M分布模型下多跳相干OFDM FSO系统的性能研究

本文在涵盖了从弱湍流到强湍流的所有信道条件,能够表征现有大多数湍流信道的M分布模型下,采用QPSK调制方式研究了多跳相干OFDM FSO系统的性能。系统在中继辅助链路的发射机和接收机之间使用DF中继协议。考虑大气湍流、路径损耗以及瞄准误差对大气信道衰落模型的联合作用,分别推导出系统的中断概率和误符号率的Meijer G形式的闭合表达式。通过仿真分析了中继链路长度、中继节点数以及子载波个数等关键因素对系统的中断性能和误符号率性能的影响。本研究为中继系统的实际应用奠定了理论基础。     

A modified model of the atmospheric effects on the performance of FSO links employing single and multiple receivers

In free space optical (FSO) communication links, atmospheric effects including absorption, scattering and turbulence have significant impacts on the quality of the laser beam propagating through the free space channel. Absorption and/or scattering due to the atmospheric particles result in optical losses, whereas turbulence contributes to the intensity scintillation which can severely impair the operation of FSO links. In this paper, using a modified model we analyze the atmospheric effects on the signal-to-noise ratio (SNR) and the bit error rate (BER) performance of an FSO system. We show that for multiple detectors with the same surface area as a single detector there is a critical link range less than which the SNR decreases for larger values of M.     

基于OFDM的大气激光通信湍流抑制关键技术研究

文章在分析无线激光通信(FSO)存在两种主要的大气信道问题的基础上,针对激光大气信道问题尤其是在复杂湍流环境下的频率选择性衰落问题和多径效应问题,提出了基于正交频分复用(OFDM)的湍流效应抑制方法,构建了FSO-OFDM系统,研究了该系统的基带模型以及信号的多载波调制与解调方法。分析了无线激光通信中存在复杂湍流环境下的大气信道问题,讨论了大气湍流影响下的平面波激光通信系统模型,建立了大气湍流影响下对数正态湍流通道的高斯光束空间光通信系统模型,推导了光波强度的概率密度函数,研究了利用信噪比概率密度函数分析各种大气湍流效应对系统性能影响的方法;设计了无线光通信系统的OFDM多载波调制方案,构建了FSO-OFDM系统基带模式模型,并基于该模型研究了其信号的调制与解调原理。最后,采用MATLAB编程实现FSO-OFDM系统,对多径干扰下的FSO通信系统进行仿真实验,进行了不同保护间隔下的误码率特性实验,验证了FSO-OFDM系统具有很强的抗多径干扰和频谱选择性衰落能力以及良好的BER性能,可有效解决码间干扰大、链路不可靠的问题,具有非常广泛的应用前景和使用价值。     

Free-space communication through turbulence: a comparison of plane-wave and orbital-angular-momentum encodings

Abstract

Free-space communication allows one to use spatial mode encoding, which is susceptible to the effects of diffraction and turbulence. Here, we discuss the optimum communication modes of a system while taking such effects into account. We construct a free-space communication system that encodes information onto the plane-wave (PW) modes of light. We study the performance of this system in the presence of atmospheric turbulence, and compare it with previous results for a system employing orbital-angular-momentum (OAM) encoding. We are able to show that the PW basis is the preferred basis set for communication through atmospheric turbulence for a system with a large Fresnel number product. This study has important implications for high-dimensional quantum key distribution systems.     

Performance analysis of free-space optical communication systems over atmospheric turbulence channels

Turbulence fading is one of the main impairments affecting the operation of free-space optical (FSO) communication systems. The authors study the performance of FSO communication systems, also known as wireless optical communication systems, over log-normal and gamma-gamma atmospheric turbulence-induced fading channels. These fading models describe the atmospheric turbulence because of its very good agreement with experimental measurement data. Closed-form expressions for the average (ergodic) capacity and the outage probability are derived for both statistical models. Another contribution of this work is a study of how the performance metrics are affected by the atmospheric conditions and other parameters such as the length of the link and the receiver's aperture diameter. The derived analytical expressions are verified by various numerical examples and can be used as an alternative to time-consuming Monte-Carlo simulations.     

