## 5 Applications and Challenges of VLC in Underwater Systems
### Challenges of Underwater VLC  
Various problems arise in the distribution of VLC systems underwater, largely because it is an uncommon environment with different conditions. Light absorption and scattering into water are major obstacles among all. Water molecules and particles scatter light and reduce the strength of the signal with greatly reduced distances. As discussed by [19], the environmental factors make this maintenance of high data rates across longer distances hard to implement. Similarly, ref. [54] also gives evidence that changing water clarity and particulate matter presence affect the transmission of the signal due to disturbance or signal distortion that can impede proper data communication. That is further reiterated by [50], where it was shown that the effective VLC range is grossly limited by a severe absorption and scattering of light, and hence establishing a reliable communication link over a long distance is a challenge.  
Apart from absorption and scattering, environmental variability is another challenge. In water, temperature, salinity, and turbidity are all quite different from each other, and even within one of these conditions, signal quality would change, according to [26]. This variability makes the communication link less stable and complicates the design of VLC systems that may be able to adapt to these changes. For instance, VLC systems underwater in shallow waters are prone to interference by ambient light sources like sunlight, which degrades the quality and increases the noisiness of the signal, especially in the VLC systems placed near the surface of the water. Research like [51] outlines how these ambient light sources, coupled with any turbulence in the water, contribute to a very unpredictable environment in which even the slightest chance of maintaining good communication becomes difficult.  
The applications of VLC systems are also limited by the small bandwidth, especially in cases involving high data rates. In contrast with the RF system, which has a wider bandwidth, VLC underwater faces limitations toward its capacity in simultaneous data transmission. Ref. [54] discusses how these bandwidth constraints further complicate applications such as video streaming or real-time data transfer that require a high data rate. Research such as [53] highlights the role of advanced modulation techniques in mitigating bandwidth limitations. However, since the underwater medium is inherently unstable and complex, the advanced modulation techniques that might be implemented on top must be expectedly challenging. The performance of reliable VLC underwater often requires sophisticated hardware and software solutions, which, in turn, results in an increase in system complexity and cost. This can be a limiting factor for large-scale deployment, according to [19].  
These issues are further compounded by the interference challenge resulting from dynamic underwater environments, where aspects such as turbidity, water currents, and the presence of particulate matter could result in changes to signals. This dynamic nature in underwater surroundings further puts extra demands on VLC systems for robustness and adaptability. Ref. [52], on the other hand, demonstrates such an environmental perturbation;in this case, turbulence works to decrease signal reliability, making a stable link for secure communication much more difficult to establish. That can be turbulence contributing to increasing the probability of an outage. In fact, as discussed in [51] the communication reliability is influenced directly by the distance and transmission conditions in these environments.