Research On Propagation Characteristics Of Broadband Wireless Channel In Underground Tunnels

With the rapid development of urbanization and the gradual expansion of urban scale, underground plays an increasingly significant role in public transportations in China. China’s underground railway transit is during a fast developing period compared with that in other developed countries, which leads to the larger security and communication requirements. Thus, it is necessary to design an novel underground communication system, aiming at ensuring train control security and providing high-quality communication services. Accurate knowledge of radio propagation channel characteristics is the basis of design and optimization of communication systems. It is therefore of importance to investigate the channel characteristics in underground tunnel scenarios. In this paper, radio wave propagation characteristics and MIMO channel characteristics in underground tunnels are studied. In order to explore impacts on channel characteristics as well as and feasibility and related influential factors of using MIMO technology in the underground tunnel, we establish analyze the channel model for subway tunnel scenarios using ray-tracing method. In this paper, its main work and innovations are presented as:1. Based on the ray tracing modeling method, the large-scale and small-scale fading characteristics in underground tunnels are deeply analyzed, and initial references for the design and evaluation of communication system are given by these analyzes.In the simulation and analysis of large-scale fading, conclusions are obtained as follows:path loss experiences a upward trend with the increase of frequency; when transceivers are close to the horizontal center position, path loss has the low level; the area of cross section is proportional to the slope of path loss, which can make it possible to choose the spacing of access point according to the area of tunnel cross section; If the cross section of the tunnel is more close to a circular, the path loss will be lower, and the polarization cross rate will be smaller, which could be beneficial for polarization diversity. In the simulation and analysis of small-scale fading, RMS delay spread of the rectangular tunnel environment for reference is given.2. MIMO channel simulation model in underground tunnels is established. Based on the proposed model, antenna correlation and capacity performance are analyzed from various aspects, providing research foundation of future application of MIMO technology in underground tunnels.In the study of MIMO antenna correlation, it can be concluded that:among three kinds of transmitting antenna array placement (the four-corner, horizontal and vertical setting), horizontal setting is the most useful one for optimization of the system performance; the larger antenna spacing leads to the lower correlation. Furthermore, correlation has the impact on the channel capacity. It shows that higher correlation causes the lower capacity, and vice versa.In the analysis of different influential factors of MIMO channel capacity in underground tunnels, it comes to conclusions as follows:when antenna array is placed vertically, the antenna correlation is high, and the channel capacity is small; while antenna array is placed horizontally, the antenna correlation is low, and the channel capacity is large; with the increasing large of antenna array unit’s interval, antenna correlation is reduced, and it brings to the increasing of channel capacity; with the multiplied growth of transceiver antenna array’s element number, the channel capacity is almost growing exponentially; in underground tunnels, applying MIMO technology’s channel capacity is far less than Rayleigh channel’s, it still brings considerable increase comparing with single antenna; the closer to circular the cross section of the tunnel is, the larger the channel capacity is.