| 1. |
- Chen, Ruilin, et al.
(author)
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Experimental demonstration of real-time optical DFT-S DMT signal transmission for a blue-LED-based UWOC system using spatial diversity reception
- 2023
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In: Applied Optics. - : Optica Publishing Group. - 1559-128X .- 2155-3165. ; 62:3, s. 541-551
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Journal article (peer-reviewed)abstract
- Underwater wireless optical communication (UWOC) has broad prospects in underwater real-time applications. We design and experimentally demonstrate a real-time discrete Fourier transform spread discrete multi-tone (DFT-S DMT) signal transmission based on a field programmable gate array for a blue-LED-based UWOC system with a data rate of up to 30 Mbps over a 15-m underwater channel. The architecture and usage of an on-chip resource as well as power consumption are analyzed and discussed. To reduce the impacts of multipath fading and received intensity fluctuation, spatial diversity reception is also introduced. Furthermore, the receiver sensitivity at a speci-fied bit error rate (BER) threshold and the quality of the images are evaluated using three types of Reed-Solomon (RS) codes. At the BER threshold of 10-4, over 2.8-dB receiver sensitivity improvement is obtained by the DFT-S DMT scheme with the RS (64, 56) code as compared to the uncoded one at the data rate of 30 Mbps. The perform-ance of BER, color difference, and structural similarity in the image transmission of DFT-S DMT is superior to that of the conventional hard clipping quadrature amplitude modulation DMT in a high-data-rate region because of the low peak-to-average-power ratio and ability to mitigate high-frequency fading in a band-limited UWOC system. With schemes of the RS code, DFT-S, and diversity reception, error-free transmission of images is achieved over a 15-m water channel. The proposed UWOC system has the advantages of low power consumption and porta-bility, which foresees a bright future in underwater applications over short to moderate distances.
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| 2. |
- Du, Ji, et al.
(author)
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Experimental demonstration of 50-m/5-Gbps underwater optical wireless communication with low-complexity chaotic encryption
- 2021
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In: Optics Express. - : The Optical Society. - 1094-4087. ; 29:2, s. 783-796
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Fei, Chao, et al.
(author)
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100-m/3-Gbps underwater wireless optical transmission using a wideband photomultiplier tube (PMT)
- 2022
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In: Optics Express. - : The Optical Society. - 1094-4087. ; 30:2, s. 2326-2337
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Journal article (peer-reviewed)abstract
- 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 (00K) 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 LIWOC 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 LIWOC experiment.
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| 4. |
- Fei, Chao, et al.
(author)
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Underwater wireless optical communication utilizing low-complexity sparse pruned-term-based nonlinear decision-feedback equalization
- 2022
-
In: Applied Optics. - : Optica Publishing Group. - 1559-128X .- 2155-3165. ; 61:22, s. 6534-6543
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Journal article (peer-reviewed)abstract
- The nonlinearity of the light-emitting diode (LED) in underwater wireless optical communication (UWOC) systems is considered the one major limiting factor that degrades the system's performance. Volterra series-based nonlinear equalization is widely employed to mitigate such nonlinearity in communication systems. However, the conventional Volterra series-based model is of high complexity, especially for the nonlinearity of higher-order terms or longer memory lengths. In this paper, by pruning away some negligible beating terms and adaptively picking out some of the dominant terms while discarding the trivial ones, we propose and experimentally demonstrate a sparse pruned-term-based nonlinear decision-feedback equalization (SPT-NDFE) scheme for the LED-based UWOC system with an inappreciable performance degradation as compared to systems without the pruning strategy. Meanwhile, by replacing the self/cross beating terms with the terms formed by the absolute operation of a sum of two input samples instead of the product operation terms, a sparse pruned-term-based absolute operation nonlinear decision-feedback equalization (SPT-ANDFE) scheme is also introduced to further reduce complexity. The experimental results show that the SPT-NDFE scheme exhibits comparable performance as compared to the conventional NDFE (nonlinear decision-feedback equalization) scheme with lower complexity (the nonlinear coefficients are reduced by 63.63% as compared to the conventional NDFE scheme). While the SPT-ANDFE scheme yields suboptimal performance with further reduced complexity at the expense of a slight performance degradation, the robustness of the proposed schemes in different turbidity waters is experimentally verified. The proposed channel equalization schemes with low complexity and high performance are promising for power/energy-sensitive UWOC systems.
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| 5. |
- Hong, Xiaojian, et al.
