基于指向性误差校正技术的自由空间光通信系统BER分析

针对指向性误差对自由空间光通信系统性能的影响如误码率以及传统指向性误差校正技术存在的实施难度大等问题,本文在焦面上,基于指向性误差校正技术对FSO系统进行BER分析。首先,讨论了指向性误差校正技术的基本原理,并得到了指向性误差与FSO系统BER之间的关系,设计了一种PID闭环控制算法。其次,搭建了基于PID算法的闭环控制系统,并进行了相关实验。实验结果表明:针对不同大气湍流扰动强度的FSO系统,所述指向性误差校正方法和PID闭环控制算法均具有较好的效果,其中弱扰动情况下的改善效果尤其明显。

车体横向振动对高铁光通信指向误差的影响

为研究在高速列车场景下车体横向振动对自由空间光(free space optical,FSO)通信指向误差的影响,首先利用TB/T 3352—2014《高速铁路无砟轨道不平顺谱》中的轨向、轨距不平顺功率谱推导出横向合成不平顺谱公式,用谱公式仿真生成横向合成不平顺数据;其次建立1/4车体3自由度横向车体振动模型,使用拉普拉斯变换推导出车体横向振动微分方程组的传递函数,搭建车体振动的Simulink仿真模型,推导车体横向位移的功率谱密度(power spectrum density,PSD),经统计得出仿真车体横向位移的概率密度函数(probability density function,PDF);最后利用车体横向位移PDF推导其导致的指向性误差的PDF。仿真验证了轨道横向合成不平顺谱公式、车体横向位移的PSD和PDF,以及横向位移导致的指向性误差的PDF。本研究获得的指向性误差的PDF可用于高铁FSO通信链路的信道建模和系统性能分析。

车体垂向振动对高铁FSO通信性能影响的研究

为了将自由空间光(free space optical,FSO)通信技术应用于高速铁路通信,研究车体垂向振动对FSO通信性能的影响。首先,根据轨道不平顺功率谱密度函数推导其在频域的关系表达式,并利用逆傅里叶变换法对轨道不平顺进行仿真;然后,根据1/4车体4自由度模型的微分方程,利用拉普拉斯变换推导传递函数搭建基于传递函数的Simulink仿真模型;最后,通过对车体垂向位移的仿真,推导其概率密度函数表达式,并在高速铁路FSO通信链路中应用动态束腰控制方案,即在发射端实时调整束腰。仿真结果验证了车体垂向位移的概率密度函数表达式,证明动态束腰控制方案在高速铁路FSO通信链路中性能优于传统的固定束腰控制方案。因此,车体垂向位移的概率密度函数和动态束腰控制方案可为高铁FSO通信研究提供理论基础。

Performance analysis of relay-aided free-space optical communication system over gamma-gamma fading channels with pointing errors

The average bit error rate(ABER) performance of a decode-and-forward(DF) based relay-assisted free-space optical(FSO) communication system over gamma-gamma distribution channels considering the pointing errors is studied. With the help of Meijer's G-function, the probability density function(PDF) and cumulative distribution function(CDF) of the aggregated channel model are derived on the basis of the best path selection scheme. The analytical ABER expression is achieved and the system performance is then investigated with the influence of pointing errors, turbulence strengths and structure parameters. Monte Carlo(MC) simulation is also provided to confirm the analytical ABER expression.

Large-scale spatial angle measurement and the pointing error analysis

A large-scale spatial angle measurement method is proposed based on inertial reference. Common measurement reter- ence is established in inertial space, and the spatial vector coordinates of each measured axis in inertial space are measured by using autocollimation tracking and inertial measurement technology. According to the spatial coordinates of each test vector axis, the measurement of large-scale spatial angle is easily realized. The pointing error of tracking device based on the two mirrors in the measurement system is studied, and the influence of different installation errors to the pointing error is analyzed. This research can lay a foundation for error allocation, calibration and compensation for the measurement system.

Performance analyses of subcarrier BPSK modulation over M turbulence channels with pointing errors

An aggregated channel model is achieved by fitting the Weibull distribution, which includes the effects of atmospheric attenuation, M distributed atmospheric turbulence and nonzero boresight pointing errors. With this approximate channel model, the bit error rate （BER） and the ergodic capacity of free-space optical （FSO） communication systems utilizing sub- cartier binary phase-shift keying （BPSK） modulation are analyzed, respectively. A closed-form expression of BER is de- rived by using the generalized Gauss-Lagueree quadrature rule, and the bounds of ergodic capacity are discussed. Monte Carlo simulation is provided to confirm the validity of the BER expressions and the bounds of ergodic capacity.

Outage performance of multihop free-space optical communication system over exponentiated Weibull fading channels with nonzero boresight pointing errors

The outage performance of the multihop free-space optical(FSO) communication system with decode-and-forward(DF) protocol is studied by considering the joint effects of nonzero boresight pointing errors and atmospheric turbulence modeled by exponentiated Weibull(EW) distribution. The closed-form analytical expression of outage probability is derived, and the results are validated through Monte Carlo simulation. Furthermore, the detailed analysis is provided to evaluate the impacts of turbulence strength, receiver aperture size, boresight displacement, beamwidth and number of relays on the outage performance for the studied system.