Performance enhancement techniques for free-space optical transmission systems

Free-space optical (FSO) communication offers many advantages including high capacity, high security, free spectrum license and easy to install and remove. However due to the inhomogeneity of temperature and pressure fluctuations, the intensity of an optical wave fluctuates when it propagates through the atmosphere. Such fluctuation is known as scintillation, which can deteriorate the FSO transmission performance severely. We have recently investigated and analyzed two effective techniques to mitigate the adverse effect of scintillation. These techniques include a dynamic detection threshold (DDT) scheme with coherent detection and a reference continuous wave (CW) light assisted detection technique. They are reviewed and discussed in detail.