The effect of turbulence on NLOS underwater wireless optical communication channels [Invited]

Conventional line-of-sight underwater wireless optical communication(UWOC) links suffer from huge signal fading in the presence of oceanic turbulence due to misalignment, which is caused by variations in the refractive index in the water. Non-line-of-sight(NLOS) communication, a novel underwater communication configuration,which has eased the requirements on the alignment, is supposed to enhance the robustness of the UWOC links in the presence of such turbulence. This Letter experimentally and statistically studies the impact of turbulence that arises from temperature gradient variations and the presence of different air bubble populations on NLOS optical channels. The results suggest that temperature gradient-induced turbulence causes negligible signal fading to the NLOS link. Furthermore, the presence of air bubbles with different populations and sizes can enhance the received signal power by seizing the scattering phenomena from an ultraviolet 377 nm laser diode.     

Performance Analysis and Relay Location Research of OFDM Free-Space Optical Communication Systems Under Moderate and Strong Turbulence

In this paper, based on the Gamma–Gamma channel model for describing moderate and strong atmospheric turbulence, we study the relay location of serial decode-and-forward relay systems and parallel decode-and-forward relay systems in free-space optical (FSO) communication. According to the orthogonal frequency-division multiplexing modulation (OFDM) and coherent detection demodulation technology, we develop a novel statistical fading channel model for relay FSO systems by incorporating the atmospheric turbulence, pointing errors, and path loss effects. Based on this channel model, we derive the closed-form expression of the outage probability in the FSO serial relay system and parallel relay system, using the Meijer G-function. The serial decode-and-forward relay system with different relay locations and parallel decode-and-forward relay system, which consider different number of links and different relay locations, are simulation analyzed under moderate and strong atmospheric turbulence. The performance of serial relay systems and parallel relay systems in free-space optical communication can be improved by optimizing the relay location.     

Effects of atmospheric turbulence and building sway on optical wireless-communication systems

Urban optical wireless communication (UOWC) systems are considered a last-mile technology. UOWC systems use the atmosphere as a propagation medium. To provide a line of sight the transceivers are placed on high-rise building. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways distort the alignment between transmitter and receiver, causing pointing errors, the outcome of which is fading of the received signal. Furthermore, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, resulting in impaired link performance. A bit-error probability (BEP) model is developed that takes into account both building sway and turbulence-induced log amplitude fluctuations (i.e., fading of signal intensity) in the regime in which the receiver aperture, D0, is smaller than the turbulence coherence diameter, d0. It is assumed that the receiver has knowledge about the marginal statistics of the signal fading and the instantaneous signal-fading state.     

Performance characterization of two-way multi-hop underwater networks in turbulent channels [Invited]

This Letter investigates the performance of the two-way multi-hop system for underwater optical wireless communications. With the decode-and-forward(DF) relaying, the two-way multi-hop system is modeled, where the effects of absorption, scattering, and oceanic turbulence are all taken into account. An exact closed-form expression for outage probability is derived under the assumption that the oceanic turbulence obeys a log-normal distribution. Numerical results demonstrate the impacts of various parameters on the outage performance and indicate that the two-way multi-hop system significantly improves the performance in comparison to both the one-way multi-hop system and the two-way two-hop system.     

Aperture-averaged scintillation index and fade statistics in weak oceanic turbulence

With the rapid demand for underwater optical communication(UOC), studies of UOC degradation by oceanic turbulence have attached increasing attention worldwide and become a research hot-spot in recent years. Previous studies used a simplified and inaccurate oceanic turbulence spectrum, in which the eddy diffusivity ratio between temperature and salinity is assumed to be unity and the outer scale of turbulence is assumed to be infinite. However, both assumptions are not true in most of the actual marine environments. In this paper, based on the Rytov theory in weak turbulence, we derive analytical expressions of "the aperture-averaged scintillation index"(SI) for both plane and spherical waves, which can clearly demonstrate how SI is influenced by several key factors in UOC. Then, typical fade statistics of the UOC system in weak turbulence is discussed including the probability of fade, the expected number of fades per time, the mean fade time,signal-to-noise ratio and bit error rate. Our results show that spherical wave is preferable in the UOC system in weak turbulence compared to plane wave, and the aperture-averaged effect has a significant impact on UOC system's performance.Our results can be used to determine those key parameters for designing the UOC system over reasonable ranges.     

