Designing and Analysis of Underwater Optical Wireless communication system

Underwater wireless optical communication (UWOC) has emerged as a key technology for next-generation underwater wireless data transmission. Semantic communication is regarded as a promising communication paradigm to improve the efficiency and robustness of UWOC systems. However, existing underwater wireless optical semantic communication (UWOSC) system lacks the capability to adapt the code rate to different channel states, leading to intelligence inadequacy and coding inefficiency. In this paper, we introduce environment semantics into the UWOSC system for the first time, and propose an environment semantics aided underwater wireless optical semantic communication (ESA-UWOSC) framework. Based on this framework, we develop a system leveraging computer vision and deep learning for adaptive rate transmission. The ESA-UWOSC system is realized by a channel state perception mechanism (CSPM) for extracting environment semantics from the channel environment image to obtain the channel gain, a link network (LinkNet) for enabling variable code rate transmission of source semantic information, and a prediction network (PreNet) for predicting the optimal code length that satisfies the transmission task requirement. The combined operation of these three modules enables the ESA-UWOSC system to actively perceive channel state and perform adaptive rate transmission accordingly. In a series of experiments conducted on an emulated UWOC experimental platform, the results show that the ESA-UWOSC system outperforms the baseline scheme, achieving superior performance with lower bandwidth consumption, especially in harsh channel environments. Moreover, the results also demonstrate that the ESA-UWOSC system can achieve adaptive rate transmission based on the perceived channel state to meet transmission task requirement.

In scientific, military, and industrial sectors, the development of robust and efficient submarine wireless communication links is of enormous interest. Underwater wireless communications can be carried out through acoustic, radio frequency (RF), and optical waves. Underwater optical communication is not a new idea, but it has recently been considered because seawater exhibits a window of reduced absorption both in the visible spectrum and long-wavelength UV light (UV-A). Compared to its bandwidth limited acoustic counterpart, underwater optical wireless communications (UOWCs) can support higher data rates at low latency levels. Underwater wireless communication networks are important in ocean exploration, military tactical operations, environmental and water pollution monitoring. Anyway, given the rapid development of UOWC technology, documents are still needed showing the state of the art and the progress made by the most current research. This paper aims to examine current technologies, and those potentially available soon, for Underwater Optical Wireless Communication and to propose a new perspective using UV-A radiation.

Underwater wireless optical communication (UWOC) has been regarded as one of the promising solutions to underwater wireless communication systems due to its advantages of high bandwidth, fast transmission and good confidentiality. In UWOC systems, after emitting from light source, photons will be scattered with random deviated angles before propagating to the receiver plane. Hence, the light beam suffers a spatial angle spread at the receiver side. In order to improve the system performance with an appropriate receiver design, it is of significance to figure out the angle of arrival (AOA) distribution of the received light beam. In addition, the UWOC channel is susceptible to other effects such as absorption, turbulence and bubbles, especially when it is exposed to the complex ocean environment. Therefore, analyzing the impacts of these factors on AOA distribution is also essential to evaluate UWOC system performance. The existing studies of AOA distribution only considered single scattering component, which is not practical for turbid water. Furthermore, there are few studies focusing on multi-source scenarios. In this paper, we first present a simple expression for AOA distribution with single light source, taking both single and multiple scattering components into account. Then we extend the work to multi-source scenarios and derive the corresponding closed-form expression of AOA distribution. Compared to the traditional single scattering case, numerical results show that the proposed AOA distribution can fit well with Monte Carlo simulation results with various water types, link distances, and the characteristics of actual light sources.

Single LED-based 46-m underwater wireless optical communication enabled by a multi-pixel photon counter with digital output

We provide an experimental study of the channel characteristics in an underwater wireless optical communication (UWOC) system given by a 35-m transmission distance in various water types. A UWOC system using a low-power 488-nm laser diode is established to comprehensively evaluate the influence of the link distance, water turbidity, receiver parameters, and link misalignment on the communication link power. The results show that the link misalignment-induced power loss is significant and the effect of receiver parameters on power is limited. However, a higher turbidity or longer signal transmission distance can help to reduce the link misalignment effect and manifest the receiver parameters effect on the received power. In particular, in turbid waters with an attenuation coefficient of 0.471 m − 1, the change of the signal reception power curve is small over a certain degree of the link misalignment in the 35-m physical distance, and the reasonable configuration of receiver parameters effectively improves the signal reception quality in the UWOC system. These results can provide theoretical guidance for optimizing the UWOC system.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is shown to be an efficient modulation scheme for underwater optical wireless communication (UOWC) systems due to its high power efficiency and excellent flexibility. Differing from conventional OFDM, the modulation parameters of OFDM-IM consist of not only M-ary constellations but also subcarrier indices. In this Letter, we propose and demonstrate an intelligent automatic modulation recognition (AMR) scheme for OFDM-IM in UOWC systems by using Swin Transformer (Swin-T). Moreover, transfer learning with a hybrid training approach is further proposed to substantially enhance the AMR performance. Experimental results successfully demonstrate the feasibility and superiority of the proposed intelligent AMR scheme based on Swin-T with transfer learning and hybrid training in UOWC systems. More specifically, a recognition accuracy of 96% can be obtained by using Swin-T with transfer learning and hybrid training for a relatively small received signal-to-noise ratio (SNR) of 3.6 dB.

