A Multi-Mode Co-Directional Orthogonal Index OTFS Modulation Algorithm

Amid the growing demand for communication in high-speed mobility and complex underwater acoustic channels, Orthogonal Time Frequency Space (OTFS) modulation has emerged as a promising modulation technique due to its robust resistance to Doppler shift and its channel diversity gain properties, making it a research hotspot in underwater communication. However, in traditional OTFS with Index Modulation (OTFS-IM) systems, only a portion of the grid is activated, leading to silent subgrids and lower spectral efficiency. To improve spectral and energy efficiency, this paper proposes an innovative multi-mode co-directional orthogonal index OTFS modulation algorithm. This algorithm introduces co-directional orthogonal index modulation in the Delay-Doppler (DD) domain subgrids to enhance both spectral and energy efficiency. Simulation results show that the multi-mode co-directional orthogonal index OTFS modulation algorithm exhibits superior bit error rate (BER) performance and spectral efficiency compared to the traditional OTFS-IM system under different modulation orders. These improvements position the proposed approach as an efficient solution for underwater communication in high-speed mobile environments, with promising potential for broader practical applications.