Abstract
Atmospheric turbulence is a practical obstacle in deployment of free space optical (FSO) communications. Due to its ease of implementation, on-off keying (OOK) is a widely-adopted modulation technique for intensity modulation with direct detection (IM/DD) in optical communications. However, for coherent detection of OOK symbols, the optical receiver requires an accurate estimate of the instantaneous channel fading coefficient. To mitigate the effect of atmospheric turbulence and pointing errors in the detection of OOK symbols, here, we propose a maximum a posteriori probability (MAP) channel estimation based on the transmission of training symbols. A major drawback of MAP estimator is its computational complexity, since it requires complex numerical computations to find the optimum instantaneous channel coefficient. To reduce the implementation complexity for practical deployments, we also propose a near MAP channel estimation method which has preserved the accuracy of the MAP estimator. Several numerical results are provided to prove the efficiency of the proposed channel estimator in the presence of practical atmospheric impairments such as turbulence and pointing errors.
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