## 2 Literature Review
### Transmitters  
Light-emitting diodes (LEDs) and laser diodes are the main candidates for transmitters considering their ability for light modulation. However, a mobile UOWC requires a significant region of coverage, where LED-based emitters with large beam-divergence angles are preferred. Therefore, unfortunately, these links have a much lower modulation bandwidth of up to several tens of MHz, limited by the cut-off ceiling of the LEDs and driver circuitry used, despite the fact that the visible light spectrum spans several THz in frequencies [4]. Such connections are called diffused line-of-sight (DLOS) links. The resultant light field's intensity distribution is often non-uniform but may take on a Gaussian, Lambertian, or even an irregular (e.g., batwing) distribution around the emitter's center axis (axis of orientation). In such cases, the angle between the receiver and the emitter's axis becomes an additional variable (Figure 1). Considering a realistic DLOS link, the beamwidth in Figure 1 at the distance of the receiver will be much larger than the receiver diameter, and the variation of optical power received along the plane of the photodetector aperture more even. To mitigate this non-uniformity, [PERSON] et al. devised a transmitter employing a blue LED array with a free-form lens to achieve a 150\({}^{\circ}\) angle of divergence with 90% uniform intensity, demonstrating a data rate of 19 Mbps over an 8 m distance with an attenuation coefficient of 0.4 m\({}^{-1}\)[5]. Similarly, [PERSON] et al. developed a quasi-omnidirectional triangular prismatic array of three LED modules, each with nine LEDs, achieving a full 360\({}^{\circ}\) intensity distribution with a peak-value illuminance of only 1 dB higher than the trough value. They achieved a data rate of 22 Mbps over a 10 m distance, suggesting applicability for swarm robotics or an underwater optical cellular network [6]. [PERSON] et al. investigated the placement of 16 LEDs, comparing a rectangular layout, a particle swarm optimized (PSO) layout, and a hybrid layout for achieving uniform power distribution across the depths of clear and turbid waters. Their results demonstrated a significant reduction in power distribution variance with the PSO layout, indicating consistent emission uniformity across the water classifications [7]. In addition to this, to maintain consistent alignment, [PERSON] and [PERSON] proposed a novel beam adaptation algorithm for UOWC receivers based on artificial intelligence reinforcement learning to optimize beamwidth and orientation in different water types for point-to-point line-of-sight (P2P-LOS) communication [8]. They compared the performance of Q-learning and State-Action-Reward-State-Action (SARSA) algorithms, with SARSA converging more quickly. Additionally, [PERSON] et al. presented work on pointing adjustable beam arrays using reflectors and lasers to enhance performance under varied water conditions, resulting in reduced path loss and improved adaptability to attenuation and turbulence [9]. As an alternative approach, [PERSON] et al. have explored the deployment of underwater reflectors to improve the gain by the inclusion of non-line-of-sight (NLOS) paths, yielding promising outcomes [10]. They indicate that using mirrors between the transmitter and receiver could potentially yield a maximum gain of approximately 3 dB in clear water, and further propose that utilizing the air/water boundary as a reflector might offer greater efficiency compared to traditional mirrors due to higher reflectance. However, the overall benefit of deploying such reflectors in mobile scenarios may be low, as they could contribute to environmental clutter and unwanted reflections. In such cases, using a tether may be more practical. Nevertheless, integrating NLOS paths could prove advantageous for UOWC applications, particularly in scenarios  
Figure 1: (**a**) The polar distribution of the emitter’s optical intensity is non-uniform, especially for diffused line-of-sight optical links; the received power is contingent on multiple factors.