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Atmospheric turbulence and pointing errors cause intensity fluctuation of free space optical communication signals and impair link performance. Several receiver structures which could mitigate the signal fluctuations were proposed in the past, but these existing receivers depend highly on the channel model and the model parameters. The performance deteriorates if the channel model or the model parameters are inaccurate. In this paper, we develop a Viterbi-type trellis-search sequence receiver based on the generalized likelihood ratio test principle that jointly detects the data sequence and estimates the unknown channel gain. This receiver requires very few pilot symbols, and therefore, does not significantly reduce the bandwidth efficiency. It is robust in that it continuously performs maximum likelihood (ML) estimation of the unknown channel gain without the knowledge of the channel model, and adapts the decision metric accordingly. It works well in a slowly time-varying environment and its error performance approaches that of ML detection with perfect knowledge of the channel gain, as the memory length used for forming the sequence detection metric increases. A new, decision-feedback, symbol-by-symbol receiver with lower implementation complexity and higher memory efficiency is obtained as an approximation to the sequence receiver. The performance improvement and implementation simplicity of our receivers compared to existing receivers are pointed out.