Symbol-Spaced Feedforward Techniques for Blind Bit Synchronization and Channel Estimation in FSO-OOK Communications

Clock recovery and channel parameter estimation are essential functions for reliable communications over a free-space optical (FSO) link. Thanks to its large bandwidth, FSO ensures high-speed transmissions with data rates similar to those enabled by optical fiber links. In such a case, synchronization procedures that operate at symbol rate are highly desirable, since signal oversampling would require expensive and power consuming analog-to-digital converters (ADCs). In this work we investigate the problem of bit synchronization and channel estimation in an FSO link that employs on-off keying (OOK) signaling and is plagued by a mixture of thermal noise, shot noise and background radiation. In doing so we resort to the least-square (LS) estimation principle and look for some feedforward solution that can operate in a non-data-aided (NDA) fashion using symbol-spaced samples. Since the true LS estimator leads to a computationally intractable multi-dimensional optimization problem, we develop suitable approximations that can be implemented with affordable complexity. The accuracy of the proposed schemes is assessed through computer simulations and compared with the relevant modified Cramér-Rao bound (MCRB). Thanks to their ability to operate at symbol rate, they represent a promising solution for blind synchronization in high-speed FSO communications.