Preface Wireless communications has become a field of enormous scientific and economic interest. Recent success stories include 2G and 3G cellular voice and data services (e.g., GSM and UMTS), wireless local area networks (WiFi/IEEE 802.11x), wireless broadband access (WiMAX/IEEE 802.16x), and digital broadcast systems (DVB, DAB, DRM). On the physical layer side, traditional designs typically assume that the radio channel remains constant for the duration of a data block. However, researchers and system designers are increasingly shifting their attention to channels that may vary within a block. In addition to time dispersion caused by multipath propagation, these rapidly time-varying channels feature frequency dispersion resulting from the Doppler effect. They are, thus, often referred to as being "doubly dispersive." Historically, channels with time variation and frequency dispersion were first considered mostly in the context of ionospheric and tropospheric communications and in radio astronomy. The theoretical foundations of rapidly time-varying channels were established by Bello, Gallager, Kailath, Kennedy, and others in the sixties of the twentieth century. More recently, rapidly time-varying channels have become important in novel application scenarios with potentially high economic relevance and societal impact. User mobility, a source of significant Doppler frequency shifts, is an essential factor in today's cellular and broadband access systems. An extreme example is given by radio access links for high-speed trains. Channels with rapid time variation are also encountered in car-to-car and car-to- infrastructure communications, which are becoming increasingly important. .