  With the blooming of Internet-of-Things (IoT), we are witnessing an explosion
in the number of IoT terminals, triggering an unprecedented demand for
ubiquitous wireless access globally. In this context, the emerging
low-Earth-orbit satellites (LEO-SATs) have been regarded as a promising enabler
to complement terrestrial wireless networks in providing ubiquitous
connectivity and bridging the ever-growing digital divide in the expected
next-generation wireless communications. Nevertheless, the harsh conditions
posed by LEO-SATs have imposed significant challenges to the current multiple
access (MA) schemes and led to an emerging paradigm shift in system design. In
this article, we first provide a comprehensive overview of the state-of-the-art
MA schemes and investigate their limitations in the context of LEO-SATs. To
this end, we propose a novel next generation MA (NGMA), which amalgamates the
grant-free non-orthogonal multiple access (GF-NOMA) mechanism and the
orthogonal time frequency space (OTFS) waveform, for simplifying the connection
procedure with reduced access latency and enhanced Doppler-robustness. Critical
open challenging issues and future directions are finally presented for further
technical development.
