Underwater Visible Light Communication Research Articles

对数正态分布衰落下的光OFDM水下可见光通信空间分集系统

Underwater visible light communication (UVLC) is an attractive solution to achieve high-speed and large-data transmission but challenging due to the impairments induced by absorption, scattering and turbulence. To combat effects of multipath and fading for the UVLC system over turbulence channels, optical orthogonal frequency division multiplexing (O-OFDM) schemes with the transceiver spatial diversity were proposed, which employed equal gain combining (EGC) at the receiver side. Underwater path loss was calculated by a generalized Lambertian formula, and the fading induced by weak turbulence was modelled as a lognormal-distribution random variable. Based on the channel model and Monte Carlo (MC) simulation, the bit error ratio (BER) performance for quadrature-amplitude modulation (QAM) asymmetrically clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) systems in the channel with and without turbulence was evaluated. Furthermore, the diversity gain was estimated for different diversity orders and scintillation indexes. The results demonstrate that the diversity scheme with EGC is an effective measure to reduce the effect of turbulence and could be useful for designing, predicting, and evaluating the performance of O-OFDM UVLC system in a weak oceanic turbulence condition.

Sherman-Morrison Formula Aided Adaptive Channel Estimation for Underwater Visible Light Communication With Fractionally-Sampled OFDM

In this paper, we investigate the channel estimation (CE) problem in an underwater visible light communication (UVLC) system invoking fractionally-sampled optical orthogonal frequency division multiplexing (FS-OOFDM). In practical UVLC scenarios, the communication links inevitably suffer from many stochastic channel effects including multi-path dispersion, scattering, turbulence, etc., and/or from the mobility of the transceiver, therefore resulting in a time-varying, location-dependent non-stationary propagation environment. Naturally, compared with the indoor visible light communication (VLC) scenario with a typical assumption being the time-flat channel models, it becomes a notable challenge for designing a low-complexity adaptive CE in the much more complicated UVLC scenarios. To solve this problem, we derive a class of Bayesian CE algorithms referred to as the Sherman-Morrison formula (SMF) based CE (SMF-CE), by exploiting the property of rank-one structure of the second-order channel statistics in the delay domain. Furthermore, an adaptive version of SMF-CE (ASMF-CE) can be obtained through updating the imperfect a priori knowledge of the channel and the noise's statistics. Simulation results demonstrate the superior performances of the proposed algorithms in comparison to existing methods, while maintaining a reduced computational complexity in comparison to the conventional linear minimum mean square error (LMMSE) scheme.

3.2 Gbps underwater visible light communication system utilizing dual-branch multi-layer perceptron based post-equalizer

In this paper, we demonstrate a novel dual-branch multi-layer perceptron (DBMLP) based post-equalizer(PE). Since the two branches of the DBMLP can compensate linear and nonlinear distortion in the received signal, respectively, the complexity of the neural network is reduced and the performance of the post-equalizer is improved. Compared with multi-layer perceptron (MLP) based PE the space complexity of DBMLP based PE reduces 59.5%, and the bit error rate (BER) at 3.1 Gbps reduces 45.5%. The reduction of spatial complexity helps the application of post-equalizer based on neural network algorithms in practical communication systems. Meanwhile, increased BER performance helps underwater visible light communication (UVLC) systems reach higher rates. We achieved 3.2Gbps in the 64 quadrature amplitude modulation (64QAM)-carrierless amplitude and phased modulation (CAP) modulated UVLC system using DBMLP based PE. To the best of our knowledge, this is the highest record of data rate in the field of one single chip blue LED based UVLC system.

