A modified synchronization scheme for coherent optical OFDM systems

Since the timing and frequency synchronizations are critical steps in the signal processing of the coherent optical orthogonal frequency-division-multiplexing(CO-OFDM) system,modified methos are proposed for timing and frequency synchronizations respectively in the CO-OFDM system in this paper.The modified timing synchronization method uses a modified training symbol weighted by a pseudo-noise(PN) sequence and solves the ambiguity problem in Park′s method,while the modified frequency synchronization method uses a single training symbol to implement both the partial and the integer frequency offset estimations.and maintains equivalent accuracy compared with schmidl′s method which needs two training symbols.The modified synchromization scheme is tested in a 20 Gbit/s 256-subcarrier quadrature phase shift keying(QPSK) modulated CO-OFDM system by Matlab simulation,and the results show that the modified scheme gives a more accurate timing estimation and a more economical frequency offset estimation in a wider frequency range.In addition,with the aid of the chromatic dispersion(CD) compensation based on the results of the CD monitoring before the synchronization steps,the modified timing synchronization works more stably,and the subcarrier recovery achieves better performance,reducing the optical signal-to-noise rate(OSNR) requirement by 3 dB at the bit error rate(BER) of 10-3.