# Designing an Adaptive Underwater Visible Light Communication System +
## 1 Introduction  
As water covers nearly two-thirds of the Earth's surface, the Internet of Underwater Things (IoUT) has attracted much research interest recently. Research in ocean exploration systems has been growing due to climatic changes and resource depletion [1]. The underwater communication system uses media like radio, sound, and optical waves to carry out wireless communication in water. Radio waves are not a good option for such communication since water acts like a conductor. Also, the equipment used is large, expensive, and requires high power, which causes operational and financial difficulties. Acoustic communication is the most widely used underwater technology but has a limited data rate despite the long link range. Figure 1 shows the absorption coefficient under the visible spectrum (400-700 nm) underwater. The figure suggests that, considering a suitable wavelength, optical waves within that specific wavelength can be a viable alternative with a reasonable data rate and are probably safe for life underwater. The experiment shows that water becomes transparent to photons under the visible spectrum [2]. In addition, it has the potential to be used in rough conditions and harsh environments as it is easy to deploy [3]. Unlike free-space optical communication, the underwater environment is highly dynamic and influenced by factors such as turbidity, temperature gradients, salinity, absorption, and scattering. Alignment challenges limit underwater optical communication. Underwater visible light communication (UVLC) can be relay-based or software-defined radio (SDR)-based, with light-emitting diode (LED) assembly at the transmitter and photodiodes at the receiver. In this paper, we aim to extend the work presented in our previous study [4]. We have developed a prototype underwater transceiver model to address these limitations, improve alignment, and enhance system performance. The novelties of this design approach are summarized below:  
* Designing a UVLC system model by combining Li-Fi (light fidelity) technology with an SDR. Li-Fi transmits the optical signal at high speeds, whereas the SDR conditions the signal using software, thus replacing traditional hardware components such as amplifiers, mixers, and modulators. * Using pulse position modulation (PPM) instead of conventional OOK (on-off keying), amplitude shift keying, frequency shift keying, and OFDM (orthogonal frequency division multiplexing) for real-time communication. * The integration of a servo motor and an Arduino microcontroller for the adaptive adjustment of the receiver position concerning the signal direction. Underwater communication follows the same pattern as free-space optical communication by using the OOK as a primary modulation technique for real-time communication due to its simplicity and high power efficiency. However, since the water channel is complex for light propagation, the UVLC system faces challenges. Research shows [5; 6] that PPM and phase shift keying (PSK) are suitable for underwater optical links since they have reasonable data rates and high power efficiency. However, PSK requires higher power and a more complex implementation. PPM and the SDR offer complementary strengths that enhance UVLC systems. PPM ensures power efficiency, while the SDR provides the flexibility to adapt to changing conditions and modulation schemes. By combining these techniques, UVLC systems can achieve improved reliability, adaptability, and efficiency, making them well suited for diverse underwater communication scenarios. The previous paper [4] measured signal quality at various angular positions by manually adjusting the Li-Fi receiver using PPM and an SDR. In contrast, this paper investigates the effect of automatically adjusting the receiver's position based on power measurements at different angles using an external setup consisting of Li-Fi, an SDR, an Arduino microcontroller, and a positional servo motor.