Demonstration of a 2.34 Gbit/s Real-Time Single Silicon-Substrate Blue LED-Based Underwater VLC System

We develop a real-time discrete multi-tone (DMT) transceiver based on field programmable gate array (FPGA) chips for a single chip silicon-substrate light-emitting diode (LED) based underwater visible light communication (UVLC). On-chip resource usages are analyzed and discussed. To improve bit error rate (BER) performance, a novel channel estimation technique utilizing hybrid inter-symbol frequency-averaging (Inter-SFA) and intra-symbol frequency-averaging (Intra-SFA) is proposed and investigated. The real-time DMT transceiver is experimentally verified in a silicon substrate blue LED-based UVLC system with a 1.2 m underwater link. By using the enhanced channel estimation, a gross bit rate of 2.34 Gbit/s real-time DMT signal over 1.2 m underwater transmission can be achieved with the BER of 3.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . What's more, multiple-symbol interleaved Reed-Solomon (RS) codes are employed to further improve BER performance. The real-time measured post-FEC BER of the DMT-UVLC with multiple-symbol interleaved RS (255, 191) codes can be improved by more than six orders of magnitude. As a result, error-free (less than 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> ) transmission is observed in our real-time experiment. Furthermore, 1.485 Gbit/s 720p high-definition video underwater transmission is successfully demonstrated. To the best of our knowledge, it is the first time to demonstrate a real-time LED-based UVLC system with DMT modulation beyond Gbit/s.

Application of Gaussian Mixture Model to Solve Inter-Symbol Interference in PAM8 Underwater Visible Light System Communication

Traditional soft or hard decisions ignoring the correlation between symbols removing linear and nonlinear damage directly may lead to lack of information and result in system performance degradation. In order to reduce the performance degradation of the system caused by the lack of information, we propose a method that use the Gaussian mixed model to consider the influence of the correlation between consecutive symbols for underwater visible light communication system. The experimental results show that the performance of the system is greatly improved after the correlation between consecutive symbols is considered. The more consecutive symbols there are, the greater the performance will be. When the forward error correction (FEC) threshold is met, the Q factor of Gaussian mixture models(GMM) that join three adjacent symbols is 1.19 dB greater than without the GMM clustering algorithm. The highest data rate that can be achieved exceeds 1.5 Gbps with GMM.

The node distance effects on the performance of the underwater VLC incremental adaptive networks

ABSTRACTIn this paper, an underwater implementation of incremental networks based on visible light communication technology is described and analysed. The underwater distance between transmitter and receiver nodes and the salinity and temperature levels of the water yield stochastic properties of the link that can be modelled with the Log-normal distribution. The incremental network performance can be expressed using excess mean square error and Mean square deviation metrics. Our findings showed that the distance between nodes must not exceed 10 metres; if it does, the network performance will diverge from its estimation goal. The performance is analysed through multiple link distances and results are presented for simulations. The impacts of various salinity and temperature levels are analysed simultaneously.

NLoS underwater VLC system performance: static and dynamic channel modeling.

In this paper, the impact of water channels under different communication link parameters is studied for underwater visible light communication (UVLC). The objective is to highlight the best results for non-line of sight (NLoS) communication links. In addition, NLoS links are studied under different parameters: LED colors, viewing angle, receiving angle, and data rates. The results are obtained and plotted using MATLAB simulation. The performance of the received power is first measured at different wavelengths and data rates. Then, the best results are further investigated at different viewing angles and receiving angles. The obtained results show that using cyan color provides more depth for the NLoS case, as well as a low bit error rate compared to the other colors. Most of the literature is concerned with unpractical configurations in underwater scenarios, such as an empty sea or assuming no human-object or blockage environment. We use the practical setup in Zemax Optics Studio to allow a precise description of ray tracing and high order of reflections inside a sea water environment. The channel impulse response (CIR) is obtained for static channel modeling, including a blockage environment to evaluate the best transmitters in sea water. Also, we are able to compare the average delay and the average delay spread of the source colors. The reflection characteristics of the sea water are considered as wavelength dependent. The CIR obtained by Zemax Solver and MATLAB indicates that cyan is the best source in sea water for different LED chips. Moreover, other previous studies assume perfect alignment scenarios between divers, which is not practical and not suitable for real channel gain results. Accordingly, we present a comprehensive dynamic channel modeling and characterization study for UVLC. Our study is based on Zemax programming language (ZPL) combined with Zemax Optics Studio. Using ZPL enables us to apply a mobility algorithm for divers and measure the channel gain variations due to random motion. We introduce a dynamic motion in a single-input single-output scenario and a single-input multiple-output scenario in the presence of blockage divers. Statistical analyses are studied for the appropriate distributions that can fit the data with various transmitter and receiver specifications. All dynamic scenarios are performed using cyan color in sea water, as it is proven to have satisfactory performance. The statistical results are beneficial for further analysis. As case studies, we consider various underwater scenarios, and the resulting parameters of statistical distributions can be used for future analysis in UVLC dynamic environments.

