## 1 Introduction
Our prior research investigated PAT for laser-based UOWC using electronic beam steering using two-dimensional OPA antennas [8]. OPA is not the only technology which can steer the optical beam electronically. Liquid crystal-based reconfigurable intelligent surfaces (RISs) are a new, emerging technology with high potential and with the ability to control the incident optical beam for steering and beam divergence control. The main reason to favor OPA over RISs is the high phase shift and response time as well as the required relaxation time of RIS [9; 10; 11], which makes RISs much slower than OPA. This is a critical limitation of RISs for the UOWC system that requires fast PAT capabilities. Our previous approach enabled precise control of the laser beamwidth during both the acquisition and communication phases, resulting in faster synchronization and improved data throughput. On the receiver side, our system concept emphasized spatially selective reception, significantly reducing interference while increasing receiver gain. In this paper, we aim to investigate comprehensive channel modeling of UOWC, including the turbulence effect and compensating for this destructive effect by real-time wavefront correction of the receiving laser beam using an LC-SLM as an adaptive optics module and a lens for aperture averaging. Additionally, we aim to cascade the previously investigated PAT system with our new adaptive optics system to enhance PAT accuracy and overall communication performance. We adopt a spherical wave extended Rytov formulation because our typical link lengths and beam parameters make it suitable for evaluating moderate-to-strong turbulence, and it integrates naturally into our system concept link budget. While Gaussian beam modeling can capture certain beam-shaping effects more precisely [12], our OPA based transmitters generate well-collimated beams with controlled divergence. Notably, our (\(16\times 16\)) OPA array has a compact footprint of approximately 3.6 \(\upmu\)m \(\times\) 3.6 \(\upmu\)m, which can be considered as a point source at practical underwater link distances, which further justifies the spherical wave approximation. The main novelty of our approach is not limited to theoretical beam modeling. Rather, we propose an integrated system design that combines the following:  
* OPA antennas for electronic beam steering which accommodates PAT. * Adaptive optics based on an LC-SLM for wavefront correction, and
* Aperture averaging with a converging lens. By jointly optimizing these components, we effectively mitigate underwater turbulence and alignment issues in a coherent UOWC link. This unification extends beyond the existing literature that focuses on one aspect in isolation, e.g., beam propagation modeling or adaptive optics alone, without addressing a full end-to-end underwater communication system. As a result, our framework tackles practical challenges such as maintaining alignment (via PAT) and minimizing turbulence-induced fading via adaptive optics and aperture averaging within a single solution. The remainder of the paper is organized as follows. Section 2 discusses the light propagation in water and highlights the optical properties of water, which result in absorption, scattering, and turbulence. Section 3 introduces the proposed system concept architecture, which is divided into two sub-systems. In Section 3.1, the PAT and beam divergence control system is discussed, and in Section 3.2, the turbulence compensation system is described in detail. Two methods for turbulence compensation are proposed: adaptive optics and aperture averaging. First, the turbulence compensation method using adaptive optics is described and explains how phase distortions caused by turbulence can be described by Zernike polynomials. Additionally, aperture averaging is explained, illustrating how larger apertures can mitigate turbulence effects. Section 4 presents numerical results, the evaluation of the performance of the proposed system, and a discussion.