Effective Diversity of OTFS Modulation

Orthogonal time frequency space (OTFS) modulation, which encodes information symbols in the delay-Doppler domain, offers a promising solution to the problem of high Doppler sensitivity of orthogonal frequency division multiplexing (OFDM) transmission. In this letter we study the diversity of OTFS assuming rectangular waveforms and a delay-Doppler channel with two paths. After introducing the concept of effective diversity (ED), which we argue to be more significant than "standard" diversity in the case of a large number of transmitted symbols, we examine the conditions under which OTFS achieves full ED for QAM symbols. We validate our analytical results through numerical simulations, which show that OTFS practically achieves full ED with sufficiently large signal constellations.