OTFS-TSMA for Massive Internet of Things in High-Speed Railway

Massive internet of things (mIoT) could play an important role in the future smart high-speed railway (HSR), where grant-free multiple access technologies are required. Recently, tandem spreading multiple access (TSMA) has been raised for mIoT without mobility which achieves high connectivity and reliability. Meanwhile, orthogonal time frequency space (OTFS) modulation shows its potential to combat high mobility in point-to-point communication systems. To this end, in this article, we jointly design OTFS and TSMA, and propose OTFS-TSMA for HSR mIoT. The principle of OTFS-TSMA transceiver is described, where OTFS and TSMA are improved respectively. Especially, two-dimension cyclic shift of DD domain elements in OTFS is transformed into cyclic shift of Doppler elements, segments, symbols and chips by the proposed novel resource allocation and interleaving schemes. Data recovery approaches of the four categories of cyclic shift are given, thus massive user interference is mitigated. Simulation results illustrate that both high user connectivity and transmission reliability in HSR massive IoT can be achieved by OTFS-TSMA.