Recent progress in and perspectives of underwater wireless optical communication

In this paper, we demonstrate a novel Gaussian kernel-aided deep neural network (GK-DNN) equalizer that can effectively compensate for the high nonlinear distortion of underwater PAM8 visible light communication (VLC) channels. The application of a Gaussian kernel can reduce the necessary training iterations to 47.06%, enabling it to outperform the traditional DNN equalizer. At the same time, a novel design strategy with respect to the structure of the GK-DNN equalizer is proposed, which can effectively save computing resources and reduce the data volume of the necessary training data set. By using the GK-DNN equalizer, a 1.5 Gbps PAM8 VLC system over 1.2-m underwater transmission is successfully demonstrated.

Gaussian kernel-aided deep neural network equalizer utilized in underwater PAM8 visible light communication system.

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.

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

In this paper, we experimentally demonstrate a 450-nm laser underwater wireless optical transmission system by using adaptive bit-power loading discrete multi-tone (DMT) and Volterra series based post nonlinear equalization. Post nonlinear equalization mitigates the nonlinear impairment of the UWOC system. By incorporating post nonlinear equalization with a 3rd-order diagonal plane kernel, the received signal-to-noise ratio (SNR) can be improved by ~2 dB compared with a linear equalization method. The measured transmission capacity of the UWOC system is 16.6 Gbps over 5 m, 13.2 Gbps over 35 m, and 6.6 Gbps over 55 m tap water channel, with bit error rates (BERs) below the standard hard-decision forward error correction (HD-FEC) limit of 3.8 × 10−3. The used electrical signal bandwidth is 2.75 GHz, corresponding to electrical spectrum efficiency of ∼6 bit/s/Hz. The distance-datarate product reaches 462 Gbps*m at 35 m tap water transmission. To the best of our knowledge, both the data rate and distance-data rate product are the largest reported for single laser diode.

16.6 Gbps data rate for underwater wireless optical transmission with single laser diode achieved with discrete multi-tone and post nonlinear equalization.

Group-III-nitride superluminescent diodes (SLDs) are emerging as light sources for white lighting and visible light communications (VLC) owing to their droop-free, low speckle noise, and large modulation bandwidth properties. In this paper, we discuss the development of GaN-based visible SLDs, and analyze their electro-optical properties by studying the optical power-bandwidth products and injection current densities. The significant progress in blue SLDs and their applications for white light VLC is highlighted. A blue SLD, with an optical power of >100 mW and large PBP of 536 mW·nm, is utilized to generate white light, resulting in a high color rendering index (CRI) of 88.2. In a modulation experiment designed for an SLD-based VLC system, an on–off keying scheme exhibits a 1.2 Gbps data rate, with a bit error rate of 1.8 × 10−3, which satisfies the forward error correction criteria. A high data rate of 3.4 Gbps is achieved using the same SLD transmitter, by applying the 16-quadrature-amplitude-modulation (16-QAM) discrete multitone modulation scheme for high-speed white light communication. The results reported here unequivocally point to the significant performance and versatility that GaN-based SLDs could offer for beyond-5G implementation, where white lighting and high spectral efficiency VLC systems can be simultaneously implemented.

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Group-III-Nitride Superluminescent Diodes for Solid-State Lighting and High-Speed Visible Light Communications

High speed underwater visible light communication system based on LED employing maximum ratio combination with multi-PIN reception

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

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.

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

A femtosecond hybrid mode-locked short cavity dye laser

Visible light communication(VLC) has become a state-of-art technology in wireless communication which take the advantages of unlimited bandwidth, green source with no pollution, combination communication with illumination. However, when we need to transmit for a long distance or at a high data rate, the underwater visible light communication (UVLC) system will suffer from severe nonlinearity. In this paper, we proposed a gray-coding square (GCS) 8QAM based underwater VLC system which performs better than normal rectangular QAM and circular (7,1) 8QAM. Different from circular (7,1) which own the maximum minimum Euclidean distance, the GCS 8QAM own the thoroughly gray-coding of every constellation point. The GCS-8QAM can achieve a higher data rate of 1.62Gb/s at 240mA bias current and 1.4Vpp than circular(7,1) and rectangle, and more operation range at bigger bias current and V pp , which prove that GCS-8QAM is more suitable for nonlinear situations and higher data rate transmission in UVLC system.

Yuanfan Liu

Papers

High speed underwater visible light communication system based on LED employing maximum ratio combination with multi-PIN reception

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

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

A femtosecond hybrid mode-locked short cavity dye laser

Mitigating Nonlinearity Characteristics of Gray-Coding Square 8QAM in Underwater VLC system