# Optimized Deployment of Generalized OCDM in Deep-Sea Shadow-Zone Underwater Acoustic Channels
## 1 Introduction  
The ocean covers more than 70% of the Earth's surface, with deep-water regions exceeding depths of 3000 m accounting for over 90% of the total ocean area. The deep-sea environment hosts unique ecosystems, vast mineral resources, and numerous opportunities for scientific exploration. However, the extreme conditions of the deep sea--including high pressure, low temperatures, and complete darkness--pose significant challenges to human access and technological operations. One of the most critical challenges in deep-sea exploration is establishing reliable communication. The underwater acoustic channel is widely recognized as one of the most challenging communication media due to its inherentcharacteristics, including multipath propagation, scattering, high ambient noise, narrow channel bandwidth, and significant Doppler shifts [1]. Communication in the deep-sea shadow zone, compared to the deep-sea sound channel axis, is particularly difficult due to the higher propagation losses and longer multipath delays, making it a formidable technological challenge. Given the importance of deep-sea communication, research in this field has expanded in recent years. For ocean-monitoring applications such as submarine earthquake detection, temperature and salinity measurements, and biodiversity studies, multiple sensors and devices are deployed in the deep-sea environment. Autonomous underwater vehicles (AUVs) also rely heavily on multi-user communication systems for operations such as multi-point sampling, synchronized mapping, and complex underwater tasks. A multi-user receiver designed for joint channel parameter estimation has been developed and tested in deep-sea environments [2]. To improve connectivity in deep-sea remote communication, a study in [3] demonstrated long-range acoustic transmission using large grazing angle acoustic waves, achieving error-free data transmission at rates of 15 and 7.5 bits per second over distances of 550 and 700 km, respectively, using OFDM modulation. To optimize communication performances under varying environmental conditions, an adaptive long-range underwater acoustic communication modulation method has been proposed [4]. Additionally, techniques such as space-time clustering [5] and time-reversal mirrors [6] have been explored for enhancing deep-sea acoustic communication. In [7], pseudo-random codes and wideband multiple access techniques were applied in a self-developed 6000 m acoustic release system, while [8] demonstrated that wide-beam transducers improve the SNR, thereby enhancing the performance of long-range deep-sea communication. Based on deep-sea communication characteristics, reference [9] models the deep-sea channel as two virtual users to perform channel estimation and data detection, effectively mitigating the inter-symbol interference. However, the existing deep-sea communication research has exclusively focused on sound channel axis communication and convergence zone communication, with virtually no studies addressing shadow-zone communication. Currently, the key technologies in underwater acoustic communication include frequency hopping, direct sequence spread spectrum, single-carrier transmission, linear frequency modulation, and multicarrier modulation. Among these, OFDM has played a crucial role in terrestrial wireless communication due to its ability to counteract frequency-selective fading [10; 11; 12] and has also found extensive applications in underwater acoustic communication. Considering the broadband nature of underwater acoustic communication compared to radio communication, a two-step channel compensation method--comprising resampling and residual Doppler compensation--has been proposed under the assumption that the underwater acoustic channel experiences a common Doppler scaling factor across all propagation paths [13].