Underwater Wireless Optical Communication Using Li-Fi Technology In Data Transmission

Guest Editorial

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.

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.

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.

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 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.

With the deepening exploration of ocean, underwater wireless communication technology has become the key to restrict its development. Aiming at the requirement of high speed and long distance underwater wireless communication, an underwater bidirectional link communication system for marine commercial was designed. The laser LD and LED dual emission light sources were adopted at the transmitting end, the system design scheme of dual emission light sources was proposed, and the feasibility of the scheme by designing the corresponding drive circuits was verified. At the receiving end, 5 mm large-area APD and high-sensitivity PMT dual receiving detector were used to receive optical signals, which were suitable for long-distance and high-speed communication. The information processing part of the system was completed by FPGA, and information interaction was carried out with PC through network communication. Finally, the whole system was designed and commercialized. The underwater simulation experiment shows that the communication rate of the system can reach 60 Mbps when the communication distance is 5 m and the bit error rate is 10−7. The communication rate of the system can reach 10 Mbps when the communication distance is 60 m and the bit error rate is 10−7.

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 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%.

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

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.

Photon counting (PhC) is an effective detection technology for underwater optical wireless communication (OWC) systems. The presence of signal-dependent Poisson shot noise and asynchronous multi-user interference (MUI) complicates the processing of received data signals, hindering the effective signal detection of PhC OWC systems. This paper proposes a novel iterative signal detection method in grant-free, multi-user, underwater PhC OWC systems with signal-dependent Poisson shot noise. We first introduce a new synchronization algorithm with a unique frame structure design. The algorithm performs active user identification and transmission delay estimation. Specifically, the estimation is performed first on a user group basis and then at the individual user level with reduced complexity and latency.We also develop a nonlinear iterative multi-user detection (MUD) algorithm that utilizes a detection window for each user to identify interfering symbols and estimate MUI on a slot-by-slot basis, followed by maximum <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a-posteriori</i> probability detection of user signals. Simulations demonstrate that our scheme achieves bit error rates comparable to scenarios with transmission delays known and signal detection perfectly synchronized.

Effects of source beam, link, and oceanic turbulence parameters on the scintillation index and bit error rate (BER) performance of cosine (cos) and cosine-hyperbolic (cosh) Gaussian light beams have been investigated in order to improve wireless optical communication link performance in oceanic turbulence. The Nikishov and Nikishov power spectrum of oceanic water and extended Huygens Fresnel principle were used in our evaluations; the results were obtained via MATLAB. The scintillation index and BER were examined versus oceanic turbulence parameters, which are the rate of dissipation of mean-square temperature, the ratio of temperature and salinity contributions to the refractive index spectrum, and the dissipation rate of kinetic energy per unit fluid mass of fluid. Further, the scintillation index and BER are investigated against the source size, propagation distance, and complex displacement parameters of cos- and cosh-Gaussian beams. This study aimed to select the suitable sinusoidal beam to be employed in order to increase the performance of underwater wireless optical communication systems operating in oceanic turbulence.

Experimental demonstration of underwater wireless optical OFDM communication system with a single SPAD receiver

Compared with the well-established techniques for underwater acoustic communication, underwater wireless optical communication provides much higher transmission bandwidth and can be an interesting alternative in short range underwater applications. We experimentally investigated the feasibility of using a directly modulated semiconductor laser as the transmitter in an underwater wireless optical communication (WOC) system. We transmitted pseudo-random binary sequence signals with four different bit rates (5Mbps, 10 Mbps, 15Mbps, 20 Mbps) through three different channels, with eye diagram being plotted and the bit error rate being calculated. No obvious signal distortion induced by water was observed.