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Optical communication has been employed in a wide range of applications, including the terrestrial, submarine, inter-satellite, and even space communication. It is particularly successful in fiber-based communication networks that vastly reshape modern life through the Internet. With the intensified activities such as undersea resource exploration, ecosystem monitoring, and recreation, underwater is an exciting new arena for optical wireless communication (OWC). In this paper, we review the recent progress of underwater and water-air OWC systems. Channel characterization, communication system design, and performance investigations are given. Critical limitations and effective mitigation methods to overcome the influence of bubbles and waves are presented. We also address the current issues for proper performance comparison under different wave conditions. With further research in channel modeling, device innovation, and system design optimization, practical and robust underwater and water-air OWC systems can be realized.

In a water-air optical wireless communication (OWC) channel, dynamic ocean waves may significantly deflect the light beam from its original direction, thus deteriorating the communication performance. In this letter, a beam tracking system to mitigate wave-induced communication degradation is experimentally demonstrated. By employing the beam tracking on the PAM6 system, a maximum of 486% improvement (from 140 Mb/s to 820 Mb/s) in throughput is realized at a 1.2-m air distance for an average wave slope changing rate of 0.34 rad/s. For PAM4 signals, the packet loss rate reduces significantly from 75% to 11%, and a maximum throughput of 1.25 Gb/s is achieved.

We propose a novel carrier phase recovery (CPR) method in probabilistically-shaped (PS) coherent systems. This method targets to make the distribution of the received samples match that of the desirable constellation, which is realized by minimizing the Kullback-Leibler (KL) divergence between these two distributions. This is in contrast to conventional blind phase searching (BPS) or maximum likelihood estimation that targets to match the received samples with the transmitted symbols. Consequently, the proposed method avoids pre-decisions or feedback of data decisions in conventional methods. The distribution of the target constellation is the same as that used in soft-decision forward error correction (SD-FEC) and can be implemented via look-up tables. This distribution is re-used in the KL-based CPR, thus avoiding additional overhead and the calculation of the Euclidean distance as in conventional BPS. Minimizing the KL divergence can be achieved via a recursive algorithm, blind phase searching, or the combination of both. Simulation and experimental results show that the proposed methods outperform principal component analysis (PCA), PCA+BPS and Kalman filtering, and achieve similar performance as conventional 2-stage BPS but with greatly reduced complexity. The proposed methods are also format transparent, suitable for parallel processing in a feedforward manner, and so are promising for CPR especially in SD-FEC based PS coherent systems.

We propose a novel subcarrier and power allocation method to improve both the throughput and user fairness in orthogonal frequency division multiplexing non-orthogonal multiple access visible light communication (OFDM-NOMA VLC) systems. The proposed method considers various constraints in practical VLC systems, including bandwidth limitation, non-negative optical signal, peak power for eye safety, and quality of service. The method adopts the log utility function to improve the user fairness and is applicable to cases where the number of multiplexed users on each subcarrier is not equal to the total user number due to the decoding complexity of NOMA. The problem is generalized as a non-convex one and is solved by the genetic algorithm. Monte Carlo simulations show that at a signal to interference noise ratio of 2.5 dB, the throughput of the proposed method is 11%, 22%, and 125% higher than the conventional enhanced power allocation, the fixed power allocation, and the gain ratio power allocation algorithms, while the fairness is improved by 23%, 30%, and 49%, respectively.

Underwater optical wireless communication (UOWC) is of great interest to the academic and the industry community. In this article, we propose a low-complexity and effective joint transmitter-side digital signal processing (DSP) including geometric shaping, time-domain tone reservation (TR), and clipping. The peak-to-average power ratio reduction performance and implementation complexity of the proposed time-domain TR are extensively analyzed. We then develop a real-time 2.2-Gb/s system, which is optimized for its shaping ratio and clipping ratio. With the help of the efficient DSPs, an 8-dB received optical power enhancement is realized under the bit-error-rate threshold of 3.8 × 10−3. We successfully demonstrate a time-multiplexed four 4K video transmission in real-time using the proposed scheme over a 3.6-m underwater and 8-m free-space channel. The implementation details, as well as the analyses of system stability, resource utilization, and latency, are presented. The results validate the feasibility and effectiveness of the proposed scheme and a 2.2-Gbit/s real-time OWC system is demonstrated for a water-air communication link.

In this paper, a real-time software-reconfigurable orthogonal frequency division multiplexing based non-orthogonal multiple access (OFDM-NOMA) scheme with dynamic power allocation is proposed and experimentally demonstrated for hybrid in-house access over single-mode fiber (SMF) and visible light communication (VLC). Such hybrid access can provide both wired and wireless transmission, which can accommodate the increasingly heterogeneous in-home network and realize seamless coverage. The fiber link is used to provide high-speed and stable transmission for a wired user whilst the VLC link can support flexible and secured wireless access. With the help of power-domain multiplexing based NOMA, the power ratio between both the fiber user and the VLC user can be adjusted in a software-reconfigurable manner, which can satisfy various user demands and also ensure user fairness. Experimental results validate that an aggregate data rate of 1.84 Gbit/s can be achieved in a 1-GSa/s hybrid access system. In addition, it is demonstrated that the power ratio between the fiber user and the VLC user can be adjusted in real-time, facilitating the optimal overall system bit error rate (BER) performance and hence the balanced user fairness. The proposed real-time software-reconfigurable dynamic power allocation scheme for OFDM-NOMA is a promising scheme for the future heterogeneous in-house network, as it allows the flexible power allocation according to users' demands and enables better user fairness.

