DenseFormer-UVLC: Intelligent hybrid deep learning framework for adaptive beam steering and modulation in underwater visible light communication

Visible light communication (VLC) has emerged as a promising communication method in the special field of 6G, such as intersatellite communication (ISC) and underwater wireless optical communication (UWOC). Green light-emitting diode (LED)-based underwater visible light communication (VLC) is considered a potential candidate for UWOC of mobile autonomous underwater vehicle (AUV) and remotely operated underwater vehicle, particularly in coastal and harbor environments. However, due to bandwidth limitations and variable underwater channels, implementing high-speed underwater VLC remains a challenge. In this paper, a neural network-based auto equalization model (NNAEM) using end-to-end learning is proposed to achieve a high-speed underwater VLC link with a bandwidth-limited green LED. The entire NNAEM includes a neural network-based channel model, a pre-equalization neural network, and a post-equalization neural network. Based on the data-driven channel model, our auto equalization (AE) method can dynamically pre-equalize the discrete multitone (DMT) modulated signal in different bandwidth-limited cases and various nonlinearity cases. An underwater VLC link with a data rate of 3.451 Gbps and a transmission distance of 1.2 m is experimentally demonstrated by employing the proposed NNAEM under the hard decision-forward error correction (HD-FEC) threshold of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . The data rate improvement is up to 30.4% compared to other advanced digital signal processing (DSP) methods. The scenario adaptation and robustness of the NNAEM are also verified in the experiments, which demonstrate that the NNAEM can serve an important role in underwater high-speed VLC transmission.

Spatial correlation analysis of imaging MIMO for underwater visible light communication

In optical wireless communication systems, the underwater visible light communication (UVLC) systems is becoming more and more popular. However, due to intrinsic properties of water body, the propagating optical channel can suffer from severe degrading effects, namely absorption, scattering, and turbulence. Here, we aim to improve the UVLC system performance. For this, we propose to utilize an adaptive modulation scheme at the optical transmitter based on the channel conditions. The performance of such system using different modulation schemes such as the pulse position modulation (PPM), the digital pulse interval modulation (DPIM) and the dual-header pulse interval modulation (DH-PIM), are demonstrated to prove the effectiveness of our adaptive UVLC systems.

Recently, Underwater Visible Light Communication (UVLC) has been attracting attention as a short-range underwater wireless communication technology within the range of 100 m. UVLC enables high-speed communication mainly using photodiodes (PD) but requires precise alignment of the optical axis between the transmitter and receiver. In this paper, we investigate the UVLC using image sensors (IS) to establish a more stable communication link easily. Furthermore, focusing on the change of attenuation rate in the visible light band due to the effect of turbidity, chlorophyll concentration, in the underwater transmission channel, we propose an IS-based UVLC using RGBLEDs as the transmitting light, and theoretically evaluate the effect of turbidity in the sea. Specifically, when the chlorophyll concentration is low and high, the lowest attenuation wavelengths are of blue and red respectively. In this paper, we consider image sensor-based UVLC using RGB-LEDs and evaluate how water turbidity affects the transmission light. Also, we evaluate theoretically the effect of turbidity in the sea on the optical RGB signal using a general full-color camera taking into account underwater channel, general convex lens characteristics, and bayer color filter and analog to digital converters (DAC) at CMOS image sensor

In recent years, underwater visible light communication (UVLC) has become a potential wireless carrier candidate for signal transmission in highly critical, unknown, and acrimonious water mediums such as oceans. Unfortunately, the oceans are the least explored reservoirs in oceanogeographical history. However, natural disasters have aroused significant interest in observing and monitoring oceanic environments for the last couple of decades. Therefore, UVLC has drawn attention as a reliable digital carrier and claims a futuristic optical media in the wireless communication domain. Counterparts of traditional communications, the green, clean, and safe UVLC support high capacity data-rate and bandwidth with minimal delay. Nevertheless, the deployment of UVLC is challenging rather than terrestrial basis communication over long ranges. In addition, UVLC systems have severe signal attenuation and strong turbulence channel conditions. Due to the fact that, this study provides an exhaustive and comprehensive survey of recent advancements in UVLC implementations to cope with the optical signal propagation issues. In this regard, a wide detailed summary and future perspectives of underwater optical signaling towards 5G and beyond (5GB) networks along with the current project schemes, channel impairments, various optical signal modulation techniques, underwater sensor network (UWSN) architectures with energy harvesting approaches, hybrid communication possibilities, and advancements of Internet of underwater things (IoUTs) are concluded in this research.

