Future wireless systems are expected to reliably communicate broadband data in high mobility environments. Such communication systems encounter fading channels whose response exhibits significant variations across both time and frequency. In this book, we study communications over such time- and frequency-selective fast-fading channels, focusing on the noncoherent scenario for which the channel's realization is a priori unavailable at both the transmitter and receiver. In particular, we analyze the fundamental limits on reliable information transfer rates and design pilot-aided transmissions...