Spectrally Efficient Time-Frequency Space Modulation: A Delay-Doppler Domain FTN Waveform for ISAC Systems

This paper introduces the spectrally efficient time-frequency space (SETFS) modulation, a two-dimensional (2D) modulation extending the concepts of spectrally efficient frequency division multiplexing (SEFDM) and orthogonal time–frequency space (OTFS) modulation by increasing the spectral overlap at two levels, i.e., between subcarriers and sub-bands. This double compression improves the use of the available spectrum. Moreover, in SETFS, the periodicity in both time and frequency is maintained through the use of a double cyclic prefix (DCP) in order to simplify channel equalization and avoid 2D inter-symbol interference (ISI). However, the presence of a spectral overlap induces 2D inter-carrier interference (ICI), that makes symbol detection more complicated. To mitigate channel estimation errors and also support high-resolution sensing, a novel channel estimator, dubbed Newton-based estimation and spectral cancellation algorithm (NESCA) is proposed. Numerical results show that SETFS achieves up to a 19% gain in the achievable communication rate with respect to OTFS at the cost of higher receiver complexity. Moreover, SETFS combined with NESCA provides near optimum performance in delay and Doppler estimation compared to other two techniques, thus offering a favorable trade-off between spectral efficiency, computationalcomplexity, and sensing accuracy.