the waveguide effect and achieve more uniform and flatter radio wave coverage in a tunnel environment.

We present a study of the competition between tera-hertz (THz) generation by optical rectification in <110> ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.

Accurate radio wave prediction models are necessary for the innovation of mobile communication technology and the deployment of wireless sensor networks in railways. The parabolic wave equation (PWE) method is one commonly used method to predict the electromagnetic characteristic of long railway tunnels. However, the advent of novel antennas and technologies (e.g., beam steering, beamforming) has posed new challenges to PWE-based models such as effective means of taking tilted beam patterns into account. This study addresses such a challenge by using tilted Gaussian beams to embed antennas with oriented patterns or tilted beam directions into PWE-based models. The proposed approach does not reduce the simulation efficiency of PWE itself. The validity and practicability of the technique are demonstrated by comparing against ray-tracing as well as experimental measurements in various tunnel environments.