Performance of a QAM/FSO communication system employing spatial diversity in weak and saturation turbulence channels

In this paper, the bit error rate (BER) performance and outage probability of quadrature amplitude modulation free space optical (QAM/FSO) communications with spatial diversity in turbulent environments are investigated. The equal-gain combining (EGC) and selection combining (SelC) diversity techniques are considered to mitigate turbulence-induced signal fading in the proposed system. The average BER and outage probability expressions are derived for EGC diversity in weak and saturation turbulence channels. The results indicate that using EGC diversity can significantly improve the system performance compared to employing the SelC diversity or single monolithic aperture schemes. Specifically, approximately 4 and 9?dB lower signal-to-noise power ratios are required for the 1?×?4 EGC diversity system than for the 1?×?4 SelC and non-diversity systems at a BER of 10^?10. In addition, the use of diversity techniques also significantly decreases the outage probability. The proposed scheme can be helpful for establishing a spatial diversity FSO system with a low error rate and high transmission rate.     

Coded-orthogonal frequency division multiplexing in hybrid optical networks

Future Internet should be able to support a wide range of services containing large amount of multimedia over different network types at a high speed. The future optical networks will therefore be hybrid, composed of different single-mode fibre (SMF), multi-mode fibre (MMF) and free-space optical (FSO) links. In these networks, novel modulation and coding techniques are needed that are capable of dealing with different channel impairments, be it in SMF, MMF or FSO links. The authors propose a coded-modulation scheme suitable for use in hybrid FSO - fibre-optics networks. The proposed scheme is based on polarization-multiplexing and coded - orthogonal frequency division multiplexing (OFDM) with large girth quasi-cyclic low-density parity-check (LDPC) codes as channel codes. The proposed scheme is able to simultaneously deal with atmospheric turbulence, chromatic dispersion and polarisation mode dispersion (PMD). With a proper design for 16-quadrature amplitude modulation (QAM)-based polarisation-multiplexed coded-OFDM, the aggregate data rate of 100 Gb/s can be achieved for OFDM signal bandwidth of only 12.5 GHz, which represents a scheme compatible with 100 Gb/s per wavelength channel transmission and 100 Gb/s Ethernet.     

Analysis of LDPC Code in Hybrid Communication Systems

Free-space optical (FSO) communication is of supreme importance for designing next-generation networks. Over the past decades, the radio frequency (RF) spectrum has been the main topic of interest for wireless technology. The RF spectrum is becoming denser and more employed, making its availability tough for additional channels. Optical communication, exploited for messages or indications in historical times, is now becoming famous and useful in combination with error-correcting codes (ECC) to mitigate the effects of fading caused by atmospheric turbulence. A free-space communication system (FSCS) in which the hybrid technology is based on FSO and RF. FSCS is a capable solution to overcome the downsides of current schemes and enhance the overall link reliability and availability. The proposed FSCS with regular low-density parity-check (LDPC) for coding techniques is deliberated and evaluated in terms of signal-to-noise ratio (SNR) in this paper. The extrinsic information transfer (EXIT) methodology is an incredible technique employed to investigate the sum-product decoding algorithm of LDPC codes and optimize the EXIT chart by applying curve fitting. In this research work, we also analyze the behavior of the EXIT chart of regular/irregular LDPC for the FSCS. We also investigate the error performance of LDPC code for the proposed FSCS.     

Exact error rate analysis of free-space optical communications with spatial diversity over Gamma–Gamma atmospheric turbulence

The error rate performances and outage probabilities of free-space optical (FSO) communications with spatial diversity are studied for Gamma–Gamma turbulent environments. Equal gain combining (EGC) and selection combining (SC) diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate (BER) expression and outage probability are derived for direct detection EGC multiple aperture receiver system. BER performances and outage probabilities are analyzed and compared for different number of sub-apertures each having aperture area A with EGC and SC techniques. BER performances and outage probabilities of a single monolithic aperture and multiple aperture receiver system with the same total aperture area are compared under thermal-noise-limited and background-noise-limited conditions. It is shown that multiple aperture receiver system can greatly improve the system communication performances. And these analytical tools are useful in providing highly accurate error rate estimation for FSO communication systems.     

Analysis of adaptive multi-rate FSO/RF hybrid systems using Málaga-ℳ distribution model in turbulent channels

Precise modeling of the optical channels with different conditions of atmospheric turbulence is an important prerequisite for analyzing the performance of Free Space Optical (FSO) communication systems. Statistical models which have been proposed to describe the optical channels, have some limitations to precisely model the channel. In this paper, The ? distribution which is a relatively new and comprehensive model, introduced by the University of Malaga, has been used to model and evaluate FSO/RF hybrid systems under the effect of atmospheric turbulence of the channels for two modes of operation, classic adaptive rate transmission, and the combination of adaptive rate transmission and automatic repeat request.The simulation results have been compared with the experimental results which have been reported by other researchers. It seems, using ? distribution model, yields more consistency between design objectives and implementation results.     