海洋湍流中光波特征参量和短期光束扩展的研究

Nikishov等建立的海洋湍流功率谱模型中,假设了海水有着稳定的分层.但是,实际海水通常不是稳定分层的,温度与盐度的涡流扩散率是不相等的.2017年,Elamassie等建立了考虑这些因素的更合理的海洋湍流功率谱模型.湍流介质中光波空间相干长度等基本特征参量在表征湍流强度和光传输相位校正技术等方面起着重要作用.本文基于Elamassie海洋湍流功率谱模型,重新推导出了海洋湍流中光波结构函数、光波空间相干长度和Fried参数的解析公式,并校验了所得公式的正确性.研究发现：当温度变化引起的光学湍流占主导地位时,Nikishov海洋湍流功率谱模型把湍流强度低估了;当盐度变化引起的光学湍流占主导地位时,Nikishov海洋湍流功率谱模型把湍流强度高估了.基于Elamassie海洋湍流功率谱模型,本文推导出了高斯光束短期光束扩展的半解析公式,并验证了其正确性.研究还表明：海水稳定分层与否,短期光束扩展差异很大.本文研究结果对水下湍流环境中的光通信、成像和传感等应用具有重要意义.     

High-speed underwater wireless optical communications: from a perspective of advanced modulation formats [Invited]

In this paper, recent advances in underwater wireless optical communication(UWOC) are reviewed for both LED-and LD-based systems, mainly from a perspective of advanced modulation formats. Volterra series-based nonlinear equalizers, which can effectively counteract the nonlinear impairments induced by the UWOC system components, are discussed and experimentally demonstrated. Both the effectiveness and robustness of the proposed Volterra nonlinear equalizer in UWOC systems under different water turbidities are validated.To further approach the Shannon capacity limit of the UWOC system, the probabilistic constellation shaping technique is introduced, which can overcome the inherent gap between a conventional regular quadrature amplitude modulation(QAM) format and the Shannon capacity of the channel. The experimental results have shown a significant system capacity improvement compared to the cases using a regular QAM.     

Light scintillation in oceanic turbulence

The scintillation index of plane and spherical light waves as well as of a Gaussian beam, propagating in the clear-water weakly turbulent ocean, is revealed. The results are of utmost importance for underwater optical communications and sensing. An analysis of the threshold between the weak and strong regimes of oceanic turbulence is made, with the accent on the contribution from salinity-induced turbulence. It is found that strong oceanic turbulence can occur at distances as short as several meters, in striking contrast with atmospheric studies for which the typical distances are on the order of a kilometer.     

Machine-vision-based acquisition,pointing, and tracking system for underwater wireless optical communications

Due to the proliferation of underwater vehicles and sensors, underwater wireless optical communication(UWOC) is a key enabler for ocean exploration with a strong reliance on short-range bandwidth-intensive communications. A stable optical link is of primary importance for UWOC. A compact, low-power, and low-cost acquisition, pointing, and tracking(APT) system is proposed and experimentally demonstrated to realign the optical link within 0.04 s, even when the UWOC transmitter and receiver are in relative motion. The system successfully achieves rapid auto-alignment through a 4 m tap water channel with a relatively large number of bubbles. Furthermore, the required minimum illumination value is measured to be as low as7.1 lx, implying that the proposed APT scheme is robust to dim underwater environments. Meanwhile, mobility experiments are performed to verify the performance of the APT system. The proposed system can rapidly and automatically align moving targets in complex and unstable underwater environments, which can potentially boost the practical applications of UWOC.     

BER and outage probability performance of log-normal distribution non-Kolmogorov turbulent optical links

We model the average bit-error rate (BER) and outage probability for an intensity-modulation/direct detection of tracked or untracked optical wireless communication (OWC) systems with on-off keying in weak turbulence horizontal channels, using the log-normal distribution models. The effects of atmospheric attenuation, beam wander and spread, misalignment and the spectral index of non-Kolmogorov turbulence on system's performance are included. The model can be evaluated the information loss and BER of ground-to-train OWC links.     

