6G networks are expected to provide extremely high capacity and satisfy emerging applications, but current frequency bands may not be sufficient. Moreover, 6G will provide superior coverage by integrating space/air/underwater networks with terrestrial networks, given that traditional wireless communications are not able to provide high-speed data rates for nonterrestrial networks. Visible light communication (VLC) is a high-speed communication technique with an unlicensed frequency range of 400-800 THz and can be adopted as an alternative approach to solving these problems. In this article, we present the prospects and challenges of VLC in 6G in conjunction with its advances in high-speed transmissions. Recent hot research interests, including new materials and devices, advanced modulation, underwater VLC (UVLC), and signal processing based on machine learning, are also discussed. It is envisaged that VLC will become an indispensable part of 6G given its high-speed transmission advantages and will cooperate with other communication methods to benefit our daily lives.

Visible Light Communication in 6G: Advances, Challenges, and Prospects

Recent progress in and perspectives of underwater wireless optical communication

Underwater wireless optical communication (UWOC) has attracted increasing interest in various underwater activities because of its order-of-magnitude higher bandwidth compared to acoustic and radio-frequency technologies. Testbeds and pre-aligned UWOC links were constructed for physical layer evaluation, which verified that UWOC systems can operate at tens of gigabits per second or close to a hundred meters of distance. This holds promise for realizing a globally connected Internet of Underwater Things (IoUT). However, due to the fundamental complexity of the ocean water environment, there are considerable practical challenges in establishing reliable UWOC links. Thus, in addition to providing an exhaustive overview of recent advances in UWOC, this article addresses various underwater challenges and offers insights into the solutions. In particular, oceanic turbulence, which induces scintillation and misalignment in underwater links, is one of the key factors in degrading UWOC performance. Novel solutions are proposed to ease the requirements on pointing, acquisition, and tracking (PAT) for establishing robustness in UWOC links. The solutions include light-scattering-based non-line-of-sight (NLOS) communication modality as well as PAT-relieving scintillating-fiber-based photoreceiver and large-photovoltaic cells as the optical signal detectors. Naturally, the dual-function photovoltaic–photodetector device readily offers a means of energy harvesting for powering up the future IoUT sensors.

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A Review on Practical Considerations and Solutions in Underwater Wireless Optical Communication

A wavelength-division-multiplexing (WDM) four-level pulse amplitude modulation (PAM4) free-space optical (FSO)–underwater wireless optical communication (UWOC) integrated system with a channel capacity of 100 Gb/s is proposed and attainably demonstrated. Analytic results reveal that 1.8-GHz 405-nm blue-violet-light and 1.7-GHz 450-nm blue-light laser diodes (LDs) with two-stage light injection and optoelectronic feedback techniques are competently adopted for 100 Gb/s PAM4 signal transmission through a 500-m free-space transmission with 5-m clear ocean underwater link. Combining dual-wavelength WDM scenario with PAM4 modulation, the channel capacity of FSO–UWOC integrated systems is significantly enhanced with an aggregate transmission rate of 100 Gb/s (25 Gbaud PAM4/wavelength × 2 wavelengths). With doublet lenses in FSO, laser beam reducer and transmissive spatial light modulator in UWOC, a sufficiently low bit error rate of 10−9 and acceptable PAM4 eye diagrams are acquired. This demonstrated 100 Gb/s PAM4 FSO–UWOC integrated system with a WDM scenario is advantageous for the enhancement of a high-speed optical wireless link with long-reach transmission.

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A WDM PAM4 FSO–UWOC Integrated System With a Channel Capacity of 100 Gb/s

InGaN Micro-LED Array Enabled Advanced Underwater Wireless Optical Communication and Underwater Charging

In this paper, we experimentally demonstrate an underwater wireless optical communication (UWOC) system using a 450-nm gallium nitride (GaN) laser and adaptive bit-power loading discrete multitone (DMT). To enhance the system capacity, a post nonlinear equalizer based on the simplified Volterra series is employed at the receiver to mitigate the nonlinear impairments of the UWOC system. By combining the adaptive bit-power loading with nonlinear equalization, 7.33-Gb/s DMT-modulated UWOC under 15-m tap water is achieved at a bit error rate below the 7% hard-decision forward error correction (FEC) limit 3.8 × 10–3. The electrical signal bandwidth is 1.25 GHz, which corresponds to an electrical spectrum efficiency of ∼6 bit/s/Hz. The capacity-distance product reaches 109.95 Gb/s-m in a single channel UWOC system with tap water. Compared with the linear equalization case, the system capacity at the FEC limit for 15-m underwater transmission is improved by ∼18% with the nonlinear equalization. Furthermore, the impact of turbidity on the performance of UWOC system is investigated by measuring the signal-to-noise ratio (SNR) under different suspension concentrations of Al(OH)3 and Mg(OH)2. The results show that significant SNR gains (>3 dB for transmission distance up to 11 m) can be obtained by the nonlinear equalization over a wide range of water turbidity levels representing “clear ocean,” “coastal ocean,” and “harbor water,” which demonstrates the robustness of the proposed scheme in various ocean environments.

