Introduction
Optical Wireless Communication (OWC) has become increasingly popular in recent years. There is a unique appeal to sending data at high speeds without a mess of wires. OWC provides an opportunity for users to do just that. It also makes it possible for underwater communication to happen. This can be applied to many things, such as environmental monitoring or underwater robotics. However, despite everything it has going for it, underwater OWC takes work. Optical turbulence is one of the biggest challenges it faces. This essay examines Optical Turbulence when used in MIMO NOMA VLCS underwater and the complexities surrounding it. By doing so, we will be able to investigate what implications optical turbulence poses and how these obstacles impede underwater communication systems that could otherwise work together.
Optical Turbulence in Underwater Communication
Optical turbulence sounds like a big word. What it means is the light that moves through water. Have you ever looked into water and seen how things may appear contorted? Optical turbulence is responsible for that. Why does this happen? Baykal et al. (2022) note that it occurs when the medium of light’s refraction changes uncontrollably. In underwater communications, it causes blur because of water column inconsistencies. For instance, think about what happens when there are changes in temperature gradients, salinity, or suspended particles. There are more problems with inconsistent refractive indices, making it even harder for light signals to be transmitted effectively- resulting in scattering, absorption, and refraction during transmission. Because of this, for Optical Wireless Communication (OWC) based underwater communications systems to be reliable and efficient, they have to tackle these challenges that come with optical turbulences. Improving on them will also contribute significantly towards the technological development of OWC in areas like exploration, monitoring, and robotics.
There is no doubt that optical turbulence affects underwater optical wireless communication (OWC) systems quite often. Still, we can summarize their effects and how they impact the whole system into two points. The first one is obvious- rapid light fluctuations mess up data being sent and lower the quality of the communication overall by causing interference or dropouts (Baykal et al., 2022). The second one, however, might be more complicated to understand but still an important point to make- beam wandering affects efficiency across the entire link. When interfaced with random changes made to water’s refractive index, light beams stray off their intended path, causing transmitters and receivers to not properly align, therefore making any data that needs transmitting impossible and very hard to send since if you do not know where you are sending it too… how are you going to do so?
MIMO NOMA VLC System
A combination of Multiple Input, Multiple Output – Non-organizational multiple Access (MIMO NOMA), and a Visible Light Communication (VLC) system is employed to beat the optical turbulence that plagues underwater communication. The MIMO technology uses spatial diversity with multiple antennas on both the transmitting and receiving sides (Dixit & Kumar, 2021). What did this mean? It implies that the system has more capacity to handle connections, and its reliability grows from decreasing the impacts of atmospheric turbulence caused by scintillation and beam wander. With many moving parts in a machine like this, you can receive signals more easily when sending data underwater. Not only does it make the process more efficient, but it drastically reduces errors as well. In addition to MIMO, this is where we introduce users to sharing time-frequency resources effectively via power domain multiplexing. Enabling users to use these resources concurrently enhances underwater communication systems’ bandwidth availability and overall spectral efficiency. This way, it does not seem so bad after all – we are merely solving problems associated with optical turbulence.
MIMO and NOMA combine their powers regarding the VLC system for underwater communication. These two features work together and create something akin to a superpower – eliminating any concerns one might have about optical turbulence (Wang et al., 2020). This type of partnership is required to adjust resource allocation dynamically, ultimately leading to an all-around efficient channel that can withstand changes in refractive. While also striving to optimize data transmissions and offer a smooth environment where many users share resources, this all-in-one approach makes it so misalignment and intensity fluctuations do not even matter anymore. All in all, this is a great feature when it comes to underwater communications. It shows us that with creativity, there will come an era where optical turbulence becomes just another factor rather than an obstacle that must be overcome at any cost.
It should also be noted that the MIMO NOMA VLC system has been used to combat the challenges caused by optical turbulence in underwater communication. By integrating multiple transmitters and receivers for diversity, scintillation, and beam wandering, problems can be solved. The MIMO technology is adaptable, allowing it to work with nearly anything found under the water at an optimal level (Dixit & Kumar, 2021). When you have a low error rate during data transfer, you create a system that is more reliant on communication. Because of this, not only are the effects of optical turbulence let go of, but a robust and efficient underwater communication regime is crafted.
In combination with all these things, though, come a lot of issues current systems face when dealing with the MIMO NOMA VLC system in an underwater environment. In addition to higher path loss, other problems, such as inter-symbol interference (ISI) and inter-user or inter-channel interference (IUI/ICI), make it difficult to maintain reliable communication links altogether. However, these problems worsen when they introduce errors and reduce ISI, IUI, and ICI data rates. It is thus necessary to address these issues effectively to ensure the resilience and efficacy of MIMO NOMA VLC systems in an underwater environment and develop innovative remedies and adaptive technologies that can reduce adverse effects relating to optical turbulence on communication quality and data transmission rates.
