# Channel Estimation and Iterative Decoding for Underwater Acoustic OTFS Communication Systems
## 1 Introduction
However, as most existing UWA communication systems are designed for low mobility and low carrier frequency scenarios, OTFS technology introduces new challenges when designing the architectures of and algorithms for transmitters and receivers. To fully leverage the potential of OTFS technology, several critical issues need to be addressed, including waveform design, channel estimation, detection techniques, and multi-antenna and multi-user designs. Solving these challenges is imperative for realizing the benefits of OTFS technology in UWA communication systems. To date, research on OTFS modulation technology has primarily been concentrated in the domain of wireless communication, and it remains at an early stage of development. Moreover, there is a paucity of studies exploring the application of OTFS in the field of UWA communication [17; 18; 19; 20; 21; 22; 23]. Nevertheless, some scholars have begun investigating its applicability in UWA communication through leveraging insights from wireless communication research, yielding promising preliminary results. These efforts have primarily been centered on reducing receiver complexity, enhancing channel estimation, and improving signal detection; the research has largely been confined to simulation analyses and field experiments, with no significant progress towards engineering implementation or real-world application. As an emerging modulation technique, OTFS technology has demonstrated significant potential, warranting further investigation to enhance the performance of UWA communnication systems. Although some studies have explored the application of OTFS in UWA communication, the body of literature remains limited and lacks diversity in research directions. The challenge of effectively integrating the performance advantages of OTFS technology in the context of wireless communication into the UWA communication domain remains an area requiring more comprehensive research. This study aims to further combine the OTFS method with UWA communication in order to design OTFS systems based on the unique characteristics of UWA channels. The main contributions of this study are as follows:  
* For the OTFS system transmitter, frame structures tailored to the characteristics of UWA channels are designed by selecting a zero-padding (ZP) frame structure design method. Additionally, the size of the DD domain grid and the length of the guard interval are optimized to enhance the communication efficiency while maintaining performance. The proposed OTFS frame design can significantly reduce the bit error rate (BER) while maintaining the same data rate through careful frame optimization. * For the OTFS system receiver, we propose a Doppler compensation method and a dual-domain joint channel estimation method to tackle the issues caused by the strong Doppler effect in UWA communication. Additionally, we propose an OTFS system detection approach based on an iterative decoding feedback-message passing detector (IDF-MP). Based on careful consideration of the characteristics of UWA channels, the proposed OTFS system receiver is shown to provide improvements, when compared with the existing methods of OFDM, Initial Estimate Iterative Decoding Feedback (IE-IDF-MRC) and two-dimensional Passive Time Reversal Decision Feedback Equalization (2D-PTR-DFE), in UWA channels. The remainder of this paper is organized as follows: Section 2 introduces the related work. Section 3 describes the UWA OTFS system, including OTFS modulation and the UWA OTFS channel model. In Section 4, we propose a comprehensive OTFS system design scheme based on the characteristics of UWA channels. In Section 5, we analyze the communication performance of the proposed UWA OTFS system using simulated UWA channels. Finally, Section 6 presents the concluding remarks and discusses directions for further research.