Bit error rate performance of underwater optical wireless communication test bed simulating the seawater conditions

A $\mathcal{T} \times \mathcal{R}$ multiple-input multiple-output (MIMO) based underwater wireless optical communication (UWOC) system employing Intensity Modulation (IM) / Direct Detection (DD) with On-Off keying (OOK) is considered. The UWOC channel is affected by underwater optical turbulence (UOT), which is modeled by Exponential Generalized Gamma (EGG) distribution. The performance of UWOC system is highly dependent on the effect of absorption, scattering and UOT. The EGG underwater optical channel model efficiently describes the optical irradiance induced due to air bubbles, temperature gradient and salinity, thus works well for the entire weak to strong turbulence regimes. The information is sent by $\mathcal{T}$ transmit apertures and received by $\mathcal{R}$ receiver apertures. The bit error rate (BER) analysis is performed for single-input multiple-output (SIMO), multiple-input single-output (MISO) and MIMO configurations for OOK modulation scheme. The closed form BER expression is derived for the SIMO case. For different channel parameters and configurations, the results for the BER performance of SIMO-UWOC and single-input single-output (SISO)-UWOC systems are obtained. For bubble level (BL) 2.4 litres/min and fresh water scenario, 15 dB performance improvement is seen at 10−2 BER for 1 × 2 SIMO with respect to SISO configuration.

There is a speedy development withinside the place of broadband wi-fi communications as the global call for in elevating for large bandwidth and extra mobility. The speedy increase withinside the previous few a long time withinside the WAN's and LAN's is particularly because of growing improvement in excessive potential and occasional loss optical fiber. But, the main downside in optical fiber cable communique is that optical fiber receives broken in a long-haul installation. Thus, underwater communication has paved the way for overcoming this disadvantagefor programs of underwater statement and sea monitoring, the importance of underwater wi-fi optical communication has expanded. Attenuation and fading of light propagation, as well as impairment in the performance of underwater wireless optical communications, are caused by elements such as underwater absorption, scattering, and turbulence (UWOC). UWOC is a short-range wireless communication system that allows for greater data throughput than classic acoustic communication systems while using less power and posing fewer computing problems. The main challenge for underwater wireless communication, on the other hand, is from the fundamental properties of ocean or sea water. Understanding the feasibility and minimising the BER, as well as boosting the reliability of high data rate underwater communication lines, is the main topic of this paper. This study's purpose is to conduct underwater optical communication research while also producing new ideas to aid in the development of future underwater communication systems.

In this paper, a low-complexity two-level chaotic encryption scheme is introduced and experimentally demonstrated to improve the physical layer security of a 450-nm laser underwater optical wireless communication (UOWC) system using discrete Fourier transform spread discrete multi-tone (DFT-S DMT) modulation. In the first encryption stage, the original bit stream is encrypted with a chaotic sequence based on a one-dimensional Logistic map. In the second encryption stage, the real and imaginary components of the DFT-S symbols are further encrypted with a pair of separate chaotic sequences, which are generated from a two-dimensional Logistic iterative chaotic map with infinite collapse (2D-LICM). The experimental results indicate that the encryption operation has no negative effect on the performance of the proposed UOWC system. For chaotic encryption, the DFT-S DMT gives a better performance than the DMT scheme under different water turbidities. 55-m/4.5-Gbps and 50-m/5-Gbps underwater transmissions are successfully demonstrated by the chaotic encrypted DFT-S DMT scheme. To the best of our knowledge, this is the first time to verify the feasibility of chaotic encryption in a high-speed UOWC system.