Underwater visible light communication (UVLC) is expected to act as an alternative candidate in next-generation underwater 5G wireless optical communications. To realize high-speed UVLC, the challenge is the absorption, scattering, and turbulence of a water medium and the nonlinear response from imperfect optoelectronic devices that can bring large attenuations and a nonlinearity penalty. Nonlinear adaptive filters are commonly used in optical communication to compensate for nonlinearity. In this paper, we compare a recursive least square (RLS)-based Volterra filter, a least mean square (LMS)-based digital polynomial filter, and an LMS-based Volterra filter in terms of performance and computational complexity in underwater visible light communication. We experimentally demonstrate 2.325 Gb/s transmission through 1.2 m of water with a commercial blue light-emitting diode. Our goal is to assist the readers in refining the motivation, structure, performance, and cost of powerful nonlinear adaptive filters in the context of future underwater visible light communication in order to tap into hitherto unexplored applications and services.

Underwater visible light communication (VLC) systems are increasingly considered as a promising alternative for communication in aquatic environments due to their high data rates and low latency. However, the performance of VLC systems in underwater environments is significantly affected by factors such as distance and salinity level, leading to channel impairments that must be accurately estimated for reliable communication. In this paper, we propose a novel approach for channel estimation in underwater VLC (UVLC) systems, where the channel model is assumed to follow a log-normal distribution based on the distance and salinity level of the water. We leverage the power of deep learning, specifically a deep neural network with Q-layers, to estimate the channel conditions dynamically. The Q-layers enable the network to model the nonlinearities inherent in the channel estimation problem, providing robust performance even in the presence of varying environmental conditions. Our results demonstrate that the proposed method achieves superior performance compared to traditional estimation techniques, offering a significant improvement in the reliability and accuracy of underwater VLC systems. Simulation results show that the proposed Q-network-based estimator achieves up to 31 % lower NMSE than ChanEstNet under high turbulence (distance &gt;15 m), and improves BER performance by nearly 18 % in long-distance UVLC conditions.

Underwater visible light communication (UVLC) has a very wide application prospect in ocean exploration. The single photon avalanche diode (SPAD) can expand system communication distance. In this paper, we study the SPAD based UVLC systems performance under dead time limited. To analyze the BER performance, we first get the model of the practical SPAD based UVLC system under the dead time limit. Then, we set up a long-distance VLC model based on the double exponentials decay. Furthermore, the maximum likelihood detection methods of different modulation schemes for the system are proposed. The simulation results show that the longest communication distance in pure sea water is 360 meters.

In this paper, we studied the probability distribution function (PDF) of the channel gain of dynamic underwater visible light communication (UVLC) model for different types of water using goodness-of-fit (GoF). We used different water channels at different system parameters with dynamic scenarios. First, the Zemax Optics Studio simulator is used to simulate dynamic UVLC channels. UVLC links are examined using Monte Carlo Ray-Tracing (MCRT) simulation for three different water channels; namely, pure sea water channel, clear ocean water channel and coastal ocean water channel at different configuration types. With the presence of blocking divers, we added a dynamic movement in a single input multiple users (SIMU) scenario. Our simulation is based on Zemax Programming Language (ZPL) in sequence with the Zemax Optics Studio. The GoF tests are used to get the degree of fitness between the simulation data and the set of well-known candidate distributions to determine the best fit. We used the R statistical programming language and applied predefined algorithms to determine the optimum degree of fitting for each statistical result. The Kolmogorov-Smirnov (KS), Chi-Square (CS), Cramer-Von-Mises (CVM) and Anderson-Darling (AD) tests are used to represent the four GoF statistical computation techniques for each channel scenario. The received power is enhanced by 35&#x0025; when the detector movement area increases from 25 m² to 100 m² in clear ocean water channel. The obtained results reveal that the UVLC is best represented by Weibull, Gamma or Lognormal distributions.

The sixth-generation mobile communication is committed to creating a large-capacity, low-latency, threedimensional communication network encompassing space, air, land, and sea. Visible light communication (VLC) uses carriers with wavelengths from 380 nm to 760 nm for information transmission, offering a spectrum of 400 THz. Besides, visible light falls within the underwater transmission window, which offers significant potential for sea-based communication network construction. However, underwater visible light communication (UVLC) faces the effects of turbulence, device nonlinearity, attenuation, and noise, which limit the increase of the capacity of UVLC. To address the effects of turbulence, considering that turbulence has different effects on different polarization directions, the received signal can be better reconstructed by increasing the observation dimension, like observing the signal in multiple polarization directions. We proposed the use of circularly polarized light for transmission with a dual-aperture receiver, where the dual apertures detect the horizontally and vertically polarized components of the light beam, respectively. For the fusion of the two received signals, and in order to resist undesirable factors such as device nonlinearity and highfrequency attenuation, we designed a sparse dual-polarization triple-branch network (S-DPTBN) for signal reconstruction and recovery. The sparsification reduces the complexity and improves the robustness of the network, and the triple-branch design can fully extract the information of the horizontal and vertical polarizations as well as the weighted sum of the two. To enhance the communication capacity, we integrated an 8-wavelength laser emission module for signal transmission. Finally, experimental results confirmed that the proposed system achieved data rates of 144.90 Gbps, 143.50 Gbps, and 140.70 Gbps in conditions of no turbulence, medium turbulence, and strong turbulence, respectively, which had an improvement of 14.00 Gbps, 14.00 Gbps, and 13.30 Gbps over traditional point-to-point systems with deep neural network-based equalizers.

