Abstract
Spatial diversity reception is an effective technology to mitigate the atmospheric turbulence effect on the performance of the coherent free space optical (FSO) communications. However, considering the real-time and fast atmospheric channel fading variation, how to combine the multiple receiving signals efficiently and adaptively has become one of the key technologies for multiple-aperture receivers. In this paper, to eliminate the time-consuming and complex estimation process of random and time-varying channel fading, a novel adaptive digital combining algorithm is proposed for coherent FSO communications with binary phase-shift keying (BPSK) and quadrature phase-shift keying (QPSK) modulation. For verification of the feasibility of the proposed combining algorithm, corresponding simulations of the proposed adaptive digital combining diversity based and traditional equal gain combining (EGC) diversity based four-aperture receiver system are carried out. The simulation results show that, with the proposed adaptive digital combining, signal quality of the combining output can be improved effectively and the required transmit power can be reduced by up to 3-10 dB under different turbulence conditions for the same BER compared to EGC.
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