Wireless channel estimation for high-speed rail communications: Challenges, solutions and future directions

With the development of High-Speed Rail (HSR), countries and individual passengers alike have enjoyed far ranging benefits as a result – economic, social, environment and in added convenience. One of the important parts of HSR construction is the signaling system, where wireless communications play a key role in the transmission of train control data. Channel estimation has a significant impact on the quality of the wireless communication, however, whose performance is degraded due to the fast mobility of HSR. This paper focuses on the channel estimation technology in HSR. We first summarize the key challenges for HSR channel estimation, especially the Inter-Carrier Interference (ICI) faced by Orthogonal Frequency Division Multiplexing (OFDM) systems. Then we provide a comprehensive review of existing pilot-aided channel estimation schemes from three points: channel model, estimation algorithm, joint channel estimation and ICI mitigation schemes. Lastly, we present the challenges of channel estimation for the Orthogonal Time Frequency Space (OTFS) system and Reconfigurable Intelligent Surface (RIS), which are promising techniques for HSR systems in the future sixth Generation (6G) wireless communication.