Performance analysis and optimization for UAV-based FSO communication systems

Recently, with the increasingly mature development of unmanned aerial vehicle (UAV) technology, free-space optical (FSO) communication with the UAV relay has gradually attracted extensive attention from scholars at home and abroad. In this paper, a hovering UAV-based FSO dual-hop decode-and-forward system with multiple sources is investigated. In the system, channel attenuation coefficient is affected by atmospheric loss, atmospheric turbulence, pointing error and angle-of-arrival fluctuation. By analyzing the probability density functions (PDF) of these four factors, we derive a tractable expression for the PDF of the total channel gain. Then, closed-form expression of the end-to-end outage probability and asymptotic outage performance bounds are derived to evaluate the influence of different factors on the system performance. Based on the derived theoretical expressions, the location of UAV and the number of source nodes are jointly optimized to improve the overall system performance and reduce the system cost. Numerical results show that the derived theoretical expressions are accurate to evaluate the outage performance of the system. Moreover, the proposed optimization algorithm significantly improves the system performance and can provide the optimal parameter selection for UAV system design.     

Analysis of free space optical link in turbulent atmosphere

Free space optical (FSO) communication is an upgraded supplement to the existing wireless technologies. FSO technology provides vast modulation bandwidth, unlicensed spectrum, cost effective deployment, low power consumption and less mass requirement. Today, researchers are preliminary focused to use the free space communication systems for inter satellites links. In this paper, the performance analysis of FSO communication link in weak atmospheric turbulence has been analyzed for different atmospheric transmission windows using OOK modulation. The analysis has been done using bit error rate as the performance metric. The effect of attenuation on the link performance has been investigated by varying distance between transmitter and receiver for a given power and data rate. Further, BER performance analysis has been carried out for varying data rate and transmitted power. Also, the effect of attenuation on received optical power has been studied. The work has been performed in OptSim environment.     

Bit error rate analysis of MISO FSO systems

Abstract

Multiple-input single-output (MISO) systems are employed in free space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. In this paper, we consider a MISO FSO system with practical transmitter and receiver configuration that consists of radial laser array with Gaussian beams and a Gaussian receiver aperture function. We have employed our previously derived formulation of the power scintillation in which Huygens–Fresnel principle was employed. Therefore, we choose system parameters within the range of validity of the wave structure functions. Using the on-off keying modulation and the log-normal probability distribution function, we quantify the average bit error rate (〈BER〉) of laser array beams in weak turbulence. It is observed that the radial array beams at the transmitter are more advantageous than the single Gaussian beam. However, increasing the number of array beamlets to more than three seems to have negligible effects on 〈BER〉 . It is further observed that 〈BER〉 decreases when the source size, the ring radius and the receiver aperture radius increase.     

Simulation research of medium-short distance free-space optical communication with optical amplification based on polarization shift keying modulation

Circular polarization shift keying (CPolSK) modulation technique has many advantages such as excellent BER performance and freedom from the alignment of polarization coordinates of the transmitter and the receiver, etc., and it turns out to be a good choice to FSO system. In this paper, a FSO system using CPolSK modulation is studied by simulation; it is found that the communication performance of the system is excellent in most weather condition. Additionally, three ways of optical signal amplification are proposed, and contrastive analysis on performance of corresponding optical amplification systems is carried out by examining SNR、BER and transmission distance with different specific attenuation. The results show that the system with optical amplifier at the transmitter have the optimum performance, and then the system with optical amplifier at the both ends with the same total gain, it is worst for the system with optical amplifier at the receiver. In addition, the safety factor for high emission power induced by optical amplification is also considered in this paper for practical application. The study above may be utilized in the system design for enhancing performance.     

光纤通信系统中差分相位调制平衡接收机系统误码率的精确计算

采用高斯函数近似方法以及傅里叶变换法分析推导出高速光纤通信技术中所采用的差分相位调制(differential phase-shift keying,DPSK)-平衡接收机系统中的差分非线性相位噪声△ψ的精确表示,把它应用于考虑了所有对接收机误码产生影响的噪声计算公式中,最终得出了DPSK-平衡接收机误码率最终完整的数学表达式,并用VPI软件进行了仿真验证.     

Optimization of free space optics parameters: An optimum solution for bad weather conditions

Free Space Optics Systems (FSO) is one of the most effective solutions, especially for atmospheric turbulence due to the weather and environment structure. Free space optics system suffers from various limitations. A well-known disadvantage of FSO is its sensitivity on local weather conditions-primarily to haze and rain, resulting in substantial loss of optical signal power over the communication path. The main objective of this article is to evaluate the quality of data transmission using Wavelength Division Multiplexing (WDM) with highlighting several factors that will affect the quality of data transmission. The results of these analyses are to develop a system of quality-free space optics for a high data rate transmission. From the result analysis, FSO wavelength with 1550 nm produces less effect in atmospheric attenuation. Short link range between the transmitter and receiver can optimize the FSO system transmission parameters or components. Based on the analysis, it is recommended to develop an FSO system of 2.5 Gbps with 1550 nm wavelength and link range up to 150 km at the clear weather condition of bit-error-rate (BER) 10⁻⁹.     