In underwater wireless optical communication (UWOC) systems, using single-photon avalanche diodes (SPAD) as a detector can improve the detection sensitivity and thus improve the transmission distance. However, the signal detection for SPAD-based systems is greatly challenged by the complex optical channel characteristics and SPAD nonlinear distortion. In order to mitigate the nonlinear distortion caused by SPAD detectors, signal-dependent noise (SDN), and the effect of underwater channel attenuation on the transmitted signal in UWOC systems, this paper proposes a signal detection scheme using deep neural networks with embedded SDN variance (DNN-SDN). This scheme performs joint equalization and demodulation of the received signal, avoiding information loss and error propagation between these two processes, thereby achieving more accurate signal recovery. In this work, we first characterize the statistical noise model for the SPAD array and establish a nonlinear relationship between the number of detected photons and the transmitted signal power. The SDN variance is integrated as an input feature during network training. Performance is quantified via a joint loss function with parameters optimized through a systematic ablation study. Finally, the performance of the proposed signal detection scheme is also compared with other schemes in different waters. Simulation results demonstrate that the scheme can effectively mitigate the effects of SPAD-induced nonlinear distortion and SDN on the signal, and the proposed method extends the transmission distance by 20%-40%.

In order to meet the reliability requirements of communication for underwater resource exploration, this study develops an underwater wireless optical communication (UWOC) system utilizing a blue semiconductor laser as the light source. At the receiver, a fully digital automatic gain control (AGC) module, implemented on a field-programmable gate array (FPGA), is designed to mitigate signal fluctuations induced by underwater turbulence. Digital filtering techniques, including median filtering (MF) and bilateral edge detection filtering (BEDF), are also employed to improve signal demodulation reliability. An improved Reed–Solomon (RS) coding scheme is further adopted to significantly reduce the bit error rate (BER). The design of a highly reliable UWOC system was realized based on the above techniques. The system’s performance was evaluated across a range of signal-to-noise ratios (SNRs) and bubble intensities. The results show that the digital AGC module can provide a gain range from −3.2 dB to 16 dB, adapting to varying signal strengths, which greatly bolsters the system’s resilience against underwater turbulence. Filtering techniques and RS coding further enhance the system’s immunity to interference and reduce the system BER. Communication experiments were conducted over various distances under three distinct water quality conditions. The results demonstrate that, within the detection range of the avalanche photodiode (APD), the system consistently maintained a BER below 3.8 × 10−3 across all water types, thereby confirming its high reliability. In clear seawater, the system demonstrated reliable information transmission over a 10 m distance at a data rate of 10 Mbps, achieving a BER of 2 × 10−8. Theoretical calculations indicate that the maximum transmission distance in clear seawater can reach 111.35 m.

The overview of the principles of organizing underwater wireless optical communication is presented in the paper. The basic features of the optical communication system design with the usage of laser and LED emitters working in the visible range, as well as using various types of modulation, are considered. A comparative review of the developments of underwater wireless optical communication systems, operating at distances up to one hundred meters and with data transfer rates up to tens of Gbit/s, is presented. In addition, a comparative review of design of underwater optical modems is considered. It is shown, that the further prospect of their development is the combination of various methods of underwater wireless communication, including the use of MIMO technology.

This paper has presented our interesting in wireless underwater communications for different ocean water types. Recent interest in ocean exploration has brought about a desire for developing wireless communication techniques in this challenging environment. Due to its high attenuation in water, a radio frequency (RF) carrier is not the optimum choice. Acoustic techniques have made tremendous progress in establishing wireless underwater links, but they are ultimately limited in bandwidth. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this paper is to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. As well as we have developed the underwater optical wireless communication systems to have shorter ranges, that can provide higher bandwidth (up to several hundred Mbit/s) communications by the assistant of exciting high brightness blue LED sources, and laser diodes suggest that high speed optical links can be viable for short range application. The received signal power, signal to noise ratio, bit error rate, transmitted signal bandwidth, and transmission bit rates are the major interesting parameters for different ocean water mediums as a criteria of the best signal transmission characteristics of short wireless optical communications over wide range of the affecting parameters.

A 25-Gbps underwater wireless optical communication (UWOC) system with a two-stage injection-locked 680-nm red-light vertical-cavity surface-emitting laser (VCSEL) transmitter to enhance the frequency response and a laser beam expander to expand the collimated beam diameter over a 5-m highly turbid harbor water link is proposed and practically demonstrated. In highly turbid harbor water link, the overall attenuation coefficient at 680 nm is smaller than that at 520 and 450 nm, thereby a 680-nm red-light VCSEL transmitter is adopted in this proposed 5 m/25 Gbps UWOC system rather than a 520-nm green-light laser diode (LD) transmitter or a 450-nm blue-light LD transmitter. A satisfactory bit error rate performance (3 × 10–9) and a clear eye diagram are acquired in real time. This proposed UWOC system with a two-stage injection-locked 680-nm VCSEL transmitter and a laser beam expander brings important improvements in the scenario characterized by high turbidity.