What is the definition of channel estimation? What's the role? thank you

The performance of wireless communication system is greatly influenced by wireless channel, such as shadow fading and frequency selective fading, which makes the propagation path between transmitter and receiver very complicated. Wireless channel is not fixed and predictable like wired channel, but has great randomness, which poses great challenges to the design of receiver. Coherent detection of OFDM system requires channel estimation, and the accuracy of channel estimation will directly affect the performance of the whole system. In order to accurately recover the transmitted signal at the receiving end, people adopt various measures to resist the influence of multipath effect on the transmitted signal. The realization of channel estimation technology needs to know the information of wireless channel, such as channel order, Doppler frequency shift and multipath delay or channel impulse response. Therefore, channel parameter estimation is a key technology to realize wireless communication system. Whether the detailed channel information can be obtained at the receiving end and the transmitted signal can be demodulated correctly is an important index to measure the performance of the wireless communication system. Therefore, the research on channel parameter estimation algorithm is of great significance.