Channel estimation-based time-frequency neural network for post-equalization in underwater visible light communication

This Letter proposes a post-equalizer for underwater visible light communication(UVLC) systems that combines channel estimation and joint time-frequency analysis, named channel-estimation-based bandpass variable-order time-frequency network(CBV-TFNet). By utilizing a bandpass variable-order loss function with communication prior knowledge, CBVTFNet enhances communication performance and training stability. It enables lightweight implementation and faster convergence through a channel estimation-based mask. The superior performance of the proposed equalization method over Volterra and deep neural network(DNN)-based methods has been studied experimentally. Using bit-power loading discrete multitone (DMT) modulation, the proposed method achieves a transmission bitrate of 4.956 Gbps through a 1.2 m underwater channel utilizing only 38.15% of real multiplication calculations compared to the DNN equalizer and achieving a bitrate gain of440 Mbps and a significantly larger dynamic range over the LMS-Volterra equalizer. Results highlight CBV-TFNet's potential for future post-equalization in UVLC systems.

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.

Advanced modulation formats for underwater visible light communications [Invited]

In this paper, we present a detailed comparison of applying three advanced modulation formats including carrierless amplitude and phase modulation(CAP), orthogonal frequency division multiplexing(OFDM), and discrete Fourier transform spread orthogonal frequency division multiplexing(DFT-S OFDM) in underwater visible light communication(UVLC) systems. Cascaded post-equalization schemes are suggested to compensate the system impairments. For the first time, a two-level post-equalizer is presented to mitigate the nonlinear effect and improve the system performance of UVLC. The first post-equalization is based on a novel recursive least square Volterra. These modulation formats are all experimentally demonstrated with corresponding digital signal processing(DSP) algorithms. The experimental results show that single carrier modulations including CAP and DFT-S OFDM can outperform OFDM. Our experiment results show that up to 3 Gb/s over a 1.2 m underwater visible light transmission can be achieved by using DFT-S OFDM 64 QAM and CAP-64. The measured bit error rate is well under the hard decision-forward error correction(HD-FEC) threshold of 3.8 × 10^(-3).

Security enhanced underwater visible light communication system based on chaotic phase scrambling and conjugate frequency hopping

We propose an encryption technique for underwater visible light communication[UVLC]based on chaotic phase scrambling[PS]and conjugate frequency hopping[CFH].The technique is experimentally tested using an 8-level pulse amplitude modulation[PAM-8]and a 1.2 m underwater link.The security key of the phase scrambling code is generated according to a logistic map,and the frequency hopping is achieved by adding the same zero frequency points to the signal spectrum.The maximum transmission rate of 2.1 Gbit/s is measured with bit-error-rate[BER]below 7%the hard-decision forward error correction[HD-FEC]threshold of 3.8×10^(-3).

Handover Mechanism Based on Underwater Hybrid Software-Defined Modem in Advanced Diver Networks

For the past few decades,the internet of underwater things(IoUT)otained a lot of attention in mobile aquatic applications such as oceanography,diver network monitoring,unmanned underwater exploration,underwater surveillance,location tracking system,etc.Most of the IoUT applications rely on acoustic medium.The current IoUT applications face difficulty in delivering a reliable communication system due to the various technical limitations of IoUT environment such as low data rate,attenuation,limited bandwidth,limited battery,limited memory,connectivity problem,etc.One of the significant applications of IoUT include monitoring underwater diver networks.In order to perform a reliable and energy-efficient communication system in the underwater diver networks,a smart underwater hybrid softwaredefined modem(UHSDM)for the mobile ad-hoc network was developed that is used for selecting the best channel/medium among acoustic,visible light communication(VLC),and infrared(IR)based on the criteria established within the system.However,due to the mobility of underwater divers,the developed UHSDMmeets the challenges such as connectivity errors,frequent link failure,transmission delay caused by re-routing,etc.During emergency,the divers are most at the risk of survival.To deal with diver mobility,connectivity,energy efficiency,and reducing the latency in ADN,a handover mechanism based on pre-built UHSDM is proposed in this paper.This paper focuses on(1)design of UHSDM for ADN(2)propose the channel selection mechanism in UHSDM for selecting the best medium for handover and(3)propose handover protocol inADN.The implementation result shows that the proposed mechanism can be used to find the new route for divers in advance and the latency can be reduced significantly.Additionally,this paper shows the real field experiment of air tests and underwater tests with various distances.This research will contribute much to the profit of researchers in underwater diver networks and underwater networks,for improving the quality of services(QoS)of underwater applications.

Study of Wireless Sensor Network Based on Optical Communication:Research Challenges and Current Results

With the rapid developments of commercial demands,a majority of ad­vanced researches have been investigated for the applications of underwa­ter wireless sensor(WSN)networks.Recently optical communication has been considered for underwater wireless sensor network.An experimental set-up for testing optical communication underwater has been provided and designed in present papers to maximize the energy coupled from these displacements to the transduction mechanism that converts the mechanical energy into electrical.The true case has been considered by measuring dif­fuse attenuation coefficients in different seas.One stand out potential opti­cal communication method,Visible Light Communication(VLC)has been talked and several communication methods are compared from many points of view,for example attenuation in salt water.The evaluation of modula­tion techniques for underwater wireless optical communications has been displayed,and further how the data collection and storage with an under­water WSN is introduced.In this paper current researches for an(UWSN)based on optical communication are studied,in particular the potential VLC method and comparisons of VLC with other optical communication approaches.Underwater challenges would be analyzed by comparing a sort of communication methods,applied in underwater.Future work will be de­veloped at last.

基于蓝光LED的水下双向通信测试系统设计

蓝光在海水中具有良好的传输特性,使得水下可见光通信的优势明显,针对水下高速无线通信的需求,提出了一种基于蓝光LED的水下双向通信测试系统设计方案,完成了系统的整体硬件设计,并进行了下水测试。结果表明,该系统实现了水下通信链路60 MHz的带宽拓展,并在水下5 m距离实现了双向可靠通信。该系统验证了水下高速可见光双向通信的可行性,为今后研究基于可见光的水下高速无线通信奠定了基础。