Performance of Frequency Division Multiplexing Readout System for AC-Biased Transition-Edge Sensor X-ray Microcalorimeters

Frequency division multiplexing (FDM) is a promising approach to read out a large format transition-edge sensor (TES) array for future astrophysical missions. We constructed a four channel FDM readout system using baseband feedback in the MHz band. We demonstrated the principle of our FDM method with an actual TES array, a multiplexing SQUID and LC band-pass filters under 100 mK. The resonant frequencies of LC filters were consistent with the design value with an accuracy of better than 3 %. We successfully obtained X-ray pulses from two TESs simultaneously but the energy resolution was degraded to about 100 eV at 5.9 keV and crosstalk effects were observed. The origin of the crosstalk effects is investigated by modified setups. Based on comparative experiments and numerical calculations, we conclude that the non-linearity of the SQUID is the cause of some of the crosstalk effects. Unlike the regular crosstalk effect from the adjoining channels, the crosstalk effect due to non-linearity observed in this paper occurs in all channels. Solving these problems will help us to obtain FDM readout with sufficient energy resolution.     

Time jitter influence on the performance of gamma–gamma turbulence FSO links with various modulation schemes

ABSTRACT

The research area of optical wireless communication links has attracted significant interest over the last years due to the significant advantages offered by this kind of technology. However, the performance of FSO communication systems depends strongly on the effects, which are related to the atmosphere along the propagation path such FSO links are using. More specifically, the weather conditions and atmospheric turbulence effects can deteriorate considerably the performance characteristics. In this work, we are studying the joint influence of atmospheric turbulence and time jitter effects on the average BER of an FSO link by presenting a new approach for various modulation schemes. Thus new closed-form mathematical expressions are derived for accurate estimation of the BER performance of the optical wireless communication systems. Finally, using the obtained expressions and typical parameter values for FSO links, the numerical results are presented and then verified through Monte Carlo simulations.     

Outage capacity and outage rate performance of MIMO free-space optical system over strong turbulence channel

Abstract

In this paper, we investigate outage capacity, outage probability, and outage rate performance of multiple-input multiple-output (MIMO) free-space optical system operating over strong turbulence channels. The MIMO optical system employs intensity modulation direct detection with on-off signaling, and equal gain combining technique at the receiver. We derived novel closed-form expressions for three system metrics, namely, outage capacity, outage probability, and outage rate. Expressions derived here are based on the generalized Gamma–Gamma channel model, which is based on scintillation theory that assumes that the irradiance of the received optical wave is modeled as the product of small-scale and large-scale turbulence eddies. The results are evaluated for different values of received signal-to-noise ratios, strong turbulence conditions, and several values of transmit/receive diversity.     

极大似然判决下大气无线光脉冲位置调制差错性能

大气湍流对无线光通信系统影响较大,基于高斯信道及门限判决方法的大气无线光通信脉冲位置调制(PPM)的误码特性分析存在局限性.针对于此,推导了湍流大气中无线光PPM系统在极大似然判决(MLD)下的差错性能计算模型,并运用蒙特卡洛方法验证了理论计算模型的有效性.仿真分析表明,MLD下PPM具有更好性能,但也较易受湍流影响,为使弱湍流信道中PPM的误比特率不高于10-6,调制阶数每增加1阶,须提高平均接收信噪比0.67 dB以上.对于峰值功率恒定PPM系统,应根据器件性能和具体通信条件,在差错性能和功率利用率之间作出平衡.     

逆向调制无线光通信空间分集分析

在逆向调制无线光通信系统中,大气湍流对系统的影响大于传统的无线光通信系统。本文研究了一种基于逆向端调制器分集的逆向调制无线光通信系统,以减小大气湍流对系统的影响。利用相位屏法,建立了弱湍流下的激光大气的传输模型,对比分析了逆向调制无线光通信系统逆向端逆向调制器分集和不分集分别所受到的大气湍流的影响。结果显示在相同情况下,逆向调制无线光通信系统在逆向端对逆向调制器进行分集能抑制大气湍流对系统的影响,使整个系统的闪烁指数下降。     

