A joint frame and carrier frequency synchronization algorithm for coherentoptical systems, based on the digital computation of the fractional Fouriertransform (FRFT), is proposed. The algorithm utilizes the characteristics ofenergy centralization of chirp signals in the FRFT domain, together with thetime and phase shift properties of the FRFT. Chirp signals are used toconstruct a training sequence (TS), and fractional cross-correlation isemployed to define a detection metric for the TS, from which a set of equationscan be obtained. Estimates of both the timing offset and carrier frequencyoffset (CFO) are obtained by solving these equations. This TS is later employedin a phase-dependent decision-directed least-mean square algorithm for adaptiveequalization. Simulation results of a 32-Gbaud coherent polarization divisionmultiplexed Nyquist system show that the proposed scheme has a wide CFOestimation range and accurate synchronization performance even in poor opticalsignal-to-noise ratio conditions.
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