Coherent Free Space Optical Digital Communication

Free Space Optical Communication (FSO) is one of the wireless communications, the carrier is optical wave. Compared with the traditional radio frequency (RF) wireless communication, FSO has the advantages of super broad bandwidth, license-free operation, high security, lesser terminal volume and weight, low cost and high flexibility. As a result, it attracts much interests. However, the performance of the FSO system is greatly affected by the propagation medium. The signal through atmosphere usually suffers from the attenuation effect by clouds or precipitation and from atmospheric scintillations. One of the most significant factors that limit the performance of FSO systems is the effect of atmospheric turbulence which mainly results in amplitude and phase fluctuation of optical signals. Based on the detection technique of optical fiber system, it can be either non-coherent (intensity modulation/direct detection,(IM/DD)) or coherent (homodyne detection) system. We know that IM/DD has such disadvantages as low spectral efficiency and low sensitivity which would weaken the performance of FSO system. Comparing to traditional intensity modulation and direct detection (IM/DD) scheme, coherent detection is a more complicated detection method but has the ability to enable higher receiver sensitivity and less receive signal power, and overcome the thermal noise effect. Optical coherent detection can also provide an effective way to mitigate the influences of atmospheric turbulence. So, this paper focuses on FSO system which is based on digital coherent receiver technology. The main content is as follows:1. The modulation of coherent FSO is studied. The principle of QPSK modulated by DPMZM is introduced and the BER performance of different modulation formats is compared, with the conclusion that QPSK has a lower BER for the system.2. The digital coherent receiver technology of FSO is analyzed, including the principle of coherent receive and noise analysis. Finally, the Sebastian Hoffmann and Viterbi&Viterbi algorithms for signal frequency and phase compensation are emphasis researched.3. The atmospheric channel fading is analyzed, and the intensity fluctuation, phase fluctuation, beam broadening caused by atmospheric turbulence are studied. The process of Gaussian beam propagations through the atmospheric channel is simulated by the approach of phase screen. Some valuable conclusions is drawn in experiments.4. The coherent FSO communication experiment platform is designed and built. Then the10GBaud indoor and outdoor FSO transmission based on QPSK is launched, the distance of FSO link indoor and outdoor are6m and600m, respectively. A BER of10"6at about-17.6dBm is achieved. The BER indoor is approximately7dB better than outdoor because of weaker turbulence. And the BER in day would be higher than it at night due to the interference of background light. After adding the compensation algorithm, the constellation of the received optical signal is significantly improved.