Narrowband Channel Measurements and Statistical Characterization in Subway Tunnels at 1.8 and 5.8 GHz

Railway wireless communications play an increasingly significant role in train control and signal transmission. Communication-based train control system (CBTC) can improve the safety, efficiency and reliability of train operations, and is widely used in subway wireless communication system. Therefore, radio propagation characterization and channel modeling in subway tunnels for CBTC are significant for designing reliable wireless communication system. This paper presents extensive narrowband propagation measurements conducted in a subway tunnel at 1.8 GHz and 5.8 GHz with different scenarios, which are straight tunnel scenario, station scenario and curved tunnel scenarios. Based on the measurements, some typical large and small-scale channel parameters are extracted and modeled, such as path loss, shadow fading and channel correlation, fading depth, level crossing rate, average fade duration and envelope distribution of fading. Effects of frequencies, polarizations, tunnel scenarios on wireless propagation are discussed in detail. Generally, higher frequency, vertical polarization, station and tunnel curvature will bring additional propagation losses and larger fluctuations of fading. Moreover, it is found that envelope distribution of small-scale fading is fitted well by Nakagami distribution, and parameter- $m$ is proportional to distance. Results can support realization of subway intelligent transportation system.