Estimation of the four-wave mixing noise probability-density function by the multicanonical Monte Carlo method

The performance of high-powered wavelength-division multiplexed (WDM) optical networks can be severely degraded by four-wave-mixing- (FWM-) induced distortion. The multicanonical Monte Carlo method (MCMC) is used to calculate the probability-density function (PDF) of the decision variable of a receiver, limited by FWM noise. Compared with the conventional Monte Carlo method previously used to estimate this PDF, the MCMC method is much faster and can accurately estimate smaller error probabilities. The method takes into account the correlation between the components of the FWM noise, unlike the Gaussian model, which is shown not to provide accurate results.     

一种新的自由空间光通信调制方式——圆偏振位移键控

自由空间光通信(FSO)在开放的大气链路中传输光信号, 不可避免地会受到大气扰动和背景光噪声的影响, 导致系统的可靠性降低。为了抑制大气扰动等对FSO系统造成的不利影响, 提出一种新的光信号调制方式——圆偏振位移键控(CPOLSK)。该调制方式利用圆偏振光的两种旋光状态进行数据的传送, 接收端结合差分方法进行光信号接收。给出了CPOLSK的装置模型, 并对其性能进行了分析。与目前广泛采用的开关键控(OOK)、脉冲位置调制(PPM)方式相比, CPOLSK很好地抑制了背景光噪声的影响, 同时对光电探测器内部噪声也有一定的抑制效果。     

实际大气中无线光通信的系统差错性能

无线光通信(FSO)系统的性能受大气湍流影响会产生剧烈波动。根据系统和大气参数评估系统差错性能的研究具有现实意义。以大气湍流信道和光电探测模型为基础,使用拟合概率分布替换常用的对数正态分布,建立了FSO系统差错性能的数学仿真模型,改进了湍流条件下系统误码率计算公式,并且进行全天的验证实验。实验结果显示,光强概率分布对系统性能有显著的影响,原有公式在某些情况下的计算结果有较大偏差,而改进公式的计算结果具有更好的适应性和准确性。该改进公式可有效评估湍流条件下FSO系统性能,并为相关理论研究提供参考。     

Error rate performance analysis of optical links between high altitude platforms with misalignment fading by Hoyt distribution

The more reality misalignments model with Hoyt distribution is used to analyze performance for optical links between high altitude platforms. Compared with the recent work in terrestrial FSO, it is proved that the variance ratio have more significant effect on error performance for optical links over long distance. Then, the expression for the pairwise error probability is analyzed and applied to obtain upper bounds on the BER performance for coded optical communications. Simulation results show that coding technique can improve error performance of optical links, but it also can enhance the influences of the variance ratio.     

Simulation and evaluation of phase noise for optical amplification using semiconductor optical amplifiers in DPSK applications

A thorough simulation and evaluation of phase noise for optical amplification using semiconductor optical amplifier (SOA) is very important for predicting its performance in differential phase-shift keyed (DPSK) applications. In this paper, standard deviation and probability distribution of differential phase noise at the SOA output are obtained from the statistics of simulated differential phase noise. By using a full-wave model of SOA, the noise performance in the entire operation range can be investigated. It is shown that nonlinear phase noise substantially contributes to the total phase noise in case of a noisy signal amplified by a saturated SOA and the nonlinear contribution is larger with shorter SOA carrier lifetime. It is also shown that Gaussian distribution can be useful as a good approximation of the total differential phase noise statistics in the whole operation range. Power penalty due to differential phase noise is evaluated using a semi-analytical probability density function (PDF) of receiver noise. Obvious increase of power penalty at high signal input powers can be found for low input OSNR, which is due to both the large nonlinear differential phase noise and the dependence of BER vs. receiving power curvature on differential phase noise standard deviation.     

Reflection assisted beam propagation model for obstructed line-of-sight FSO links

A non-line-of-sight (NLOS) infra-red reflection based beam propagation model is proposed as a supplement to conventional terrestrial free space optical (FSO) communication system. This ray propagation model lets tactically positioned optical reflectors to smartly exploit the aggregated advantages of mirror characteristics to bridge the existent communication gap between two FSO nodes due to inclined or obstructed line-of-sight view. Additionally, a numerical framework of the proposed system is presented that analytically explores the optical losses induced by harmonic distortions and the resultant beam divergence at the receiver. The impact of the different reflectors on the traversing beam is then investigated through an experimental FSO test-bed set in an outdoor environment in terms of phase shifts, divergence loss, noise margin and maximum achievable link length. Matlab based simulations, based on the experimental outcomes, envisages that concave reflectors can effectively compensate the turbulence induced signal fading and restrict the beam divergence loss; thereby, improving the maximum achievable NLOS FSO link length.     