Common-anode LED on a Si substrate for beyond 15 Gbit/s underwater visible light communication

Visible light communication based on light-emitting diodes (LEDs) has become a promising candidate by providing high data rates, low latency, and secure communication for underwater environments. In this paper, a self-designed common-anode GaN-based five-primary-color LED (RGBYC LED) on a Si substrate is proposed and fabricated. The design of a common anode is used to mitigate the saturation effect for a low-frequency component. Additionally, compared with commercially available LEDs that suffer from nonlinearity distortion, applying the designed LED can provide much better and broader linearity according to the measurement results. Therefore, the modulation depth and system performance can be further improved to implement a high-speed underwater visible light communication (UVLC) system. There is no nonlinearity compensation algorithm applied due to the good linearity of the proposed LED; thus, the offline digital signal processing is simplified. We experimentally demonstrate 14.81 Gbit/s 64 quadrature amplitude modulation (QAM)-discrete multitone (DMT) and 15.17 Gbit/s bit-loading-DMT transmissions through a 1.2-m-long underwater channel based on the proposed RGBYC LED with an intrasymbol frequency-domain averaging channel estimation and zero-forcing equalization. As far as we know, this is the highest data rate for an LED-based UVLC system.

Enhanced Performance of PAM7 MISO Underwater VLC System Utilizing Machine Learning Algorithm Based on DBSCAN

After visible light communication drawing increasing attention, underwater visible light communication (UVLC) has attracted more interest in the research community nowadays. As multiple input single output (MISO) is getting increasingly widely used to improve the transmission speed in UVLC system, the unbalance between multiple transmitters’ power is still a common phenomenon, which leads to the unequal spacing between each adjacent level and damages the system performance. In this paper, we study and analyze the unbalance between the two transmitters. Compared to a traditional hard decision, a density-based spatial clustering of applications with noise (DBSCAN) of a machine learning method is employed to get the actual center of each cluster and distinguish each level of PAM7 signals. In this way, a new decision curve substitutes traditional standard straight line as a constellation discrimination method. The experimental results show that up to 1.22 Gb/s over 1.2 m underwater visible light transmission can be achieved by using DBSCAN for PAM7 MISO signals. The measured bit error rate is well under the hard decision-forward error correction threshold of 3.8 × 10−3.

Two tributaries heterogeneous neural network based channel emulator for underwater visible light communication systems.

This paper proposes a novel two tributaries heterogeneous neural network (TTHnet) based channel emulator, which is suitable for both estimating single-carrier and multi-carrier modulated channels of underwater visible light communication (UVLC). Compared to traditional neural networks, the TTHnet channel emulator has only 1932 trainable parameters, which is only 0.8% of multilayer perceptron (MLP) based channel emulator and 1% of a convolutional neural network (CNN) based channel emulator. Furthermore, it provides a more accurate estimation of the UVLC channel and greater interpretability than MLP and CNN. The experiments in this paper use carrier-less amplitude/phase modulation (CAP) and discrete multi-tone modulation (DMT) as representative examples of single-carrier and multi-carrier modulation, respectively. The experiment proves that the TTHnet based channel emulator could effectively emulate the channel response of UVLC systems both in time and frequency domain. To the best of our knowledge, this is the first time that the single-carrier and multi-carrier modulated UVLC channel is emulated by the deep neural networks based channel emulator, which will effectively accelerate the research progress of UVLC and reduce research costs of UVLC systems.

3.075 Gb/s underwater visible light communication utilizing hardware pre-equalizer with multiple feature points

We propose a flexible method for multiple feature frequencies-based bridge T hardware pre-equalizer that can better match the channel of an LED-based visible light communication (VLC) system and dramatically increase the system capacity. Based on this method, three kinds of equalizers are fabricated with the same maximum attenuation but different envelopes of frequency response. The performance of the three equalizers is experimentally compared in an underwater VLC system. The results show that the different shapes of the pre-equalizer can significantly impact the system SNR as well as the system capacity. By utilizing one of the optimally designed equalizers, an overall data rate of 3.075 Gb/s underwater VLC system can be achieved based on 64 QAM discrete multitone modulation over 1.2-m water channel. The measured bit error rate is under the hard decision-forward error correction threshold of 3.8 × 10 − 3.

