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With the blooming of Internet-of-Things (IoT), we are witnessing an explosionin the number of IoT terminals, triggering an unprecedented demand forubiquitous wireless access globally. In this context, the emerginglow-Earth-orbit satellites (LEO-SATs) have been regarded as a promising enablerto complement terrestrial wireless networks in providing ubiquitousconnectivity and bridging the ever-growing digital divide in the expectednext-generation wireless communications. Nevertheless, the harsh conditionsposed by LEO-SATs have imposed significant challenges to the current multipleaccess (MA) schemes and led to an emerging paradigm shift in system design. Inthis article, we first provide a comprehensive overview of the state-of-the-artMA schemes and investigate their limitations in the context of LEO-SATs. Tothis end, we propose a novel next generation MA (NGMA), which amalgamates thegrant-free non-orthogonal multiple access (GF-NOMA) mechanism and theorthogonal time frequency space (OTFS) waveform, for simplifying the connectionprocedure with reduced access latency and enhanced Doppler-robustness. Criticalopen challenging issues and future directions are finally presented for furthertechnical development.

Grant-Free NOMA-OTFS Paradigm: Enabling Efficient Ubiquitous Access for LEO Satellite Internet-of-Things