## 2 Literature Review
### Receivers  
However, research works in addressing those obstacles from the receiver's perspective appear to be comparably lacking. This is particularly critical for mobile receivers, where adjustments at the receiver could theoretically reduce misalignments, improve gain, and allow the device a greater degree of autonomy. At the material level, halide perovskite (HP) polymer-based photodetectors have garnered increasing attention in recent years due to their cost-effective manufacturing and favorable semiconductor properties [11]. These detectors offer expansive FOV and are readily scalable, overcoming the trade-offs between modulation bandwidths, detection regions, and viewing angles [12]. [PERSON] et al. demonstrated the broadband photo-response of a custom-designed HP, indicating enhanced stability of the photodetector in water, along with superior optical performance, achieving a peak external quantum efficiency of 630% at 644 nm and a remarkably high responsivity of 3.27 A/W [11]. Similar performance results are exhibited by [PERSON] et al., where a photo-response of 3.93 A/W for 520 nm, and an extremely low noise level of \(1.26\times 10^{-12}\) A/\(\surd\)Hz, was observed, although at a lower baud rate of 128 kbps [13]. Moreover, [PERSON] et al. demonstrated that HP-based polymer fibers could serve as omni-directional and flexible \"photon-collectors\", with an Avalanche Photodiode (APD) serving as the receiver. Employing a non-return-to-zero on-off keying (NRZ-OOK) and quadrature amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM) techniques, data rates of 23 Mbps and 152.5 Mbps were achieved across a 1 m long tank, respectively [12]. [PERSON] et al. have further demonstrated that HPs could be enhanced with self-healing properties underwater, showing that they may be very beneficial for long-term underwater deployments [14]. Performance results are provided for UOWCs while in damaged conditions and post-self-healing [14]. Nonetheless, the use of HPs for photodetection is a novel research field and it has yet to mature for integration into prototypes for field tests in mobile UOWCs. As alternative high-gain devices with large FOVs and reduced alignment requirements, photomultiplier tubes (PMTs) offer promise. However, their susceptibility to ambient light and vibrations renders them impractical for mobile underwater solutions without additional interventions [15]. Alternatively, semiconductor-based photodetectors such as PIN photodiodes (PIN-PDs) or APDs are preferred. They feature planar detection regions sensitive to the angle of incidence (AOI) of photons. Therefore, in dynamic mobile communications scenarios where transmitter-receiver configurations are non-coaxial, design decisions are crucial to maintain an appropriate AOI within the detector's FOV. Many existing studies have explored the use of such photodetector arrays, typically in fixed geometric orientations that are either flat or curved/spherical. [PERSON] et al. investigated the feasibility of employing a green-centered InGaN micro-LED array as both transmitter and receiver, achieving a modulation bandwidth of 251.3 MHz and a data rate of 660 Mbps in a full-duplex link at 450 nm spanning 2.3 m [16]. They also demonstrated increased photodetector sensitivities with negative biasing and proposed utilizing this array as a photovoltaic cell in a charging system which may help prolong underwater deployments. However, despite the many advantages, they demonstrate a quantum efficiency of 8.20% (\(-5\) V bias and responsivity of 0.0259 A/W) for a light power density of 56.8 W/m\({}^{2}\), which seems very inefficient if immediately adopted for mobile UOWCs. In another study, [PERSON] et al. designed a receiver featuring a Fresnel lens array to enhance coverage, achieving a maximum modulation bandwidth of 50 MHz and a data rate of 100 Mbps with a pre-equalization circuit specifically tailored for mobile UOWCs [17]. Despite this, detailed performance metrics regarding receiver orientation have not been provided.