Spatial correlation analysis of imaging MIMO for underwater visible light communication

Absorption and scattering are the main problems affecting the transmission of large capacity information data in underwater visible light communication system(UVLC). To address these problems, we propose the imaging multiple input multiple output (MIMO) system for the underwater communication in this paper. Furthermore, the proposed system uses imaging lens to separate the light signal resulting in that decreasing disturbance of the proposed system is better than that of non-imaging MIMO. In this paper, the spatial correlation caused by imaging lens structure of underwater visible light imaging MIMO communication channel is analyzed. In addition, the influence of the spatial position for transmitting and receiving ends of the imaging system such as light-emitting diode (LED) light source array spacing, and the distance between LED and photodiode (PD) on spatial correlation are modeled. Moreover, the imaging MIMO channel capacities under different channel conditions are derived. The upper bounds of channel capacities under different spatial correlations are solved by equal power allocation method and water injection power allocation method. Finally, the simulation results show that the imaging MIMO system achieves a 12 dB gain at the same bit error rate (BER) compared to the non-imaging MIMO system. Furthermore, the capacity and BER performance of the proposed system are the best in the case of the weakest spatial correlation, and the performance is greatly improved as the spatial correlation becomes weaker.

The phase estimation of geometric shaping 8-QAM modulations based on K-means clustering in underwater visible light communication

With more and more human underwater activities carrying out, underwater visible light communication has received increasing attention worldwide. To solve the problem of noise and nonlinearity in underwater visible light communication systems, we proposed a phase deviation correction algorithm based on K-means clustering to correct the phase deviation of special-shaped 8-QAM constellations. The experimental results indicate the performance of the circle has the maximum improvement after using the phase deviation correction algorithm. Before the use of the K-means phase correction algorithm, the highest data rate that circle can achieve is 1.3875 Gbps. The highest data rate increases to 1.4625 Gbps after applying the algorithm.

Computer-vision-based intelligent adaptive transmission for optical wireless communication.

Optical wireless communication (OWC) has been presented as a promising candidate for future space-air-ground-ocean-integrated communication. However, the OWC is quite sensitive to the variation of the channel transmission characteristics. The light beam absorption and the scattering in the transmission media affect not only the channel feature, but also the imaging quality. Thus, there is an inherent relationship between the OWC performance and the optical imaging quality. Based on this consideration, we firstly present the idea of introducing computer vision mechanisms into the OWC systems, and then propose a computer vision-based multi-domain cooperative adjustment (CV-MDCA) mechanism's functional modules to realize the intelligent adaptive transmission in OWC systems. The CV-MDCA mechanism are specifically designed, with the emphasis on how to quantitatively determine the exact on-line channel quality from the captured images by using effective computer vision schemes. Two groups of experiments, the indoor-simulated underwater visible light communication and the outdoor-practical atmospheric free-space optics, are implemented in order to evaluate the presented CV-MDCA mechanism's performance. The results not only validate the feasibility to determine the channel quality, according to the captured channel images, but also reveal the presented three computer vision-based criteria's limitations.

Generalized Likelihood Block Detection for SPAD-Based Underwater VLC System

In the underwater visible light communication (VLC) system, the single-photon avalanche diode (SPAD) can expand communication distance. However, the output signals from practical SPADs under dead time limit are not Poisson distributed. In this paper, the generalized likelihood block detection (GLBD) receiver is developed for practical SPAD-based underwater VLC system with on-off keying modulation. The proposed receiver can detect the data sequence without any prior knowledge of the channel and the background radiation. Correspondingly, a fast search algorithm for GLBD receiver is also proposed. In addition, a block coding scheme is utilized to solve the error floor problem. Simulation results show that the bit error rate (BER) performance of the proposed receiver is closer to the BER low bound compared to the existing receiver. Moreover, the fast search algorithm reduces the computational complexity without any performance loss.

