# Optimized Deployment of Generalized OCDM in Deep-Sea Shadow-Zone Underwater Acoustic Channels
## 1 Introduction
Low-density parity-check codes, known for their efficiency and flexibility, have also been integrated into OFDM-based underwater acoustic communication, facilitating iterative carrier frequency offset estimation, channel estimation, detection, and decoding [14]. To achieve high-data-rate transmission, MIMO technology has been incorporated into underwater acoustic communication, significantly improving both quality and reliability [15]. Subsequently, methods leveraging the sparsity of underwater acoustic channels for channel estimation have been developed [16]. When combined with equalization techniques to reduce inter-carrier interference (ICI), compressed sensing algorithms can effectively mitigate Doppler-induced channel distortions. Additionally, an ICI self-cancellation method based on index modulation has been proposed in order to further enhance underwater acoustic communication [17]. With growing interest in underwater wireless sensor networks, recent studies have explored trade-offs between underwater channel characteristics, wireless communication techniques, and overall communication quality [18]. Recently, a novel OCDM technique based on the Fresnel transform has emerged [19]. As a multicarrier modulation technique, OCDM uses chirp signals as subcarriers, achieving the same bandwidth efficiency as OFDM while maintaining compatibility with existing OFDM systems, with only a slight increase in system complexity. OCDM has been widely applied across various domains. For example, in [20], OCDM was first introduced in coherent optical communication, demonstrating superior resistance to dispersion, fading, and noise compared to OFDM. OCDM has also shown significant promise in underwater acoustic communication. In [21], an OCDM-based system was evaluated and compared with traditional OFDM systems in underwater acoustic channels, and the experimental results show that OCDM offers superior robustness. To further optimize its performance for underwater applications, various techniques have been integrated with OCDM. For instance, a data-pick-based RAKE receiver has been designed to maintain reliable communication even with short guard intervals [22], while the incorporation of index modulation techniques has improved the system's interference resistance [23]. Furthermore, combining OCDM with MIMO technology has been shown to enhance diversity, effectively mitigating channel fading [24]. Given its advantages, OCDM has been successfully applied to various underwater acoustic communication applications, including the underwater Internet of Things (IoT) [25] and sonar communication integration [26]. However, similar to OFDM, OCDM also suffers from a high PAPR. This issue arises because the superposition of multiple subcarriers in multicarrier modulation schemes can create peaks at certain moments, placing significant demands on the power amplifier. When the signal peaks enter the nonlinear region of the power amplifier, distortion occurs, leading to performance degradation. Additionally, the noise power in oceanic environments is generally higher than in wireless communications, and the received signal power in shadow-zone communication is particularly unstable and significantly lower than that in sound channel axis communication. This necessitates a higher transmission power, further emphasizing the importance of PAPR reduction to ensuring an efficient power amplifier operation and improving the overall system performance. Several methods have been proposed to mitigate PAPR, including amplitude modulation, partial transmit sequences, virtual carriers, PAPR constraints, and precoding techniques [27; 28; 29]. A modulation scheme known as OSDM, which divides the transmission frame into multiple vectors, has been shown to effectively reduce PAPR [30; 31; 32; 33; 34]. OSDM achieves partial frequency diversity on each subcarrier, whereas OCDM provides frequency diversity across the entire bandwidth for each subcarrier. Recently, a new modulation scheme, GOCDM, has emerged [35].