Some limitations on optical communication reliability through Kolmogorov and non-Kolmogorov turbulence

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received signal, affecting link performance.Influence of Kolmogorov and non-Kolmogorov turbulence statistics on laser communication links are analyzed for different propagation scenarios, and effects of different turbulence spectrum models on optical communication links are presented. Statistical estimates of the communication parameters such as the probability of fade (miss) exceeding a threshold dB level, the mean number of fades, and BER are derived and examples provided. The presented quantitative data suggest that the non-Kolmogorov turbulence effects on lasercom channels are more severe in many situations and need to be taken into account in wireless optical communication. Non-Kolmogorov turbulence is especially important for elevations above the boundary layer as well as for even low elevation paths over water.     

大气激光通信链路的性能仿真

大气衰减和大气湍流严重影响着大气激光通信的链路质量。建立了大气信道的激光通信链路模型，研究了衰减信道和湍流信道中光链路的传输影响，对最大通信速率、链路功率余量和误码率进行了分析和计算。结果表明，大气湍流严重影响系统误码率，当大气闪烁指数斫是0．07时，可达到的最小误码率为10^-9。分析结果可为系统设计提供参考依据。     

Improved free space optical communications performance by using time diversity

A laser beam propagating through turbulence experiences random amplitude and phase fluctuations, which can severely degrade the performance of free space optical communication systems. It this letter, time diversity is demonstrated as a technique which can decrease turbulence influence. Statistically, laser propagation along an atmospheric path is uncorrelated with an earlier-time path for a time interval greater than the atmospheric turbulence correlation time. To estimate time diversity system performance, a 2.2-km optical link is set up for comparing the fade probability of a system using time diversity with a system not using time diversity. The experimental results obtained under different turbulence conditions are shown which are in good agreement with the theory.     

地物散射和大气湍流导致的激光闪烁统计

通过分析大气湍流和地物散射对传输激光束的调制机理，运用双随机统计理论研究了空－地－空传输路径空载探测器接收到光闪烁统计问题。闪烁统计由大气湍流和地物（岩石，海洋和森林等）散射的联合效应给出，分析中采用最速下降法处理概率积分，获得了与岩石，海洋和森林二大类地物散射体对应的闪概率密度函数，广义Ｃｈｉ－Ｓｑｕａｒｅｄ地面散射与大气湍流综合调制效应明显地压缩了概率分布的宽度并使分布函数的峰值移向归一化强度     

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.     

On the use of wavelength and time diversity in optical wireless communication systems over gamma–gamma turbulence channels

Optical wireless communication or free space optical systems have gained significant research and commercial attention in recent years due to their cost-effective and license-free high bandwidth access characteristics. However, by using the atmosphere as transmission media, the performance of such a system depends on the atmospheric conditions that exist between transmitter and receiver. Indeed, for an outdoor optical channel link, the existence of atmospheric turbulence may significantly degrade the performance of the associated communication system over distances longer than 1 or even 0.5 km. In order to anticipate this, particular attention has been given to diversity methods. In this work, we consider the use of wavelength and time diversity in wireless optical communication systems that operate under weak to strong atmospheric turbulence conditions modeled by the gamma–gamma distribution, and we derive closed form mathematical expressions for estimating the system's achievable outage probability and average bit error rate. Finally, numerical results referred to common practical cases are also obtained in order to show that wavelength and time diversity schemes enhances considerably these systems’ availability and performance.     

1.5410.6-μm eyesafe laser rangefinders performance under adverse weather conditions

In this investigation, the performance of a CO₂ laser rangefinder at 10.6 μm and an erbium:glass rangefinder at 1.54 μm are evaluated and compared under various atmospheric conditions. Both systems and the trial site are described. The effect of atmospheric extinction and scattering is discussed. Also, the maximum range capability of the two laser rangefinders under these degraded weather conditions is assessed. The results show that the tested CO₂ laser rangefinder performed better than the Er:glass LRF used for the trials under all weather conditions. However, comparing two state-of-the-art systems does not lead necessarily to the same conclusion.     