大气环境下基于目标照明回光的视轴误差校正实验研究

理论分析了激光瞄准系统中视轴误差与目标照明回光的关系. 介绍了同时具备激光束发射与瞄准偏差校正功能的自适应光学器件––自适应光纤准直器的原理. 搭建了光束经200 m水平大气传输的激光瞄准实验平台. 基于二维目标和三维目标的照明回光, 利用随机并行梯度下降算法分别实现了不同初始视轴误差下的瞄准闭环校正. 实验结果表明, 闭环后二维目标和三维目标的视轴校正残差评价参数分别小于6%和10.8%, 校正精度均在理论范围内. 最后, 分析了算法参数对动目标瞄准的影响.     

Hybrid LD and LED-based underwater optical communication: state-of-the-art,opportunities,challenges,and trends [Invited]

In this paper, the current research of an underwater optical wireless communication(UWOC) network is reviewed first. A hybrid laser diode(LD) and light-emitting diode(LED)-based UWOC system is then proposed and investigated, in which hybrid cluster-based networking with mobility restricted nodes is utilized to improve both the life cycle and throughput of the UWOC network. Moreover, the LEDs are utilized for the coarse alignment, while the LDs are used for high-precision positioning to reduce the difficulty of optical alignment. Finally, challenges and trends for UWOC are pointed out to provide some insight for potential future work of researchers.     

Underwater wireless optical communication:why,what, and how? [Invited]

Foreseeing the proliferation of underwater vehicles and sensors, underwater wireless optical communication(UWOC) is a key enabler for ocean exploration, with strong competitiveness in short-range bandwidth-intensive applications. We provide a tutorial on the basic concepts and essential features of UWOC, as well as an overview of work being conducted in this field. Research challenges, arising from the characteristics of underwater channels, and possible roadmaps are discussed in detail. This review is expected to be of great use for the link designers of this field.     

Probability of fade for airborne laser communication system over exponentiated Weibull distribution under aperture averaging

In this paper, the probability of fade is performed for the airborne laser communication systems considering both the atmospheric and aero-optic effects. The atmospheric fluctuation is characterized by the exponentiated Weibull (EW) fading channel. The novel analytical expression is derived for probability of fade according to Meijer’s G function. The probability of fade is demonstrated to the simulation data with the Gamma–Gamma and log-normal distributions. The probability of fade is obtained for different flight altitudes and propagation distances. The probability of fade is analyzed in the airborne laser communication systems under the effect of aperture averaging in weak-to-strong turbulence regime. Results suggest that the proposed EW model is valid in airborne laser communication with high altitude, especially in the lower values of the irradiance under atmospheric turbulence and aero-optics effect. The fading of outage performance can be effectively mitigated by aperture averaging. Furthermore, this work is helpful for the compensate technique of system performance on airborne optical communication system.     

Adaptive detection technique for optical wireless communication over strong turbulence channels

Optical wireless communication (OWC) systems use the atmosphere as a propagation medium, so the atmospheric turbulence effects lead to fading related with signal intensity. The received signal of OWC over strong turbulence channels is assumed to be a mixture of K-distributed fading and Gaussian distributed thermal noise. Second-order spectral analysis is unable to separately estimate the mixed signal. In order to mitigate the fading induced by turbulence, the decision threshold-updating algorithm based on second and higher order cumulants is proposed and is able to operate in an unknown turbulence environment. The performance of the adaptive processing scheme has been evaluated by means of Monte Carlo simulations. Simulation results show the improvement of the bit error rate (BER) performance.     

汉克-贝塞尔光束在海洋湍流信道中的螺旋相位谱分析

携带轨道角动量的汉克-贝塞尔(Hankel-Bessel,HB)光束具有无衍射和自聚焦特性,用来作为信息传输的载体有望增大信息传输容量.基于Rytov近似理论,推导得到了HB涡旋光束经过海洋水平弱湍流信道后的螺旋相位谱的解析表达式,并利用数值仿真方法研究了海洋湍流参数对轨道角动量模式探测概率的影响.结果表明,海洋湍流导致发射轨道角动量模式的探测概率下降,出现模式串扰和螺旋相位谱扩展.海洋湍流对HB涡旋光束的负面影响随着轨道角动量模式数、传输距离、温度方差耗散率的增加而增强,随湍流动能耗散率的增加而减弱.HB涡旋光束受以盐度波动驱动的海洋湍流的负面影响更大.另外,在弱湍流及几十米传输距离条件下,HB涡旋光束的传输性能要差于最佳束腰大小设置的拉盖尔-高斯涡旋光束.这些结果有望为海洋环境水下光通信链路的实现提供一定的参考价值.     