In this article, we propose and experimentally demonstrate the full-duplex channel-aware adaptive physical-layer network (APNC) coding over orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) to boost the capacity of relay-assisted VLC networks. The network encoding is realized by detecting signals coming from different terminal nodes simultaneously with a single photodiode at the relay node, which then achieves the amplify-and-forward function. At the receiver of each terminal node, the desired signal can be reconstructed by applying the corresponding APNC decoding to the detected interfered signal from the relay node. The use of adaptively loaded OFDM not only addresses the inherent frequency fading issue in VLC systems but also relaxes the synchronization issue at the relay node. The detailed principles of the proposed full-duplex channel-aware APNC scheme are presented, and its performance is experimentally investigated in a two-way-relay (TWR) VLC network. Experimental results show that the proposed scheme exhibits superior BER performance to other existing network scheduling schemes. For a TWR-VLC network with a fixed 600-Mb/s throughput, we achieve around two orders of magnitude reductions in the bit-error-rate (BER) by using the proposed channel-aware APNC scheme compared to other schemes, and BERs of 3.96 × 10−4 and 5.32 × 10−4 are attained for the two transmission links in the TWR network, respectively.

Photovoltaic (PV) modules have been employed in visible light communication (VLC) for simultaneous energy harvesting and data reception. A PV-based receiver features easy optical alignment and self-powered operation. It is commonly assumed that PV modules in VLC have a linear optical-electrical response, which is generally true under high illumination levels. This paper will illustrate the exacerbated PV's nonlinear distortion when under typical indoor illumination. The nonlinearity of a PV module for different numbers of PV cells is also characterized. We investigated the transmission performance of a 1-Mbit/s PAM4 signal under different illuminance. Experimental results show that the bit-error rate (BER) first decreases and then increases with the increasing illuminance. Thus, an optimal illuminance to minimize BER exists. In addition, we demonstrated two distortion mitigation methods, namely localized distortion compensation lighting and post-distortion compensation. BER reduction from 3.2x10-1 to 2.6x10-3 and 2.8x10-2 to 1.5x10-2 are achieved with the two respective schemes., 4 pages, 8 figures

To alleviate the error propagation issue of non-orthogonal multiple access (NOMA) based visible light communication (VLC) systems, we propose and experimentally demonstrate the subcarrier pairwise coding (PWC) based NOMA-VLC. Without extra coding overhead, the computation-efficient subcarrier PWC can significantly improve the overall system performance by pairing the subcarriers of high signal-to-noise ratios (SNRs) with those of low SNRs. Under different transmitted baud rate from 150 MSa/s to 225 MSa/s, the bit error rate (BER) performance with and without PWC is investigated in a NOMA-VLC system. Experimental results show that with the optimized power ratio between the NOMA users, up to 3.3-dB Q2-factor improvement can be achieved by the proposed scheme.

In a mobile phone-based optical camera communication (OCC) system, the spatial and time dispersions, due to inter-pixel crosstalk and exposure time overlapping, deteriorate the transmission performance considerably. In this letter, a double-equalization method is studied and experimentally demonstrated in a rolling-shutter based OCC system, showing a record-high transmission data rate for a single white light-emitting diode (LED) system. Moreover, we further propose an improved technique to reduce the overhead of the signal extraction process while achieving similar performance. The effect of the normalization factor in the gamma correction processing is also investigated.

We propose and investigate discrete-circulant transform spread orthogonal frequency division multiplexing (DCrT-S-OFDM) for bandwidth-limited optical systems. We derive the conditions for optimal designs of DCrT-S-OFDM in the noise limited scenarios and verify its bit error rate advantage over conventional OFDM. We also analytically investigate the performance of DCrT-S-OFDM under the effect of inter-symbol interference (ISI) and inter-carrier interference (ICI), and prove its performance advantage over discrete-Fourier-transform spread OFDM (DFT-S-OFDM). Based on the developed theory, we propose several novel designs of DCrT-S-OFDM to achieve optimal or near-optimal performance. Extensive simulations have been performed in a visible light communication (VLC) system to validate the findings. The results agree with the theory and verify the performance advantage of DCrT-S-OFDM over other schemes. Finally, a ∼650-Mbit/s VLC system with ∼70-MHz 3-dB system bandwidth was experimentally demonstrated. The results also confirm that DCrT-S-OFDM outperforms conventional OFDM and DFT-S-OFDM, making the proposed DCrT-S-OFDM a promising solution for optical systems using low-bandwidth devices to achieve high data rates.

We propose and demonstrate a real-time software reconfigurable dynamic power-and-subcarrier allocation scheme for orthogonal frequency division multiplexing based non-orthogonal multiple access (OFDM-NOMA) in visible light communications (VLC) systems. The key digital signal processing algorithms for real-time OFDM-NOMA implementations are illustrated in details. With the help of the software reconfigurable technique, the power ratios of different sub-bands can be dynamically adjusted according to the channel conditions, resulting in superior user fairness and optimal transmission performance. Moreover, the subcarrier number of each user can also be flexibly controlled to fulfill the corresponding capacity demands. To validate the effectiveness of the proposed scheme, we consider different user scenarios with various path losses for extensive performance evaluations. Experimental results show that 1.84-Gbit/s real-time OFDM-NOMA transmission can achieve stable operation over several hours, wherein the BER is below the hard-decision forward error correction threshold of 3.8×10−3. The proposed scheme provides a promising solution for practical implementations of multi-user VLC, as it is markedly adaptive to dynamic capacity demands while maintaining better user fairness and system flexibility and stability.