自由空间宽谱部分相干光通信系统

大气湍流对自由空间光通信系统所造成的影响是不可忽略的，为了减弱湍流对空间光通信系统带来的影响，实验搭建了一套通信距离为900 m的真实大气信道宽谱部分相干光通信系统。系统采用皮秒脉冲泵浦高非线性光纤产生超连续谱并滤波得到部分相干宽谱脉冲，对其调制后完成通信。在测试过程中，实验专门设置了一条参考链路，保证了测试环境的一致性。实验结果表明，在中等湍流条件下，系统光强闪烁指数为0.035 8，相比窄线宽通信系统提升了23%，最低探测灵敏度达到了-23.35 dBm，相比窄线宽通信系统提升了42%。与窄线宽激光通信系统相比，宽谱部分相干光通信系统可以明显降低湍流引起的光功率抖动，并提升自由空间光通信系统通信性能。     

Effect of non-Kolmogorov turbulence on beam spreading in satellite laser communication

In the past half a century, satellite laser communication has caught the attention of scientists due to its distinct advantages in comparison with conventional satellite microwave communication. For ground-to-satellite and satellite-to-ground data links, the atmosphere is a part of the communication channel; thus, atmospheric turbulence severely degrades the performance of satellite laser communication systems. In general, the Kolmogorov turbulence model is used to study the effect of atmosphere turbulence on satellite laser communications since it has been confirmed by numerous direct measurements of temperature and humidity fluctuations in the atmospheric boundary layer. However, increasing experimental evidence and theoretical investigations have shown that the Kolmogorov theory is sometimes inadequate to describe atmospheric statistics properly, in particular, in some domains of the atmosphere. We analyze the joint influence of Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the spot size associated with the uplink and downlink propagation channels for a satellite laser communication system in the geosynchronous orbit, using a power spectrum of non-Kolmogorov turbulence with power law ?5 that describes the refractiveindex fluctuations in the atmosphere above 6 km and considering the combined power spectrum of Kolmogorov and non-Kolmogorov turbulence. Before this analysis, we study the joint influence of the Kolmogorov turbulence from the ground to 6 km and non-Kolmogorov turbulence above 6 km on the scintillation indices of laser beams.     

大气激光通信的实验测量

设计了大气激光通信信道测量系统，利用此系统在不同气象条件下进行了近地视距实验测量并采集了不同天气条件下的实验数据，详细分析讨论了所得实验数据并进行了误差分析．同时实验中获得了大量数据，为进一步完善大气激光信道模型提供了有力的依据．     

大气激光通信的实验测量(英文)

设计了大气激光通信信道测量系统,利用此系统在不同气象条件下进行了近地视距实验测量并采集了不同天气条件下的实验数据,详细分析讨论了所得实验数据并进行了误差分析.同时实验中获得了大量数据,为进一步完善大气激光信道模型提供了有力的依据.     

Bit error rate analysis with real-time pointing errors correction in free space optical communication systems

Pointing errors caused by the atmospheric turbulence will degrade the performance of free space optical (FSO) communication systems, especially the bit error rate (BER). In this paper, we innovatively analyze the relationship between BER and pointing errors by the probability density functions (PDFs) and intensity displacement in focal plane under the On-Off Keying (OOK) modulation conditions. The closed-loop experimental system is set up in laboratory, where the fast steering mirror (FSM) is real-time controlled by embedded controller with the parallel processing technology and the atmospheric turbulence is simulated by a turbulence simulation box. The results of repeated experiments show that the method of pointing errors correction we proposed is efficient under the conditions of atmospheric turbulence. By utilizing our method, the BER can decrease from nearly 10⁻³ to nearly or even below 10⁻⁹, thus improving the performance of FSO communication systems significantly.     

Performance comparison of various modulation schemes over free space optical (FSO) link employing Gamma–Gamma fading model

Free Space Optics (FSO) is an emerging line-of-sight technology intending to provide last-mile solution to the network problem where fiber technology is not feasible. The use of Wavelength Division Multiplexing (WDM) technology for FSO is inspired due to the demand for broadband communication. This technique has brought a revolution because the system data capacity is enhanced by simply adding more number of channels and reducing the channel spacing without having the need of more than one FSO link. By reducing the channel spacing to an appropriate level, Dense Wavelength Division Multiplexing (DWDM) based FSO systems are also be attained and are reported by various research works. FSO finds applications in vast areas like backhaul networks for cellular communication, disaster recovery, LAN–LAN connectivity, high-definition TV, MAN-extension, video transmission, medicine industry and surveillance. However, its usage is limited due to the serious challenges of link vulnerability to weather and atmospheric turbulence-induced fading. This paper is based on a WDM-FSO system. An 8-channel WDM based FSO system is proposed and performance is evaluated on widely accepted modulation schemes under weak, moderate and strong turbulence conditions. Gamma–Gamma fading model is employed for atmospheric turbulence modelling. The system is simulated on OptiSystem 14.0.     

