OTSM -Index Modulation: A Novel Approach for Underwater Acoustic Systems

Underwater acoustic (UWA) communication at high rates presents unique attributes such as prolonged delay spread, rapid temporal changes, pronounced Doppler effects, restricted bandwidth availability. Orthogonal Time Sequency Multiplexing (OTSM) has emerged as a promising technique for reliable communication in such doubly dispersive UWA channels, offering better error performance than Orthogonal Frequency Division Multiplexing (OFDM) and comparable Bit Error Rate (BER) to Orthogonal Time Frequency Space (OTFS) with reduced complexity. In this paper, a new transmission scheme, called OTSM with index modulation (OTSM-IM), is proposed to achieve a balance between spectral and energy efficiency while delivering improved BER performance under high Doppler spread channel. In this scheme, the information is conveyed through both constellation symbols and indices in the Delay-Sequency domain, with grid activation governed by the input bit sequence. For detection, the study presents a Maximum Likelihood (ML) detector and two reduced-complexity log-likelihood ratio (LLR)-based detectors. Theoretical analysis derives an upper bound for average bit error probability (ABEP) using Pairwise Error Probability (PEP) with ML detection. Simulation results show that OTSM-IM exhibits superior BER performance and improved tolerance to high Doppler effects compared to OTSM, Pairwise Sequency Index Modulation with OTSM (PSeIM-OTSM) maintaining comparable or better spectral efficiency. Additionally, the results confirm that OTSM - IM surpasses conventional Orthogonal Frequency Division Multiplexing with Index Modulation (OFDM-IM) in underwater acoustic communication, delivering significantly improved BER performance. The results validate that the proposed LLR detectors achieve near-optimal BER performance comparable to the ML detector across high SNR (signal-to-noise Noise Ratio) regimes while significantly reducing computational complexity.