Demonstration of 15-M 7.33-Gb/s 450-nm Underwater Wireless Optical Discrete Multitone Transmission Using Post Nonlinear Equalization

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.

Probabilistic constellation shaping (PCS) is utilized to approach the channel capacity limit in discrete multitone (DMT) transmission for underwater optical wireless communication (UOWC) system. A fixed quadrature amplitude modulation (QAM) format with various probabilistic distributions is individually allocated for different subcarriers to obtain achievable maximum channel capacity in accordance with the pre-estimated signal-to-noise ratio. By using a 450-nm directly modulated laser diode (LD) with an available modulation bandwidth of ∼2.75 GHz, DMT with PCS technique is experimentally realized with a net data rate of 18.09 Gbit/s over 5 m, 17.21 Gbit/s over 25 m, and 12.62 Gbit/s over 35 m underwater transmission, giving substantial capacity improvement of 32.22%, 30.03%, and 27.55%, respectively, in comparison with the widely used regular QAM formats in DMT with bit-power loading scheme. The figure of merit of the UOWC system in terms of entropy, generalized mutual information (GMI), and normalized GMI are also presented. To the best of our knowledge, this is the first time to employ PCS-QAM-DMT in a UOWC system, and it is also the highest data rate ever reported for a single LD in UOWC.

Discrete multitone transmission for underwater optical wireless communication system using probabilistic constellation shaping to approach channel capacity limit.

In this paper, a low-complexity two-level chaotic encryption scheme is introduced and experimentally demonstrated to improve the physical layer security of a 450-nm laser underwater optical wireless communication (UOWC) system using discrete Fourier transform spread discrete multi-tone (DFT-S DMT) modulation. In the first encryption stage, the original bit stream is encrypted with a chaotic sequence based on a one-dimensional Logistic map. In the second encryption stage, the real and imaginary components of the DFT-S symbols are further encrypted with a pair of separate chaotic sequences, which are generated from a two-dimensional Logistic iterative chaotic map with infinite collapse (2D-LICM). The experimental results indicate that the encryption operation has no negative effect on the performance of the proposed UOWC system. For chaotic encryption, the DFT-S DMT gives a better performance than the DMT scheme under different water turbidities. 55-m/4.5-Gbps and 50-m/5-Gbps underwater transmissions are successfully demonstrated by the chaotic encrypted DFT-S DMT scheme. To the best of our knowledge, this is the first time to verify the feasibility of chaotic encryption in a high-speed UOWC system.

Experimental demonstration of 50-m/5-Gbps underwater optical wireless communication with low-complexity chaotic encryption

In this paper, a wideband photomultiplier tube (PMT)-based underwater wireless optical communication (UWOC) system is proposed and a comprehensive experimental study of the proposed PMT-based UWOC system is conducted, in which the transmission distance, data rate, and attenuation length (AL) is pushed to 100.6 meters, 3 Gbps, and 6.62, respectively. The receiver sensitivity at 100.6-meter underwater transmission is as low as -40 dBm for the 1.5-Gbps on-off keying (OOK) modulation signal. To the best of our knowledge, this is the first Gbps-class UWOC experimental demonstration in >100-meter transmission that has ever been reported. To further minimize the complexity of channel equalization, a sparsity-aware equalizer with orthogonal matching pursuit is adopted to reduce the number of the filter coefficients by more than 50% while keeping slight performance penalty. Furthermore, the performance of the proposed PMT-based UWOC system in different turbidity waters is investigated, which shows the robustness of the proposed scheme. Thanks to the great sensitivity (approaching the quantum limit) and a relatively larger effective area, benefits of misalignment tolerance contributed by the PMT is verified through a proof-of-concept UWOC experiment.

Chao Fei

Papers

Demonstration of 15-M 7.33-Gb/s 450-nm Underwater Wireless Optical Discrete Multitone Transmission Using Post Nonlinear Equalization

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

Discrete multitone transmission for underwater optical wireless communication system using probabilistic constellation shaping to approach channel capacity limit.

Experimental demonstration of 50-m/5-Gbps underwater optical wireless communication with low-complexity chaotic encryption

100-m/3-Gbps underwater wireless optical transmission using a wideband photomultiplier tube (PMT).