大气湍流中空间光至单模光纤耦合的跟踪算法

 通过模拟湍流波前的振幅起伏及相位起伏,研究了采用CCD探测器的湍流波前倾斜校正系统的空间光到单模光纤耦合功率的衰落特性,对CCD图像的跟踪算法进行了研究。仿真结果表明,在接收孔径直径与Fried 参数的比值较大时,采用质心跟踪算法的倾斜校正系统的性能很差,耦合光功率会产生严重衰落。提出了一种高斯模板匹配的倾斜校正跟踪算法,在湍流较强时,可以有效地降低耦合光功率的衰落深度,并且可以避免采用质心算法可能出现的大孔径接收性能低于小孔径的不利情况。     

Capacity of wander and spread beams in log-normal distribution non-Kolmogorov turbulence optical links

We model the average channel capacity of optical wireless communication systems in weak turbulence horizontal channels, using the log-normal distribution models. The effects of beam wander and spread, pointing errors, turbulence inner scale, turbulence outer scale and the spectral index of non-Kolmogorov turbulence on system's performance are included. The model can evaluate the influence of the atmospheric turbulence conditions in the performance of a ground-to-train optical wireless communication system.     

Relay selection and nonlinear energy harvesting in full-duplex multi-relay cooperative network

Energy harvesting (EH) is a promising technique to extend the lifetime energy-constrained devices in wireless networks. This paper analyses the significance of relay selection and non-linear EH in evaluating the performance of Full-duplex (FD) decode-and-forward (DF) multi-relay cooperative network. The relay selection is considered under three scenarios based on channel state information (CSI) availability. We compare the EH performance of a hybrid power-time-splitting (PTS)-based non-linear energy harvester with time-spitting (TS) and power-splitting (PS) EH at the relay node. We derive the expression of outage probability (OP) of FD DF multi-relay cooperative network over independent and identically distributed Rayleigh fading channels. Numerical results show that the outage performance of the system is significantly enhanced by deploying an ideal relay selection scheme. This paper also gives insights about the combined effect of saturation threshold of non-linear EH and residual self-interference (RSI) on the system performance. Unlike linear EH systems, the non-linear EH causes the outage floor even when the RSI is significantly small. The response of the system is also analysed for the impact of TS and PS ratios. Furthermore, we also investigate the system’s performance in terms of other system parameters like the number of relays and data transfer rate. Monte Carlo simulations are used to verify the analytical expressions.     

Absorption and scattering effects of Maalox,chlorophyll, and sea salt on a micro-LED-based underwater wireless optical communication [Invited]

In this work, a blue gallium nitride(GaN) micro-light-emitting-diode(micro-LED)-based underwater wireless optical communication(UWOC) system was built, and UWOCs with varied Maalox, chlorophyll, and sea salt concentrations were studied. Data transmission performance of the UWOC and the influence of light attenuation were investigated systematically. Maximum data transmission rates at the distance of 2.3 m were 933, 800, 910,and 790 Mbps for experimental conditions with no impurity, 200.48 mg/m~3 Maalox, 12.07 mg/m~3 chlorophyll,and 5 kg/m~3 sea salt, respectively, much higher than previously reported systems with commercial LEDs. It was found that increasing chlorophyll, Maalox, and sea salt concentrations in water resulted in an increase of light attenuation, which led to the performance degradation of the UWOC. Further analysis suggests two light attenuation mechanisms, e.g., absorption by chlorophyll and scattering by Maalox, are responsible for the decrease of maximum data rates and the increase of bit error rates. Based on the absorption and scattering models,excellent fitting to the experimental attenuation coefficient can be achieved, and light attenuation by absorption and scattering at different wavelengths was also investigated. We believe this work is instructive apply UWOC for practical applications.