Contents 1. Introduction
Because of the high spectral efficiency and the resistance to chromatic dispersion, coherent optical orthogonal frequency division multiplexing (CO-OFDM) has attracted many researcher's attention. However, the sensitivity of symbol timing and carrier frequency offset (FO) is one of the major drawbacks of CO-OFDM systems [1]. Several methods have been proposed to estimate the timing and carrier frequency offset [2]–[4]. Compared to the schemes with pilot or cyclic prefix aided, using training sequence for synchronization is one of the most important synchronization methods because it can provide higher accuracy [5]. The most popular training sequence aided synchronization methods were proposed by Schmidl and Minn in [6]–[7]. The Schmidl's algorithm used two training sequences to estimate timing and carrier frequency offsets, however the timing metric has a peak platform with a length of about cyclic prefix size. To eliminate this peak platform, Minn improved the Schimidl's algorithm by changing the structure of the training sequence, but this algorithm has large side lobes. It can be affected by the cyclic prefix, even the multi peak phenomenon will occur which may disturb its timing performance seriously. Yuan Liu redesigned the configuration of the preamble to improve Minn's algorithm [8]. In this paper, we present a timing synchronization algorithm by a kind of new timing metric function as a modification. Simulation results show the mean square error (MSE) of this algorithm is lower, and it has a better tolerance to the chromatic dispersion.
