## 2 Related Works
In 2022 [28], authors established a high-capacity wireless millimeter-wave (mm-wave) signal delivery system at the W-band using photonics-aided mm-wave generation and polarization division multiplexing (PDM). The authors successfully transmitted PDM-QPSK, PDM-16 QAM, and PDM-PS-64 QAM mm-wave signals at 10 Gbaud over distances up to 4.6 km. The proposed system achieved a record-breaking data rate of 91.6 Gb/s for single-carrier PDM-PS-64 QAM over the wireless distance, surpassing the soft-decision forward error correction (SD-FEC) threshold. This is a significant advancement in wireless mm-wave signal delivery at the W-band. A study in 2023 [29] presented free-space optical communication systems and their challenges. A hybridized model called SS-WDM-PDM is proposed to maximize link capacity and spectral efficiency. The effects of atmospheric attenuations, such as rain and fog, are evaluated. The performance of the SS-WDM-PDM-FSO system is analyzed using parameters like OSNR, BER, and Q factor. Although PDM has been widely used in optical wireless communication, it is still unexploited in UWOC systems. In this work, we propose a PDM-based UWOC system in the turbulent underwater environment. Two channels, each carrying 1 Gbps of NRZ/RZ encoded data, were transmitted over a distance of 10.5 m in the fresh river water environment where salinity is 0.2 ppt. Further, the system was tested in the ocean environment with salinity up to 35 ppt. The numerical simulation results verify that the scheme can effectively suppress the high attenuation caused by the salinity of the oceans. The main contributions of our work lie in the utilization of two polarization states for un derwater optical wireless communication (UWOC) systems. By employing two orthogonal polarization states, our proposed PDM-UWOC system offers three significant advantages:  
1. Increased Transmission Capacity: The use of PDM allows for the simultaneous transmission of multiple data streams over the same optical channel. As a result, the data rate of our proposed PDM-UWOC system is significantly higher than existing UWOC systems that rely on single-polarization transmission. This enhancement in transmission capacity opens up new possibilities for high-data-rate underwater communication applications. 2. Improved Spectral Efficiency: With the simultaneous transmission of data in two polarization states, our PDM-UWOC system achieves improved spectral efficiency. By efficiently utilizing the available optical spectrum, our system can transmit more data within the same bandwidth, making it a promising solution for increasing the efficiency of underwater communication links. 3. Mitigation of Polarization-Induced Fading: The implementation of PDM helps mitigate the adverse effects of polarization-induced fading, which is a common issue in underwater communication. By transmitting data in two orthogonal polarization states, we enhance the robustness and reliability of the communication link, leading to improved overall system performance. The rest of the paper is divides as follows: Section 3 defines the UWOC Channel modeling, and Section 4 defines the system architecture of the proposed PDM-based UWOC link, followed by results and discussion in Section 5. Section 6 describes the contribution of our work to sensor and actuator networks. Finally, the conclusion is presented in Section 7.