Quasi-monochromatic bound on ultrashort light-pulse transmission through fog

The use of ultrashort (femtosecond duration) light pulses for line-of-sight free-space optical (FSO) communication through fog is receiving increasing attention. Assuming that the transmitter power is low enough to preclude nonlinear interactions, and that scattering-induced multipath spread is less than the reciprocal of the scattering-induced Doppler spread, it is shown that the average transmitter-to-receiver fractional energy transfer of an ultrafast FSO system cannot exceed that of a quasimonochromatic (nanosecond pulse duration) system operating at the optimum wavelength within the ultrafast system's spectrum. Thus, an ultrashort-pulse system is not a solution for high-data-rate FSO communication through fog, because, at best, it will reproduce on average the energy-transfer performance of a wavelength-optimized quasimonochromatic system.     

Probability distribution functions of derivatives and increments for decaying burgers turbulence

A Lagrangian method is used to show that the power law with a -7/2 exponent in the negative tail of the probability distribution function (PDF) of the velocity gradient and of velocity increments, predicted by E et al. [Phys. Rev. Lett. 78, 1904 (1997)] for forced Burgers turbulence, is also present in the unforced case. The theory is extended to the second-order space derivative whose PDF has power-law tails with exponent -2 at both large positive and negative values and to the time derivatives. PDF's of space and time derivatives have the same (asymptotic) functional forms. This is interpreted in terms of a random Taylor hypothesis.     

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.     

Performance of phase compensated coherent free space optical communications through non-Kolmogorov turbulence

The bit-error-rate (BER) performance of coherent free-space optical (FSO) links employing phase compensation techniques is investigated in weak non-Kolmogorov turbulence. Assuming that the amplitude fading and phase fluctuation follow lognormal model and Gaussian distribution respectively and using the expression of non-Kolmogorov turbulence in terms of Zernike polynomials, the signal-to-noise ratio (SNR) at the coherent receiver is analyzed and as a special case, a new closed-form expression using chi-square distribution is obtained. Thus, the influence of different compensation modes and normalized receiver diameter on BER performance is evaluated and an optimum normalized receiver diameter is suggested to achieve the minimum BER. Moreover, the impact of outer scale L₀ and the exponent value α in non-Kolmogorov spectrum is studied with the optimum diameter, which reveals that the BER has an obvious decrease with larger values of L₀ and α.     

Determination of rain attenuation parameters for free space optical link in tropical rain

Free space optics (FSO) is a promising communication technique for various types of services in the optical access network. Single beam FSO system in tropical rainy weather is vulnerable to atmospheric rain attenuation, so it is necessary to have precise power law parameters of rain attenuation in tropical regions. In this study, the power law parameters k, and α are estimated as 2.03 and 0.74, respectively for the FSO applications in tropical South-East Asian weather. These parameters were evaluated by using least square mean equation (LSME) method with Levenberg–Marquardt optimization based on the one year collected heavy rain data. The obtained parameter values for tropical weather are contributed to improve link performance for high-speed networks.     

Quantum-correlation-based free-space optical link with an active reflector

We present a quantum-correlation-based free-space optical(FSO) link over 250 m using an outdoor active reflector 125 m from the transceiver station. The performance of free-space optical communication can be significantly degraded by atmospheric turbulence effects, such as beam wander and signal fluctuations. We used a 660 nm tracking laser to reduce atmospheric effects, by analyzing the fast beam wander and slow temporal beam drift, using this information to correct the quantum channel alignment of the 810 nm signal photons. In this work, the active reflector consisted of a mirror, a 6-axis hexapod stage, and a long-range wireless bridge. The slow drift of the beam path due to outdoor temperature changes was steered and controlled using wireless optical feedback between the receiver units and the active reflector. Our work provides useful knowledge for improved control of beam paths in outdoor conditions, which can be developed to ensure high quality quantum information transfer in real-world scenarios, such as an unmanned FSO link for urban quantum communication or retro-reflective quantum communication links.     

Enhancement of free space optical link in heavy rain attenuation using multiple beam concept

Free space optics (FSO) has attracted a lot of attention for a variety of applications in telecommunications area, and it is dream of every researcher and telecommunication society to make it a real alternative solution for the last mile problem, to replace fiber optics. FSO is much preferred because of its low maintenance cost and deployment time. FSO with single-beam system is vulnerable to atmospheric attenuation, so to overcome this, a multiple-beam FSO transceiver system has become prominent and is usually used. In this paper, average rain attenuation is evaluated from the collected rain intensity data which are collected for a period of seven months, and implemented in the study concerning results relating link distance, and received optical power of using multiple-beam FSO system in tropical rainy weather. Comparison is made in terms of received optical power, geometrical losses, atmospheric losses, and bit error rate (BER) on using different number of optical beams, based on simulation at data rate of 1 Gb/s. From the results it is clear that the quality of received power is improved by using up to four beams, along with link distance up to 1141.2 m as compared to one-beam, two-beam, and three-beam, with link distances 833.3 m, 991.0 m, 1075.4 m, respectively.