## 3 Proposed System Concept Architecture
### Turbulence Compensation  
Turbulence in underwater environments can cause significant fluctuations in the refractive index, leading to beam scintillation and degradation of communication link performance. To maintain error-free communication, compensating for turbulence effects is crucial. Turbulence degrades our UOWC system in two significant ways. Firstly, turbulence reduces the received signal intensity, directly affecting communication performance. Secondly, it causes errors in the angle of arrival (AoA) estimation for the PAT system. Given that the aperture size of the assumed \((16\times 16)\) OPA system on the photonics integrated circuit is only \(3.6\,\upmu\)m\({}^{2}\)[8], it can be considered as a point receiver. Even minor phase distortions in the wavefront of the incoming signal can lead to inaccurate AoA estimations. Incorrect AoA estimation results in the PAT system optimizing the antenna array gain in the wrong direction, leading to severe degradation or complete loss of the communication link. In general, there are three methods that can be used to compensate for the turbulence: spatial diversity, aperture averaging, and adaptive optics. In the spatial diversity method, multiple transceivers are employed to mitigate the fading effect of turbulence by averaging the signals received from different spatial locations [27; 28; 29]. Aperture averaging involves using a larger receiver aperture to collect light over a wider area, which helps to reduce the impact of turbulence-induced intensity fluctuations by averaging out the variations in the received signal [30; 31; 32]. Adaptive optics use real-time phase correction of the distorted wavefront, thereby improving the quality of the received signal [33; 34; 35]. In addition to the electronic beam control options provided by the OPAs, we propose integrating an LC-SLM as an adaptive optics module for real-time compensation. While it is true that conventional adaptive optics systems like deformable mirrors are designed for phase correction, LC-SLMs can be configured to modulate both phase and amplitude [36; 37]. This dual modulation capability allows the LC-SLM to correct for amplitude fluctuations, helping to reduce scintillation. In addition, we are aware of the challenges associated with implementing turbulence compensation on the receiver side. In harsh underwater environments with high absorption, scattering, and turbulence, a pre-compensation approach (on the transmitter side) is even less feasible due to the lack of a reliable feedback link for real-time correction. Therefore, our proposed system, which utilizes the LC-SLM together with aperture averaging, is well suited to these conditions. Recent advances in LC-SLM technology have significantly improved their performance, particularly concerning bandwidth. Although traditional LC-SLMs were limited to a bandwidth of approximately 40 Hz, state-of-the-art devices now achieve bandwidthsup to 120 Hz or higher [38; 39; 40]. For instance, commercially available high-speed LC-SLMs, such as those from Santec and Meadowlark Optics, have demonstrated refresh rates exceeding 1400 Hz due to advancements in liquid crystal response times [39]. Recent studies have also reported sub-millisecond response times and refresh rates higher than 2000 fps, enabling information bandwidths up to 190 Gb/s [41]. Additionally, LC-SLMs have been successfully characterized in adaptive optics testbeds for reproducing phase screens, demonstrating their utility in evaluating reconstruction and control algorithms [42]. Moreover, advanced calibration techniques now enable reference-free phase modulation calibration of LC-SLMs, which improves their performance and usability for high-precision applications [43]. These developments narrow the performance gap between LC-SLMs and deformable mirrors, making LC-SLMs suitable for real-time turbulence correction in dynamic environments [44] While it is true that our study focuses on the correction of only seven modes, the high spatial resolution of LC-SLMs remains advantageous.