García-Zambrana Antonio, Castillo-Vázquez Carmen, Castillo-Vázquez Beatriz
Department of Communications Engineering, University of Málaga, Málaga, Spain.
Opt Express. 2011 Jul 4;19(14):13480-96. doi: 10.1364/OE.19.013480.
Atmospheric turbulence produces fluctuations in the irradiance of the transmitted optical beam, which is known as atmospheric scintillation, severely degrading the performance over free-space optical (FSO) links. Additionally, since FSO systems are usually installed on high buildings, building sway causes vibrations in the transmitted beam, leading to an unsuitable alignment between transmitter and receiver and, hence, a greater deterioration in performance. In this paper, the outage probability as a performance measure for multiple-input/multiple-output (MIMO) FSO communication systems with intensity modulation and direct detection (IM/DD) over strong atmospheric turbulence channels with pointing errors is analyzed. Novel closed-form expressions for the outage probability as well as their corresponding asymptotic expressions are presented when the irradiance of the transmitted optical beam is susceptible to either strong turbulence conditions, following a negative exponential distribution, and pointing error effects, following a misalignment fading model where the effect of beam width, detector size and jitter variance is considered. Obtained results show that the diversity order is independent of the pointing error when the equivalent beam radius at the receiver is at least twice the value of the pointing error displacement standard deviation at the receiver. Simulation results are further demonstrated to confirm the analytical results. Additionally, since proper FSO transmission requires transmitters with accurate control of their beamwidth, asymptotic expressions here obtained for different diversity techniques are used to find the optimum beamwidth that minimizes the outage performance.
大气湍流会使传输光束的辐照度产生波动，这被称为大气闪烁，会严重降低自由空间光（FSO）链路的性能。此外，由于FSO系统通常安装在高层建筑上，建筑物的晃动会导致传输光束产生振动，导致发射器和接收器之间的对准不合适，从而使性能进一步恶化。本文分析了在存在指向误差的强大气湍流信道上，采用强度调制和直接检测（IM/DD）的多输入多输出（MIMO）FSO通信系统的中断概率这一性能指标。当传输光束的辐照度易受强湍流条件（服从负指数分布）以及指向误差影响（服从失准衰落模型，其中考虑了光束宽度、探测器尺寸和抖动方差的影响）时，给出了中断概率的新颖闭式表达式及其相应的渐近表达式。所得结果表明，当接收器处的等效光束半径至少是接收器处指向误差位移标准差的两倍时，分集阶数与指向误差无关。仿真结果进一步证明了分析结果。此外，由于适当的FSO传输需要发射器精确控制其光束宽度，这里针对不同分集技术获得的渐近表达式被用于找到使中断性能最小化的最佳光束宽度。