Chaotic free-space laser communication over a turbulent channel

The dynamics of errors caused by atmospheric turbulence in a self-synchronizing chaos-based communication system that stably transmits information over an approximately 5 km free-space laser link is studied experimentally. Binary information is transmitted using a chaotic sequence of short-term pulses as a carrier. The information signal slightly shifts the chaotic time position of each pulse depending on the information bit. We report the results of an experimental analysis of the atmospheric turbulence in the channel and the impact of turbulence on the bit-error-rate performance of this chaos-based communication system.     

Probability of fade estimation for FSO links with time dispersion and turbulence modeled with the gamma–gamma or the I-K distribution

In this work, we study the influence of the time dispersion and atmospheric turbulence effects on the performance of a free space optical communication system, which is using longitudinal Gaussian pulses as information bit carriers. The scintillation effect, is modeled with the gamma–gamma or the I-K distribution, which, both, are suitable to emulate irradiance fluctuations due to weak to strong atmospheric turbulence conditions. Additionally, we estimate the influence of the time dispersion effect on chirped pulse's propagation. Taking into account these effects, closed form mathematical expressions for the evaluation of fade's probability of the wireless optical link, are derived. Additionally, the possibility of improving the availability results by choosing the appropriate link and pulse's parameter values is further investigated. Finally the numerical results from the derived mathematical expressions are presented.     

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.     

Pulse state interval modulation and open loop communication experiment in the hostile environments

Based on several atmospheric channel field tests and analyses, the modulation model of wireless laser communication in hostile weather is investigated, for enhancing the link rate of wireless laser communication. Firstly, the conversion relationship between SER and BER under different conditions is studied. Afterwards, by analyzing atmospheric channel tests result, the importance of atmospheric channel bandwidth was featured during test. On the basis of this, an efficiency pulse state-interval coded modulation is present for the first time and its characteristics are analyzed. In order to verify whether this modulation model is feasible and effective as well as its robustness, a wireless laser communication feasibility experiment in the hostile environments was launched. According to the results, a prospect is pointed out that atmospheric channel properties feedback technique in real time may be a key and precondition in all-weather wireless optical communication.     

Reliability analysis of an auto-tracked FSO link under adverse weather condition

In this paper, the effects of auto-tracking subsystems on the Bit-Error-Rate (BER) and reliability of free-space optical (FSO) communication links are investigated. Considering environmental influences on the laser beam intensity distribution in receiver, we study the propagation properties of Gaussian laser beams through a complete optical path. Then it is tried to derive an analytical formula for intensity distribution along the complete optical path as well as on the receiver's detector plane based on the Collins integral. Based upon the intensity formula, we calculate the values of signal to noise ratio (SNR), BER, and power while considering the effects of atmospheric losses due to absorption, scattering and turbulence on them. Using mentioned values, the role of auto-tracking subsystems on link reliability improvement in adverse weather condition is described. The related results are illustrated by graphs obtained by calculations and simulations.     

大气弱湍流下星地光链路衰落特性影响因素分析

研究了弱湍流条件下星地光通信链路的衰落概率、平均衰落次数以及平均衰落时间3个重要参数的统计特性,并通过数值仿真分析了链路天顶角、光波长以及接收机海拔高度对链路衰落特性的影响。结果表明:减小天顶角和提高接收机探测灵敏度可以改善链路的衰落特性;1.55 m激光较0.85 m、1.06 m激光更适合于星地光通信;地面接收机应尽量建在2 000 m以上的高海拔地区。     

大气信道对红外激光通信系统性能影响的实验研究

针对近地面、准水平红外激光大气通信系统的应用,利用微脉冲激光雷达、大口径激光闪烁仪在济南市区进行了为期一周的昼夜连续观测,获得了影响红外激光通信系统性能的关键大气参量的信息.着重分析了大气衰减信道、大气湍流信道对一公里通信链路系统性能的影响,考查了通信设备在复杂大气条件下应用的可行性及可靠性.最后就减弱大气信道对通信性能影响的方法进行了总结和探讨.