Receiver Design for OOK Modulation Over Turbulence Channels Using Source Transformation

For coherent detection of ON-OFF keying (OOK) symbols in free-space optical (FSO) communications, the receiver requires the instantaneous channel fading coefficients. To accurately detect OOK symbols over FSO channels without requiring transmission of pilot symbols, in the first part of this letter, we propose an expectation-maximization (EM)-based sequence detection (SD) method with low implementation complexity which is shown to be particularly suitable for fast FSO communication systems. In the second part of this letter, to remove the error floor and to improve the accuracy of the proposed detector, by utilizing a source information transformation we propose an EM-based SD for this state which significantly outperforms than classical EM-based method.