BER analysis of DPSK–SIM over MIMO free space optical links with misalignment

Free-space optics (FSO) is a safe, license-free, high speed and high-bandwidth cordless technique for broadcasting signals at the same speed as fiber optics. But the main drawback of FSO is the dissolution of the signals due to atmospheric turbulence and thus degrading the link performance. So, to enhance the bit error rate (BER) performance, spatial diversity is employed over the FSO links for single-input multiple-output FSO (SIMO-FSO) systems. In this paper, we analyze the BER performance of SIMO-FSO links with spatial diversity and multiple-input multiple-output FSO (MIMO-FSO) systems over gamma–gamma atmospheric turbulence fading channel with misalignment (pointing errors). The closed form expressions are derived for the same with various combining schemes in terms of the Meijer G function and also evaluate the results with 2D and 3D graphs.     

Influence of atmospheric turbulence on states of light carrying orbital angular momentum

We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.     

A new method of calculating the orbital angular momentum spectra of Laguerre-Gaussian beams in channels with atmospheric turbulence

Studying orbital angular momentum(OAM) spectra is important for analyzing crosstalk in free-space optical(FSO)communication systems. This work offers a new method of simplifying the expressions for the OAM spectra of Laguerre-Gaussian(LG) beams under both weak/medium and strong atmospheric turbulences. We propose fixing the radius to the extreme point of the intensity distribution, review the expression for the OAM spectrum under weak/medium turbulence,derive the OAM spectrum expression for an LG beam under strong turbulence, and simplify both of them to concise forms.Then, we investigate the accuracy of the simplified expressions through simulations. We find that the simplified expressions permit accurate calculation of the OAM spectrum for large transmitted OAM numbers under any type of turbulence. Finally,we use the simplified expressions to analytically address the broadening of the OAM spectrum caused by atmospheric turbulence. This work should contribute to the concise theoretical derivation of analytical expressions for OAM channel matrices for FSO-OAM communications and the analytical study of the laws governing OAM spectra.     

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.     

Performance enhancement of the power penalty in DWDM FSO communication using DPPM and OOK modulation

This paper presents the design and performance enhancement of the power penalty (PP) in a dense wavelength division multiplexing based on free space optical communication (FSOC) link using digital pulse position modulation (DPPM) and on–off keying (OOK) modulation. Such a system has a high performance, low cost, robust and power efficient, reliable, excessive flexibility, and higher data rate for access networks. The system performance is evaluated for an 8-channel wavelength-division-multiplexing for hybrid fiber FSOC system at 2.5 Gbps on widely accepted modulation schemes under various atmospheric turbulence (AT) regimes conditions. The performance of system is introduced in terms of PP, bit-error rate (BER), transmission distance and the average received optical power. The numerical results shows that the improvement of the PP using DPPM modulation of 0.2–3.0 dB for weak turbulence (WT) regimes for BER of 10^?6 and above 20, 25 dB for strong turbulence (ST) regimes are reported for BER of 10^?6 and 10^?9, as respectively (depending on the AT level). Further, we develop of improvement the PP caused by multiple-access interference about 6.686 dB which is predicted for target BER of 10^?9 in WT and 1 dB at target BER of 10^?6 in ST when the 8 user are active on the system of optical network units. Additionally, the optical power budget and margin losses of a system are calculated with different link length. The proposed approach of DPPM merges superiority with higher enhancement of PP about 0.8 dB for BER equal 10^?9 at FSO link length l_fso?=?2000 m compared to OOK at 1 dB for WT. An improvement of 2 dB is observed using the DPPM scheme over an OOK due to capability of detect pulses under background noise conditions with increased receiver sensitivity.     

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.     

基于高阶累计量的大气光通信自适应信号处理

针对强湍流信道下信号衰落的特点,分析了对数正态分布模型与K分布模型的适用范围.基于K分布模型建立大气光通信接收信号模型,并给出了自适应最优门限检测方法.采用四阶和六阶累计量对强湍流信道参量进行估计,采用二阶累计量对其它高斯噪音进行估计,得到K分布参量及高斯噪音统计量的预测值,实现自适应门限更新.基于Monte Calro算法进行仿真,给出了门限更新算法对通信系统误码率的影响,同时分析了信号采样率对估计参量偏差的影响.计算表明,在强湍流情况下,大气光通信系统的误码率性能得到极大的改善,优于基于MLSD检测的接收机.     

A design of novel photonic crystal fiber with low and flattened dispersion for supporting 84 orbital angular momentum modes

A dual-guided photonic crystal fiber(PCF) with low and flattened dispersion is designed, which can support a large number of orbital angular momentum(OAM) modes. The properties of the proposed PCF are systematically analyzed through the finite element method. The results show that the proposed PCF can support up to 84 OAM modes with 600 nm bandwidth ranging from 1000 to1600 nm. All values of mode purity are above 91.7%, the isolation parameters are larger than 67 dB and the maximum value up to 145 dB, the lowest confinement loss is only 5×10~(-13) dB·m~(-1).More importantly, the values of dispersion for all modes are less than 40 ps·km~(-1)·nm~(-1), and the lowest dispersion variation is only 0.7 ps·km~(-1)·nm~(-1). These superior optical properties make the proposed PCF have great advantage in stable transmissions of data and long-distance optical fiber communication system with large capacity.     

