In next-generation radio communication systems, the use of higher frequency bands and the massive multiple-input-multiple-output (MIMO) systems has turned into hot research topics because they have the potential to increase network capacity significantly by exploiting the available narrowband and broadband spectrums. Therefore, the narrowband channel measurements are executed at the following five potential frequency bands, including 2.6 GHz, 3.5 GHz, 5.6 GHz, 10 GHz, and 28 GHz in the Shanghai subway tunnel environment in order to fulfill the latest standards of fifth generation (5G). Moreover, in the broadband channel measurements, the center frequency is 3.5 GHz and 5.6 GHz and the bandwidth is considered as 160 MHz, respectively. At the transmitter (Tx) side, a uniform rectangular antenna array composed of 32 elements is fixed on the platform near the tunnel walls. The receiver (Rx) is equipped with a uniform cylindrical antenna array consisting of 64 elements, which is set on a trolley along the track. Based on the acquired massive MIMO channel impulse responses, delay spread, angle spread, eigenvalue and channel capacity are analyzed. The results reveal that the multipath delay in the tunnel scenario is quite short, the delay spread and angle spread drop rapidly as the distance between Tx and Rx increases and the channel matrix gradually becomes serious. This research provides a reference for the deployment of future 5G systems in the subway tunnel.

Aiming to address the problem of limited transmission distance in applying reconfigurable intelligent surface (RIS) technology, this study leverages a tunnel simulation platform to investigate RIS-assisted wireless communication systems. Through theoretical derivation, we propose a path loss model formula specifically applicable to tunnel scenarios. Simulation results demonstrate that the proposed model accurately reflects the communication performance characteristics of RIS in tunnel scenarios, verifying the capability of RIS technology to enhance signal transmission distance within tunnels in rail transit engineering applications. This finding highlights the significant engineering potential and value of RIS technology.

This letter studies the effect of ultra-high mobility on the performance of multiple-input multiple-output (MIMO) wireless communication system in tunnel scenarios. It is found that the Doppler frequency shift generated by the ultra-high mobility reduces the correlation coefficient. Importantly, it is observed that there exists a key inflection point distance in the ultra-high speed tunnel scene. When this distance is exceeded, the channel correlation of MIMO can be significantly reduced. Moreover, it is perceived that the inflection point distance is relatively stable and is less affected by the communication frequency. On this basis, we propose a high-quality communication area with low MIMO channel correlation. For the ultra-high speed of 720km/h, the inflection distance is about 95m, and the reduction of MIMO channel correlation at 3.5GHz and 28GHz can reach up to 52.7% and 41.2%, respectively. The findings are extremely useful for the applications of the mobile MIMO communication system in tunnel and railway scenarios.

With the rapid growth in demand for high-speed wireless communication, terahertz (THz) has become one of the most promising techniques. Both atmospheric turbulence and pointing errors are important factors in degrading the performance of THz propagation. We study the performance of a multiple-input/multiple-output (MIMO) system in the THz band under the combined influences noted above. Especially, we take the impact on amplitude and phase caused by turbulence into consideration. We adopt the Padé approximation to analyze the probability density function of the channel coefficient in equal gain combining and derive the bit error rate by the Meijer-G function. The curve-fitting results of theoretical analysis are in good agreement with the actual measurements in the THz band. Therefore, it can be deduced that the exponentiated Weibull model can also be applied in the THz band. Then, we verify the theoretical results by Monte Carlo simulation. We find that turbulence is a more significant cause, which deteriorates communication performance in a larger scale of the MIMO system.

In fifth-generation (5G) wireless communications, large-scale arrays pose a challenge to the accuracy of signal models based on the plane wavefront. In this paper, a novel method for 3D near-field direction of arrival (DOA) estimation is proposed based on large-scale uniform rectangular array (URA). First, the near-field signal model based on the vertical rectangular array and the delay phase shift of the received array is presented. Afterwards, the proposed method divides the complete parameters set into multiple-parameters subsets, and only estimates one of them in each iteration, leaving the others in the fixed subset. As a result, we can obtain the maximum convergence rate of the deterministic maximum likelihood (DML) algorithm. Finally, the simulation results demonstrate that the root mean square errors (RMSEs) of the proposed algorithm are closer to the Cramer-Rao lower bounds and converge faster than those of the DML algorithm, confirming its effectiveness and superiority.

The use of fifth-generation (5G) communication technology in high-speed railway (HSR) holds significant potential in meeting the existing need for high data speeds. Therefore, an accurate modeling of the HSR wireless channel is vital to effectively design and evaluate the performance of the 5G system. In this letter, an abrupt increase in path loss is observed in the simulation using the proposed channel model for the 2.6 GHz HSR scenario at a particular base station height for both direct and reflected signals. In order to explain this phenomenon, a theoretical model of field strength for the two-path scenario is derived, and simulation results validate the theoretical model’s accuracy. Finally, the simulation results using the theoretical model demonstrate that an abrupt increase in path loss is attributed to total refraction in the ground reflection, specifically where the angle of incidence is also known as Brewster’s angle.