Coherent Polarization Shift Keying Modulated Free Space Optical Links Over a Gamma-Gamma Turbulence Channel

Problem statement: The optical signal propagating through the Free Space Optical (FSO) channel suffers from irradiance and phase fluctuations caused by the atmospheric turbulence, which results in Bit Error Rate (BER) performance degradation. Approach: In this study the performance of the Multilevel Coherent Polarization Shift Keying (M-POLSK) based FSO communication system operating over the gamma-gamma turbulence channel is investigated. To mitigate the turbulence induced fading, the convolutional coding and spatial diversity techniques are employed. The upper BER bounds are derived using the transfer function technique. Results: For example, with a SNR of 30 dB, the BERs for uncoded and coded M-POLSK are 0.047 and 1.4×10^-4, respectively in the weak turbulence regime. When the Maximum Ration Combining (MRC) technique employing four detectors are used in the receiver, the power gains of ~31.4, ~29.5 and ~57.9 dB are achieved for weak, moderate and strong turbulence regimes, respectively. Conclusion: We have also shown that the spatial diversity offers increased link margin as the scintillation level rises. Compared to the angular modulation, the proposed M-POLSK scheme offers high immunity to the phase noise, thus reducing the power penalties.