混沌与湍流大气中的光通信

目前, 光无线通信的质量主要受到大气信道环境的制约, 大气信道中混沌介质与湍流的强烈扰动使得通信质量很差, 甚至通信中断. 提出了一种面到点的光无线通信机理: 利用面阵各单元的光信号在混沌介质中传输通道的空间非相干性, 通过桶探测器收集通过混沌介质的光信号的能量和, 平均各传输通道的交叉干扰, 降低混沌介质对光无线通信的影响; 利用随机噪声与随机编码的空间非相干性, 经过二阶相关运算, 构建新的信号传输方程, 减弱大气湍流及背景光对信号解码的干扰, 使得接收端并不需要窄带光学滤波器. 数值仿真和演示实验表明, 该光无线通信机理在混沌与湍流大气中的误码率为10^-4-10^-2, 能够实现复杂大气环境中的光通信, 在军事、抢险救援等方面具有重要应用价值.     

High speed,long reach OFDM-FSO transmission link incorporating OSSB and OTSB schemes

Free Space Optics (FSO) link is extremely responsive to the diverse climate state of affairs that bound the FSO range. A demonstration of fading resistant FSO system using a simulated test-bed employing OFDM scheme is reported in this work to realize the prolonged FSO link with acceptable SNR and BER with the highest stream rate of 5 Gb ps under the impact of diverse weather conditions. Simulations point toward that the proposed hybrid OFDM-FSO transmission system incorporating OTSB- and OSSB-schemes promises significantly enhanced FSO link compared to conventional FSO systems.     

BER analysis of BPSK-SIM-based SISO and MIMO FSO systems in strong turbulence with pointing errors

Free space optics (FSO) is one of the sprouting technologies in optical communication systems domain. It can be employed as an alternative for the conventional radio frequency (RF) links to work out the current limitations in communication systems. But, the major drawback in FSO communication is the effect of random environment conditions on its performance. In this paper, we analyze the bit error rate (BER) and outage performance of single-input single-output (SISO) and multiple-input multiple-output (MIMO) FSO systems in strong atmospheric turbulence using binary phase shift keying subcarrier intensity modulation (BPSK-SIM) signaling technique. The closed-form expressions are derived and the results are realized in terms of 2D and 3D plots.     

Fiber-based multiple-access frequency synchronization via 1f–2f dissemination

Considering the reference frequency dissemination requirements of the Square Kilometre Array telescope(SKA)project,on the basis of the 1f–2f precision frequency synchronization scheme,we propose and demonstrate a fiber-based multiple-access frequency synchronization scheme.The dissemination reference frequency can be recovered at arbitrary nodes along the entire fiber link.It can be applied to antennas close proximity to the SKA central station,and will lead to a better SKA frequency synchronization network.As a performance test,we recover the disseminated 100-MHz reference frequency at an arbitrary node chosen as being 5 km away from the transmitting site.Relative frequency stabilities of2.0×10~(-14)/s and 1.6×10~(-16)/10~4 s are obtained.We also experimentally verify the feasibility of a frequency dissemination link with three access points.     

Effects of multi-Schell beams on orbital angular momentum of entangled signal photons in low-order turbulence aberration channels

We model the detection and crosstalk probability of orbital angular momentum (OAM) states of the entangled signal photon in the Kolmogorov channels of the low-order turbulence aberrations and by the Rytov approximation. The results show that lower OAM mode number of signal photons and larger sub-beam number of multi-Gaussian Schell-model pump beam, the less susceptible of the detection probability of the signal photon to spatial coherence of source and turbulence aberrations is achieved. The maximum crosstalk probability is decrease as the decreasing of the sub-beam number of multi-Gaussian Schell-model. Enlarging OAM difference value or decreasing sub-beam number of multi-Gaussian Schell-model pump beam results in a lower crosstalk probability of the OAM of entangled signal photons.     

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.     

Infrared absorbance cross-sections for dinitrogen pentoxide vapour

Laboratory measurements of the ν₁₂ absorption band of gaseous N₂O₅ have been made using Fourier transform infrared (FTIR) spectroscopy at seven different temperatures between 233 and 293 K. Integrated band strengths and absorbance cross-sections per molecule show no significant temperature dependence. Their average values are S_int = (4.09±0.17) × 10^-17 cm molecule ^-1 and σ_peak = (1.90±0.08) × 10^-18 cm² molecule^-1.     

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。     

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.     

Joint influence of heterogeneous stochastic factors on bit-error rate of ground-to-ground free-space laser communication systems

Research methods for the formation of BER in the ground-to-ground Free-Space Laser Communication Systems (Free-Space Optics – FSO) with increased analytical effectiveness have been suggested. They correspond to physically rightful approximations with approbated quantity effectiveness. The methods have been used to study the influence of heterogeneous stochastic factors, such as: quantum noises, atmospheric transmittance fluctuations, mechanical vibrations of the transmitting laser antenna, atmospheric turbulence, on BER of FSO.

An algorithm for quantity estimation of the joint influence of all stochastic factors has been created. Numerical examples, illustrated by graphic material, have been shown.     

A cost effective 100 Gbps hybrid MDM–OCDMA–FSO transmission system under atmospheric turbulences

FSO or free space optics is a familiar name used in a wide array of applications in the area of telecommunications. Due to its features of low maintenance cost and deployment time, most of the applications consider FSO as the alternative solution for appropriately replacing fiber optics. In this work, we have designed 100 Gbps FSO system by combining mode division multiplexing (MDM) and optical code multiple access scheme (OCDMA). Ten channels, each carrying 10 Gbps data, are transported over 8 km FSO link by using MDM of two Laguerre Gaussian modes and random diagonal codes. Moreover, the performance of proposed MDM–OCDMA–FSO system is also investigated under atmospheric turbulences.