Orthogonal Time Frequency Code Space Modulation Enabled Multiple Access Under Compactness-Reduced Channel Spreading Function

Orthogonal time frequency space (OTFS) modulation is a promising technology for communications under high mobility in the sixth-generation (6G) communications system. To enable machine-type communications (MTC) with high mobility in 6G, researchers have considered designing multiple access (MA) technologies based on OTFS modulation. The reliability and connectivity of OTFS-MA are highly correlated with the characteristics of the channel spreading function. In the spectrum-limited MA system where the channel spreading function is not sparse and compact enough, the system device capacity of OTFS-MA schemes is limited. To this end, a grant-free MA scheme, named orthogonal time frequency code space modulation enabled multiple access (OTFCSMA) is proposed in this article. In general, orthogonal code domain resources are introduced into OTFCSMA to enhance device connectivity and transmission reliability of MA systems based on OTFS modulation. In detail, firstly, the characteristic of the realistic channel spreading function is described, especially the reduced sparsity and compactness of the channel in the spectrum-limited system. Secondly, the principles for designing OTFCSMA are described, including orthogonal spreading/despreading, data interleaving/deinterleaving, device identification, two-stage channel estimation, and data recovery strategies. Thirdly, the system device capacity and the system complexity of OTFCSMA are analyzed. Fourthly, a date-block-wise device connectivity scaling-up scheme for OTFCSMA is proposed, based on which the exponential system user capacity growth is realized. Finally, the performances of OTFCSMA on transmission reliability and device connectivity are demonstrated, and the gain brought by orthogonal spreading is analyzed.