## 1 Introduction  
Interest in autonomous and connected mobile systems for underwater applications is growing. With the rising global human population and demand for food, energy, and other resources, the utilization of oceans for economic activities, also referred to as the blue economy, has garnered the attention of researchers and governments globally. Open-water aquaculture, offshore energy farms, deep-sea mining, and coastal tourism are some examples. The U.S. Department of Energy has forecasted growth of USD 3-8 bn from 2020 to 2030 in the local underwater vehicle docking and recharging market to allow unhindered year-round underwater observations without the need to surface for data collection or recharging [1]. It is preferred to use untethered data transmission with unmanned underwater vehicles (UUVs) to reduce costs and entanglement, and avoid otherwise possible hydrodynamic forces on cables that affect maneuverability [2]. While acoustic communication methods are mature and capable of covering vast distances, they offer low channel bandwidth and data rate. On the other hand, radio frequency signals can be severely attenuated due to the high conductivity of seawater and large antennas needed for lower-frequency and higher-power transmissions are physically cumbersome for compact UUVs with limited energy budgets. This leaves underwater optical wireless communication (UOWC) as a suitable candidate from a size-, bandwidth-, data-rate, and efficiency-based perspective [3]. However, optical links are prone to link loss due to misalignments due to line-of-sight (LOS) requirements. Further, their signal attenuation coefficient is higher than that of acoustic links and a function-of-light wavelength, implying that a robust mobile optical link may need continuous determination of channel state even for slow-moving UUVs. Although the effects of seawater on the attenuation of the optical signal cannot be easily circumvented, a large field of view (FOV) or large receiver aperture could aid in reducing geometrical losses. This could be supplemented by enhancing the gain through wireless communication diversity strategies and dynamic intelligent alignment between receiver and transmitter. However, the independent movements of both receiver and transmitter introduce another layer of complexity to the problem. Addressing these challenges requires solutions that account for the unique characteristics of transmitters and receivers and the constraints of the underwater environment. The goal of this contribution is threefold:  
* To provide a thorough review of the existing literature and commercial products of UOWC transmitter and receiver designs, with a focus on mobile UOWCs. * To devise a model for estimating the received power distribution of diffused line-of-sight mobile optical links that accurately reflects the intensity distribution profiles of LEDs with non-uniform or irregular polar-intensity distributions. * To conduct a spatial analysis using the above model to investigate the parametric influence of the placement, orientation, and angular spread of PD positioning on array-based receivers for mobile UOWC in different Jerlov seawater types. We assess this for wavelengths within the visible spectrum (400-700 nm), and we compare the results for the overall received power and bit-error rates, employing modulation schemes commonly used for diffused line-of sight optical links. The results highlight the significance of designing dynamically shape-changing receiver arrays. The rest of the paper is organized as follows. In Section 2, we review the related works to date on transmitter and receiver designs aimed at mitigating misalignments from a physical design perspective. Specifications of existing commercial devices have also been presented here. In Section 3, we introduce the preliminary concepts and channel models used for UOWCs. In Section 4, we delineate our channel model, and then, in Section 5, we use this model to outline the parameters, limitations, and design of the geometric analysis.