(author)
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Experimental Demonstration of 55-m/2-Gbps Underwater Wireless Optical Communication Using SiPM Diversity Reception and Nonlinear Decision-Feedback Equalizer
- 2022
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In: IEEE Access. - : Institute of Electrical and Electronics Engineers (IEEE). - 2169-3536. ; 10, s. 47814-47823
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Journal article (peer-reviewed)abstract
- Underwater wireless optical communication (UWOC) is considered as an enabling technology with a mass of potential applications. The silicon photomultiplier (SiPM) exhibits a bright prospect for UWOC thanks to the traits of low-light detection capability, low-voltage operation, and superior operability. However, the performance of the SiPM-based UWOC system is severely degraded by the dead-time caused nonlinear response. In this paper, to mitigate the dead-time induced nonlinear distortion and explore the achievable capacity of the newly developed SiPM, we propose and experimentally demonstrate a 55-m / 2-Gbps UWOC system by virtue of SiPM diversity reception and nonlinear decision-feedback equalizer (NDFE). The performance of NDFE is superior to that of the conventional decision-feedback equalizer (DEE), and NDFE with a pruning factor of 5 declares similar performance as that without pruning strategy, while the number of the nonlinear equalizer can be reduced by similar to 31.8%. Significant performance improvement is also obtained by the proposed scheme under different turbidity waters. The measured data rate is pushed from 1 Gbps to 2 Gbps with a receiver sensitivity as low as -41.96 dBm, which to the best of our knowledge is the largest data rate ever achieved using the off-the-shelf SiPM among the reported UWOC works. In accordance with the receiver sensitivity and the model of optical propagation in the water channel, the maximum attainable distance/data rate is predicted to be 147 m/ 1 Gbps and 128 m/2 Gbps with the proposed scheme. The research results are promising for long-reach and high-speed UWOC.
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| 6. |
- Jiang, Chao, et al.
(author)
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An interpretable framework of data-driven turbulence modeling using deep neural networks
- 2021
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In: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 33:5
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Journal article (peer-reviewed)abstract
- Reynolds-averaged Navier-Stokes simulations represent a cost-effective option for practical engineering applications, but are facing ever-growing demands for more accurate turbulence models. Recently, emerging machine learning techniques have had a promising impact on turbulence modeling, but are still in their infancy regarding widespread industrial adoption. Toward their extensive uptake, this paper presents a universally interpretable machine learning (UIML) framework for turbulence modeling, which consists of two parallel machine learning-based modules to directly infer the structural and parametric representations of turbulence physics, respectively. At each phase of model development, data reflecting the evolution dynamics of turbulence and domain knowledge representing prior physical considerations are converted into modeling knowledge. The data- and knowledge-driven UIML is investigated with a deep residual network. The following three aspects are demonstrated in detail: (i) a compact input feature parameterizing a new turbulent timescale is introduced to prevent nonunique mappings between conventional input arguments and output Reynolds stress; (ii) a realizability limiter is developed to overcome the under-constrained state of modeled stress; and (iii) fairness and noise-insensitivity constraints are included in the training procedure. Consequently, an invariant, realizable, unbiased, and robust data-driven turbulence model is achieved. The influences of the training dataset size, activation function, and network hyperparameter on the performance are also investigated. The resulting model exhibits good generalization across two- and three-dimensional flows, and captures the effects of the Reynolds number and aspect ratio. Finally, the underlying rationale behind prediction is explored.
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| 7. |
- Wang, Yuan, et al.
(author)
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Spectrally Efficient Non-Orthogonal Discrete Multi-Tone Transmission for Underwater Wireless Optical Communication With Low-Complexity High Performance ICI Mitigation
- 2023
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In: Journal of Lightwave Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0733-8724 .- 1558-2213. ; 41:19, s. 6288-6299
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Journal article (peer-reviewed)abstract
- In this article, a novel spectrally efficient non-orthogonal discrete multi-tone (NODMT) system is proposed for the bandwidth-starved underwater wireless optical communication (UWOC) to compensate for the insufficient bandwidth of components. Different from other non-orthogonal systems with complex-valued correlation matrices, the designed NODMT system has a real-valued correlation matrix, which is significant in reducing the complexity of decoders, e.g., the complexity of the proposed NODMT with iterative detection (ID) algorithm can be reduced by 50% as compared to the conventional non-orthogonal system with ID. However, in circumstances of higher spectrum efficiency (SE), the non-orthogonality is intensified and inter-carrier interference (ICI) becomes severer, leading to the sharp deterioration of ID decoding capability. The traditional sphere decoding (SD) algorithm cannot solve this issue because Cholesky decomposition only works with the positive definite matrix. Therefore, we propose a novel SD algorithm based on QR decomposition, named QRSD, which effectively compensates for the insufficient decoding capability of the ID algorithm in the case of higher SE. By combining ID and QRSD methods, over 40% complexity reduction and nearly identical BER performance with QRSD are achieved. Through a proof-of-concept UWOC experiment, a 3.31-Gbps NODMT transmission with up to 24.44% data rate promotion is demonstrated, and the feasibility/effectiveness of the proposed ID-QRSD algorithm in different turbidity water is verified. Moreover, we have experimentally realized 75-m/2.66-Gbps and 60-m/3.02-Gbps underwater transmission with 15% and 25% bandwidth savings, respectively. This UWOC system with high SE shows great potential in the bandwidth-limited UWOC and underwater internet of things (UIoT).
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