Research On Sampling Clock Synchronization In Coherent Optical Orthogonal Frequency-division Multiplexing Systems

Orthogonal frequency-division multiplexing(OFDM)technology can be considered as a kind of multi-carrier modulation technology,has already been employed in many communication standards due to its high spectrum efficiency,strong anti-interference ability,modulation and demodulation can be implemented by fast Fourier inverse transformation(IFFT)and fast Fourier transform,(FFT).Optical Orthogonal frequency-division multiplexing merges the orthogonal frequency-division multiplexing technology with the optical fiber communication technology,has the advantages of dispersion tolerance.Coherent optical orthogonal frequency-division multiplexing(CO-OFDM)system has received more attention due to its large capacity,high rate and long distance.However,the sampling clock at transmitter and receiver in CO-OFDM system are different.In this way,it will produce the Inter-Carrier Interference(ICI)and the phase rotation of subcarriers.As a result,the performance of a coherent optical orthogonal frequency-division multiplexing system will be seriously degraded.In this thesis,sampling clock synchronization of CO-OFDM systems is mainly studied.The main research results are as follows:Firstly,sampling clock frequency offset(SCFO)in CO-OFDM systems is theoretical analysis,and a pilot-aided scheme is adopted for estimation and compensation of sampling clock frequency offset in CO-OFDM systems.SCFO will mainly cause ICI and the phase rotation of carriers.Due to the influence of the former is small,it should not be considered in the thesis,and mainly considering the phase rotation of subcarriers.Through analysis,the phase rotation of carriers is linear relationship with subcarrier index.According to the pilots inserted in transmitter,through the comparison between transmitted pilots and received pilots after FFT,it can get the phase rotation of pilot subcarriers.Then,by the least squares(LS)algorithm,phase rotation factor can be calculated.Finally,in the frequency domain,the pilot-aided scheme can directly compensate the phase rotation of all subcarriers.Consequently,the sampling clock synchronization can be completed only by DSP,and the loss of optical signal-to-noise ratio is less than 1dB.Therefore,it can reduce the cost of system.The simulated results show that the method can work quite effectively even with large sampling clock frequency offsets.Secondly,the location of pilots will impact the performance of the pilot-aided scheme,through the theoretical analysis,the optimal location of pilot inserted is obtained.Compared with five different location of pilots inserted,the simulated results show that the average insertion of pilots is optimal in generally,however,with increasing sampling frequency offset,if the method cannot work well,the pilots should be inserted in the low frequency subcarriers.