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Next-generation wireless systems will offer integrated sensing andcommunications (ISAC) functionalities not only in order to enable newapplications, but also as a means to mitigate challenges such asdoubly-dispersive channels, which arise in high mobility scenarios and/or atmillimeter-wave (mmWave) and Terahertz (THz) bands. An emerging approach toaccomplish these goals is the design of new waveforms, which draw from theinherent relationship between the doubly-dispersive nature of time-variant (TV)channels and the environmental features of scatterers manifested in the form ofmulti-path delays and Doppler shifts. Examples of such waveforms are thedelay-Doppler domain orthogonal time frequency space (OTFS) and the recentlyproposed chirp domain affine frequency division multiplexing (AFDM), both ofwhich seek to simultaneously combat the detrimental effects of doubleselectivity and exploit them for the estimation (or sensing) of environmentalinformation. This article aims to provide a consolidated and comprehensiveoverview of the signal processing techniques required to support reliable ISACover doubly-dispersive channels in beyond fifth generation (B5G)/sixthgeneration (6G) systems, with an emphasis on OTFS and AFDM waveforms, as those,together with the traditional orthogonal frequency division multiplexing (OFDM)waveform, suffice to elaborate on the most relevant properties of the trend.The analysis shows that OTFS and AFDM indeed enable significantly improvedrobustness against inter-carrier interference (ICI) arising from Doppler shiftscompared to OFDM. In addition, the inherent delay-Doppler domain orthogonalityof the OTFS and AFDM effective channels is found to provide significantadvantages for the design and the performance of integrated sensingfunctionalities.

From OTFS to AFDM: A Comparative Study of Next-Generation Waveforms for ISAC in Doubly-Dispersive Channels