Research On Synchronization Technologies Based On FH Sequence For CO-OFDM System

High speed and long distance are the challenging requirements of communication systems.In order to meet this requirement,coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology has become one of the most promising technologies for optical communications.CO-OFDM technology combines the advantages of coherent optical communication system and OFDM technology.CO-OFDM not only possesses the high sensitivity,long distance transmission of coherent optical communication technology,also has the OFDM technology resistance dispersion,polarization mode dispersion optical fiber channel,high spectrum efficiency and other advantages.This paper focuses on the CO-OFDM system and analyzes the problems that CO-OFDM systems need to solve.Timing synchronization and frequency synchronization are the key to achieve reliable transmission of CO-OFDM systems.The traditional classical synchronization algorithms that based on PN sequences have some drawbacks,such as the platform effect of the Schmidl algorithm and the sidelobe blurring of the Park algorithm.In view of this situation,this paper uses ZC(Zadoff-Chu)sequences and FH(Frank-Heimiller)sequences with strong autocorrelation as training sequences,and proposes timing synchronization algorithms based on ZC sequences and timing synchronization and frequency synchronization based on FH sequences.The algorithm overcomes the disadvantages of the classic synchronization algorithm.The main innovation work of this paper as follows:(1)The timing synchronization algorithm based on ZC sequence is proposed for CO-OFDM system.The ZC sequence is a kind of CAZAC sequence and has excellent characteristics of the CAZAC sequence.By utilizing the even symmetry feature of even numbered ZC sequences,the timing synchronization algorithm based on ZC sequence is proposed.It's timing metric function only obtains a very sharp single peak at the correct peak point,which avoids the peak platform effect of the Schmidl timing synchronization algorithm and eliminates the side lobe ambiguity of the Park timing synchronization algorithm.Proposed timing synchronization algorithms based on ZC sequences can achieve accurate timing synchronization.Timing synchronization comlements frequency synchronization,so this paper explores the impact of system frequency offset on the timing synchronization algorithm based on ZC sequences.Theoretical analysis and experimental simulations show that when the normalized frequency deviation is 13.5,the timing metric function has multiple pulses,which causes timing synchronization to be inaccurate.(2)The timing synchronization and frequency synchronization algorithm based on FH sequence is proposed for CO-OFDM system.The timing synchronization algorithm based on ZC sequences has weak performance of anti-frequency.Using the strong autocorrelation of FH sequences to design the training sequence structure and defining the timing metric function,a timing synchronization algorithm based on FH sequences is proposed.The timing synchronization algorithm based on FH sequence has the advantage of being robust to frequency offset,and its timing metric function has a sharp single peak at the correct peak point.On the basis of obtaining accurate timing,frequency synchronization is achieved using FH training sequences.The fractional and integer frequency offset estimate is realized by using only one training sequence.