Effects of diversity schemes and correlated channels on OWC systems performance

The salient features and advantages of free-space optical (FSO) system are very appealing for different applications in a number of communication network sectors. In spite of the advantages and diverse applications of FSO communication, its extensive use is hindered by the atmospheric turbulence-induced fading in real-life scenarios. Spatial diversity technique is one of effective means of mitigating turbulence-induced fading and, consequently, improves the system performance. In this paper, we study the spatial diversity schemes for mitigating turbulence-induced fading in the FSO communication systems using the bit error rate (BER) as a performance metric. The schemes considered are based on orthogonal space–time block codes and repetition codes (RCs). We derive simple approximate closed-form expressions for the error probability of the log-normal FSO links with intensity modulation and direct detection (IM/DD). Furthermore, we also investigate the effects of spatial correlation between the transmit apertures on the system performance. We achieve this using the exponential model for determining the correlations between the apertures. We observe that the proposed BER expressions are able to quantify the effects of spatial diversity schemes and spatial correlations on the system.     

OAM光通信技术研究进展

涡旋光束携带的轨道角动量(OAM)为光波的空间域提供了新的维度资源,吸引了越来越多研究人员的关注。由于具有不同OAM模式值的涡旋光束相互正交,因此将OAM模式引入传统光通信领域,衍生出两种新的应用机制--OAM键控(OAM-SK)与OAM复用(OAM-DM),这为未来实现高速、大容量及高频谱效率的光通信技术提供了潜在的解决方案。本文将从OAM光束的类别和产生方法等基本概念理论出发,对这两种通信应用机制相关的典型研究案例做简要概述,并重点论述三种关键技术,包括OAM光束复用技术、OAM光束解调技术以及OAM光通信的大气湍流效应抑制技术。最后,对OAM光通信技术的未来发展趋势及其前景进行了分析与展望。     

Modified moment-matching method for estimating pointing parameters in the presence of atmospheric turbulence

An accurate pointing system is required in free-space optical (FSO) communication links. Low energy-transmission efficiency caused by pointing errors would decline the communication system's performance. The statistics of the detected signal or return signal values could be used to estimate the pointing parameters, whereas atmospheric turbulence brings in serious challenges. A modified moment-matching estimation method is presented in this paper. The irradiance fluctuation caused by the atmospheric turbulence is considered, and the probability density function (PDF) in a weak turbulence condition is assumed to be lognormal. This modified approach is evaluated with wave-propagation simulation data and shows significant improvement over the conventional approach. The estimation accuracy and the properties of this new approach are also discussed. Although our method is based on lognormal irradiance PDF under a weak turbulence condition, the irradiance PDF would tend to be lognormal with aperture averaging effect under moderate to strong turbulence, and the ideas can be extended with appropriate PDF models to satisfy different conditions.     

涡旋光束轨道角动量在大气湍流传输下的特性分析

从拉盖尔-高斯涡旋光束表达式出发,基于瑞利衍射理论,通过研究涡旋光束在大气湍流中传输时的旋转相干函数的变化规律,总结了涡旋光束在大气湍流中传输时各轨道角动量之间的串扰情况,使用了拓扑荷数探测概率描述串扰规律,并推导了拓扑荷数探测概率的解析表达式。研究了涡旋光束通过湍流后的拓扑荷数的分布情况,并将结果与涡旋光束通过大气随机相位屏的数值仿真结果进行了对比,给出了理论与仿真的拓扑荷数的探测概率随湍流强度以及初始涡旋光束拓扑荷数大小的关系图对比,验证了推导的拓扑荷数探测概率解析表达式的正确性。通过该表达式可进一步研究大气湍流与涡旋光束相互作用从而影响涡旋光束轨道角动量散射的本质,为涡旋光束的空间光通信中选择合适的拓扑荷数间隔,以及在不同湍流强度下选择合适束腰大小以减少串扰带来的误码率提供了理论依据。     

Response analysis of holography-based modal wavefront sensor

The crosstalk problem of holography-based modal wavefront sensing (HMWS) becomes more severe with increasing aberration. In this paper, crosstalk effects on the sensor response are analyzed statistically for typical aberrations due to atmospheric turbulence. For specific turbulence strength, we optimized the sensor by adjusting the detector radius and the encoded phase bias for each Zernike mode. Calibrated response curves of low-order Zernike modes were further utilized to improve the sensor accuracy. The simulation results validated our strategy. The number of iterations for obtaining a residual RMS wavefront error of 0.1λ is reduced from 18 to 3.