I. Introduction
Nowadays, a large number of public communication systems, including Global System for Mobile Communications, third-generation, Wi-Fi, World Interoperability for Microwave Access, and Long-Term Evolution (LTE), are expected to support continuous service in subway tunnels to guarantee user experience [1]. For dedicated communication systems, such as Terrestrial Trunked Radio [2], [3], Global System for Mobile Communications for Railway (GSM-R) [4], communication-based train control system [5], LTE for Railway [6], etc., high reliability, availability, maintainability, and safety [7] must to provide safety-critical data transmissions for train control in subway tunnels [8]. In the near future, various high-data-rate applications, such as real-time high-definition video surveillance, will be required to enable face recognition and emotion analysis [9]– [11] for security concerns (e.g., terrorist attacks, riots, etc). For public wireless communication systems or signaling and train control communication systems, a full understanding of the propagation is mandatory for their deployments in subway tunnels.