  This paper investigates the massive connectivity of low Earth orbit (LEO)
satellite-based Internet-of-Things (IoT) for seamless global coverage. We
propose to integrate the grant-free non-orthogonal multiple access (GF-NOMA)
paradigm with the emerging orthogonal time frequency space (OTFS) modulation to
accommodate the massive IoT access, and mitigate the long round-trip latency
and severe Doppler effect of terrestrial-satellite links (TSLs). On this basis,
we put forward a two-stage successive active terminal identification (ATI) and
channel estimation (CE) scheme as well as a low-complexity multi-user signal
detection (SD) method. Specifically, at the first stage, the proposed training
sequence aided OTFS (TS-OTFS) data frame structure facilitates the joint ATI
and coarse CE, whereby both the traffic sparsity of terrestrial IoT terminals
and the sparse channel impulse response are leveraged for enhanced performance.
Moreover, based on the single Doppler shift property for each TSL and sparsity
of delay-Doppler domain channel, we develop a parametric approach to further
refine the CE performance. Finally, a least square based parallel time domain
SD method is developed to detect the OTFS signals with relatively low
complexity. Simulation results demonstrate the superiority of the proposed
methods over the state-of-the-art solutions in terms of ATI, CE, and SD
performance confronted with the long round-trip latency and severe Doppler
effect.
