Time-varying communication channels: Fundamentals, recent developments, and open problems

In many modern wireless communication systems, the assumption of a locally time-invariant (block-fading) channel breaks down due to increased user mobility, data rates, and carrier frequencies. Fast time-varying channels feature significant Doppler spread in addition to delay spread. In this tutorial paper, we review some characterizations and sparse models of time-varying channels. We then discuss several models and methods recently proposed for communications over time-varying wireless channels, and we point out some related open problems and potential research directions.