Enhanced performance of odd order square geometrical shaping QAM constellation in underwater and free space VLC system

With the rapid development of light emitting diode (LED), visible light communication (VLC) becomes an important technique for information transmission including underwater applications. However, accurate channel estimation for underwater VLC is still challenging due to the complex environment of the underwater VLC channel. In this paper, by utilizing a proper approximation, where the channel attenuation is linear with the frequency, a new compressive sensing (CS) based channel estimation approach is proposed. Utilizing the sparse property of the reflection path length for the underwater VLC channel, the CS framework is modeled to estimate the reflection path length, which can further recover the underwater VLC channel. Moreover, a Bayesian CS recovery algorithm is investigated to overcome the problem of high coherence for the sensing matrix which outperforms the conventional greedy algorithm such as orthogonal matching pursuit (OMP). Simulation results illustrate that our proposed channel estimation for underwater VLC systems has a superior performance which can significantly reduce the pilot overhead, improve the spectral efficiency, and enhance the estimation accuracy.

Ongoing advances in the research and development of visible light communication (VLC) systems has opened up many opportunities for using the concept of light based communication in underwater medium. Water presents a favourable medium for communication of optical wavelengths, particularly in the blue-violet regions of the visible spectrum and parts of ultraviolet wavelength. Prior work in designing underwater VLC systems and exploration of underwater optical wireless channels have largely been limited to theoretical propositions. Experimentation work in this space have not studied the impact of water quality factors on communication, and there exists no clear database of underwater VLC channel studies. Prior work have also largely focused only on horizontal configurations, where the transmitter and receiver are at the same depth underwater. To bridge this gap between existing work and advancement of underwater VLC systems, this paper presents the development of an empirical model for underwater VLC across the vertical configuration and across different water quality factors such as turbidity, salinity and temperature.

Underwater Visible Light Communication (UVLC) is promising due to its relatively strong penetration capability in water and large frequency bandwidth. Visible Light Communication (VLC) is also considered a safer wireless communication paradigm as light signals can be constrained within the area of interest with obstacles such as walls, reducing the chance of potential attack. However, this intuitional security assumption is not true anymore in underwater environment. Recent research shows that the eavesdropping risk of UVLC is more severe than we thought, attributed to the divergence and scattering effects of light beams in water. In this paper, we harness the dynamic nature of underwater environments as a true random resource to extract symmetric keys for UVLC. Specifically, the proposed AquaKey system incorporates instantaneous bidirectional channel probing, computation of relative channel characteristics, and an environment-adaptive quantization algorithm. The above design addresses unique challenges caused by the dynamic underwater environment, including self-interference, high-frequency disturbance, and mismatch, ensuring the practicality and applicability of AquaKey. Additionally, AquaKey has negligible impact on communication and has no effect on the illumination function. Through extensive real-world experiments, we show that AquaKey can achieve reliable key extraction with cheap hardware, generating a 512-bit key in just 0.5-1 seconds.

Visible light communication (VLC) has emerged as a promising communication method in the domain of underwater communication. However, the transmission rate is limited by the modulation bandwidth and the nonlinear effect of the LED. In this paper, we proposed a post-equalization method base on Bidirectional Gate Recurrent Unit (BGRU) and high order modulation format Amplitude phase shift keying (64APSK) is applied in underwater visible light communication (UVLC) for the first time. The excellent performance of nonlinearity compensation has been verified through experimental demonstration. With 64APSK and BGRU post-equalization, the transmission rate of underwater 1.2m distance is up to 3.19 Gbps. Compared to the traditional Least Mean Square (LMS) algorithm, the transmission rate increases 140 Mbps with VPP of 0.7V, current bias of 150mA and Q factor increases 2.4dB at the maximum with VPP of 0.9V, current bias of 150mA and bit rate of 3 Gbps. Compared to the Normal-64QAM, the transmission rate increases 65 Mbps and 64APSK shows a larger working range. All the results are demonstrated with 7% FEC of 3.8E-3 as a BER threshold. To the best of our knowledge, it is the first time 64APSK and BGRU post-equalization are used in UVLC system and the bit rate exceeds 3 Gbps. BGRU is shown effective to deal with the nonlinear distortion and APSK modulation format is suitable for underwater high-power transmission, which are proved as a promising method in future UVLC.