# O2O: An Underwater VLC Approach in Baltic and North Sea
## 1 Introduction  
The massive water reservoir oceans are the source of life on Earth and contribute the most significant role in the environment and ecological systems. Numerous categories of water sources are classified according to water quality and geographical availability. Most of the oceans are highly turbid and harsh in comparison with rivers, lakes, dams, and other small-scale shallow water mediums. The deep seas and oceans are very far-fetched to envisage and critically challenging to explore due to low temperature, low pressure, and limited light extents transparency. Nevertheless, in nature, the two different water medium meet, but do not mix due to distinctive physio-chemical properties of waters such as salinity, kinematic viscosity, density, and thermal conductivity, etc. [1]. The water physio-chemical properties are considered the main cause of underwater optical turbulence as signal fading in [2, 3]. The strong turbulence zone underwater could be found in the form of layers where the two different water mediums mishmash. The Baltic and North oceans mingle in the Jutland Peninsula near Skagen, which is the northernmost town in Denmark, as depicted in Figure 1. Nevertheless, the sea clash separates both of the oceans. These water reservoirs are widely used for shipping, and transportation of goods delivered within the European Union (EU) [4]. From a scientific point of view, the North ocean was found to be boisterous, stormy, and darkish-colored. On the contrary, the Baltic sea has clear water and seems calm compared to the North ocean. The clash of the Baltic and North oceans is a mysterious puzzle and attracts attention among scientists. Due to the highmobility of transportation, the necessity is to develop the most reliable communication system for continuous real-time monitoring of military and underwater human activities. In recent years, the demand for underwater wireless communication (UWC) is raised for oceanographic data collection and further analysis to support naval tactical operations, natural disaster prevention, coastal securities, and long range communication. Therefore, the UWC system in heterogeneous water types is challenging to deploy. Traditional acoustic communication has been widely used for long-range signaling in underwater environments. However, acoustic communication extensively suffers due to low bandwidth, low propagation speed, and high latency. Additionally, the electromagnetic (EM) waves propagate over limited ranges in underwater mediums. Consequently, the visible light communication (VLC) has recognized a potential wireless carrier technology for improving quality of services (QoS), and connectivity amongst network nodes. In order to increase the communication capacity and transmission data rate, an integration of spatial modulation (SM) with space-time block codes (STBCs) scheme is proposed as in [5]. In [5], the authors have proposed the Alamouti STBC-SM approach to an arbitrary number of active light-emitting diodes (LEDs) under the M-ary pulse amplitude modulation (PAM) scheme. Another study is also summarized and used Alamouti-type space-time coding (STC) to improve the demodulation performance of an image sensor communication (ISC) using a rotary LED transmitter [6]. Most recently, plenty of applications are enabled and connected with VLC networks. For instance, an impressive approach is proposed as a wheeled mobile robot using visible light with a camera and an array of LEDs as in [7]. Alternatively, VLC has become a potential wireless carrier candidate rather than the acoustic and EM (in terms of RF) waves [8]. Underwater visible light communications (UVLC) fulfilled the requirement of desired data rate and bandwidth over the moderated ranges in existing research. However, plenty of VLC systems are investigated for underwater signaling [9; 10]. Apart from that, most of the existing works are proposed over long ranges to fulfill the desired communication in terms of high data rate with low latency.