Spatial Multiplexing Aided OTFS with Index Modulation

In this paper, spatial multiplexing aided orthogonal time-frequency space with index modulation (OTFS-IM) is conceived, which combines the benefits of both OTFS-IM and vertical-bell laboratories layered space-time (VBLAST) techniques. Specifically, in the proposed VBLAST-OTFS-IM, information bits are divided into groups, each of which is modulated by OTFS-IM before being transmitted via different antennas. This allows the proposed system to fully exploit the spatial multiplexing gain. To further analyze its theoretical performance gain over the original OTFS-IM, we summarize the mathematical input-output relations of OTFS-IM and VBLAST-OTFS-IM in the form of equivalent channel matrices. Based on that, the ABEP upper bounds are derived for both original OTFS-IM and VBLAST-OTFS-IM systems by exploiting the moment-generating function (MGF) to bypass the dismantling of the sparse structure of OTFS-IM. Finally, the superiority of VBLAST-OTFS-IM and the validity of the derived ABEP upper bounds are verified by our simulation results.