Moreover, it is also worth noting that the combined use of NOMA and MIMO techniques in a single system is powerful, but when combined, they boost each other’s performance. NOMA enhances spatial diversity by acting as a booster for MIMO, which is already really good. Spectral efficiency is achieved through power domain multiplexing, which boosts overall system performance and drives down the cost of the whole system. These methods are not just helpful to overcome challenges we have never faced. Still, it also takes underwater communication up a notch in reliability and effectiveness within the link. Hence, optimizing both of those things makes sense to make things even better. By using MIMO and NOMA together, you will create an interplay that gives you transformative strategies, pushing the limits of undersea communication capabilities—ultimately giving us systems that perform way better than we currently have.
Research and Experimental Studies
The challenge of optical turbulence is a considerable burden to the underwater Multiple-Input Multiple-Output (MIMO) Non-Orthogonal Multiple Access Visible Light Communication (VLC) systems, so there have been countless attempts to address this. However, Wang et al. (2019) did something exciting and proposed a new adaptive modulation and coding scheme that can cut down on scintillation from optical turbulence. This study has shown that by increasing the strength of the signal, you can get around the quick change in light intensity caused by turbulent fluctuations in water. There were noticeable improvements in both Bit Error Rate (BER) and throughput metrics, proving the proposed scheme was effective. Being dynamic means it can change over time depending on how much turbulence is present, which makes it forward-looking because its goal is to reduce difficulties caused by optical turbulence, hence making underwater VLC systems more reliable and robust.
Moreover, the work of Wang et al. (2019) is essential because it suggests that underwater communication systems can be improved. They found a way to recognize the problem and inhibit its effects. This method allows for the improvement and enhances the system’s reliability: MIMO NOMA VLC system. Now, more research can be done to find alternatives to adaptive techniques. These can adapt information as conditions change so that efficient communication is created. The data transmission integrity in those situations where optical turbulence significantly threatens it can seem impossible, but that might not be the case.
Similarly, Li et al. (2020) have made strides in the development of underwater Multiple-Input Multiple-Output (MIMO) Non-Orthogonal Multiple Access (NOMA) Visible Light Communication (VLC) systems in their work by focusing on beam wandering. Beam wandering makes communication a mess, and underwater fluctuations in refractive index cause it. The authors of this study have given a new approach: an adaptive beamforming technique. What it does is change the direction of the transmitted beam according to changes that are currently happening with the refractive index. This adaptive beamforming technique fixes beams that drift and affect the link quality link to respond positively to changes in a moisture-filled environment. The results from Li et al.’s experiments give us proof that their adaptive beamforming techniques work. Less misalignment means more potential for more reliable and stable MIMO NOMA VLC systems underwater. This type of finding will be the starting point for better roadmaps to overcome obstacles tied to optical communications and provide better underwater communication techniques. The future can be bright if it is both robust and efficient enough.
Conclusion
Optical turbulence is a massive problem in optical wireless communication, especially underwater. However, thanks to some genius new technology, we can solve that. We can fix all this with the help of Multiple Input Multiple Output (MIMO) and Non-Orthogonal Multiple Access (NOMA) systems; the two technologies go hand in hand and have proven highly reliable until now. They improve our beamforming methods by significantly using adaptive modulation, coding schemes, and adaptive beamforming methods. Therefore, what does this mean? Well, for starters, our underwater communication systems will get much better and more sophisticated than they are now. We will see better performance and reliability due to the dynamic adjustment methods in transmission parameters and optimizing beamforming for environmental conditions. This can change how we communicate underwater to have seamless data exchanges in harsh aquatic environments.
Bibliography
Baykal, Y., Ata, Y. and Gökçe, M.C., 2022. Underwater turbulence, its effects on optical wireless communication and imaging: A review. Optics & Laser Technology, 156, p.108624.
Dixit, V. and Kumar, A., 2021. An exact error analysis of multi-user RC/MRC based MIMO-NOMA-VLC system with imperfect SIC. IEEE Access, 9, pp.136710-136720.
Li, X., Li, W., Yang, Y., & Xu, L. 2020. Adaptive beamforming for underwater MIMO NOMA VLC system in turbulence. IEEE Access, 8, 145376-145387.
Wang, H., Wang, F. and Li, R., 2020. We are enhancing the power allocation efficiency of NOMA-aided-MIMO downlink VLC networks—Optics Communications, 454, p.124497.
Wang, T., Guo, Y., Chen, J., Xu, P., & Wu, Y. 2019. Adaptive modulation and coding scheme for underwater MIMO NOMA VLC system under turbulence. IEEE Photonics Journal, 11(3), 1-10.
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