Large-coverage underwater visible light communication system based on blue LED employing equal gain combining with integrated PIN array reception.

In this paper, we proposed a large-coverage underwater visible light communication (VLC) system utilizing quadrature amplitude modulation-discrete multitone (QAM-DMT) and integrated 2×2 positive-intrinsic-negative (PIN) array reception to achieve equal-gain combining (EGC) receiving. The transmitter consists of a lens group and blue-emitting silicon substrate light-emitting diode peak emission, the wavelength of which is 458nm. After 1.2m underwater transmission, a data rate of over 1Gbit/s is successfully achieved with a spot of 25cm in diameter and 11cm alignment tolerances. A data rate of 1.8Gbit/s is also achieved when using parallel light reception. Both are the highest data rates to our knowledge so far in a large-coverage underwater VLC system, which shows the benefit and feasibility of using an integrated PIN array with QAM-DMT modulation and EGC.

Nonlinear adaptive filters for high-speed LED based underwater visible light communication [Invited

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.

Asymmetric Satellite-Underwater Visible Light Communication System for Oceanic Monitoring

In this paper, we investigate the performance of the proposed oceanic monitoring system that connects the oceanic life with the terrestrial life. For continuous real-time monitoring and ubiquitous coverage, the communication system is aided with a satellite link. Multiple sensor nodes (SN) are deployed at different water levels that collect sensor data and transmit it to underwater vehicles (UV) using underwater visible light communication (UVLC). The UVLC system provides higher data rates at lower latency as compared to existing radio frequency (RF) and acoustic wave alternative for underwater communication (UWC). The UWC system comprises of horizontal haul (HH) and vertical haul (VH) UVLC links modelled using turbulence induced fading. The vertical haul links are modelled as the concatenation of successive non-mixing turbulent links to take into account the change of turbulence with the change of water level. The UVs and submarines communicate with the floating vessels (FVs) using vertical haul UVLC link. The UVs collect the data from the low power sensor nodes and offloads it to the FVs, which further beams it to the satellite on the RF carriers. The novel expressions of performance metrics such as outage probability and average bit error rate are derived. Further, the performance of the system is analysed for various system and channel parameters to prove the feasibility of the proposed communication system.

Performance Characterization of Underwater Visible Light Communication

In this paper, we investigate the performance limits of underwater visible light communication (UVLC) systems. We first develop a closed-form path loss expression as a function of transceiver parameters and water type. We then utilize this new expression to determine the maximum achievable link distance for UVLC systems in pure sea, clear ocean, coastal water, and harbor water. Our results demonstrate that the maximum achievable distance is limited to a few tens of meters. This necessitates the deployment of relay-assisted UVLC systems to extend the transmission range. We consider both detect-and-forward and amplify-and-forward relaying. For each relaying method, we first consider a dual-hop UVLC system and determine optimal relay placement to minimize the bit error rate (BER). Then, we consider a multi-hop system with equidistant relays and determine the maximum achievable distance for a given number of hops to satisfy a targeted end-to-end BER.

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

Visible light communication(VLC) employing light-emitting diodes(LED) as light source has become a promising field for underwater wireless communication. However, due to the complex underwater environment and scattering, absorption and diffusion effects during optical transmission in water, the underwater VLC system will suffer from poor SNR and highly nonlinear distortion. In this paper, we proposed and experimentally demonstrated a novel Square Geometrical Shaping(SGS) odd order QAM discrete multitone(DMT) and carrier-less amplitude phase(CAP) modulation based VLC system. Unlike to traditional odd order QAM constellation aiming at the minimum Euclidean distance but without Gray coding at edge constellation points, the new SGS constellation boasts acceptable Euclidean distance and thoroughly Gray coding for every constellation point. Therefore, the system performance can be effectively enhanced by SGS constellation for low SNR and high nonlinearity cases. Both numerical and experimental results verify that SGS 32QAM and 128QAM constellations can outperform traditional QAM constellation by mitigating nonlinearity in a blue LED based underwater and free space VLC system using DMT modulation or CAP modulation. By employing SGS 128QAM, we successfully demonstrate 2.534Gb/s 1.2meter underwater transmission at the driving voltage of 1.1Vpp.