Time Domain Synchronous OFDM System For Optical Fiber Communications

With the continuous improvement of the degree of social information and the continuous advancement of communication technology,high-speed optical fiber communication has become more and more dominant in the field of modern communication.Coherent optical orthogonal frequency division multiplexing(CO-OFDM)technology has become a mainstream solution for long-distance,large-capacity transmission systems,due to its high sensitivity and good anti-dispersion.However,the current widely used CO-OFDM system mostly uses the cyclic prefix(CP)as the guard interval.In order to ensure the reliable transmission of the signal in the fiber channel,CP-OFDM needs to add a large number of pilots into the effective signal for synchronization and channel estimation.In other words,it improves system transmission performance by sacrificing bandwidth.Time domain synchronous orthogonal frequency division multiplexing(TDS-OFDM)uses a pseudo noise(PN)sequence as the guard interval,combining the training symbols with the guard interval,avoids destroying the integrity of the valid data.The redundant data is reduced;thereby the spectrum efficiency of the system is improved.Meanwhile,TDS-OFDM uses the correlation characteristics of the PN sequence to realize the functions such as timing synchronization;carrier frequency offset(CFO)estimation and channel estimation,which has its unique superiority.First,this paper introduces the key technologies of OFDM such as modulation and demodulation,Inverse Discrete Fourier Transformation and Discrete Fourier Transform(IDFT/DFT),the design of guard interval and so on in detail.We further explain the basic principles of TDS-OFDM system,and analyze the correlation characteristics of PN sequences.Then we elaborate the principles and the features of CO-OFDM systeIIm.Based on the MATLAB sirmulation platform,we design CO-TDS-OFDM system scheme,modify the frame structure of OFDM symbol,build the system simulation model,and introduce the key digital signal processing(DSP)module needed to realize CO-TDS-OFDM transmission system at last.Second,this paper analyzes the influence of timing synchronization error on the performance of OFDM system carefully.The timing synchronization algorithms based on training sequence are deeply studied.We explain the principle of Schmidl&Cox algorithm,Minn algorithm and Park algorithm respectively,and propose an improved timing synchronization algorithm based on the sliding correlation characteristics of PN sequence.The above algorithms are analyzed and compared through the MATLAB simulation platform.The results show that the proposed timing synchronization algorithm in this paper solves the problems of the traditional algorithms.There is no peak platform,the timing curve is sharp,and the side lobe interference is disappeared.The proposed timing synchronization algorithm has high timing accuracy and maintains stable performance in the fiber dispersion channel.Third,this paper analyzes the influence of carrier frequency offset on the performance of OFDM system carefully.The frequency offset estimation algorithm of Schmidl&Cox is studied in depth.An improved CFO estimation and compensation algorithm based on the correlation characteristics of PN sequence is proposed.The proposed algorithm guarantee the accuracy of the frequency offset estimation;meanwhile improve the communication capacity of the system.We compare and analyze the performance of the two frequency offset estimation algorithms by the MATLAB simulation platform.As the simulation results show,the frequency offset error of the proposed algorithm is smaller than that of the Schmidl&Cox algorithm in QPSK and 16QAM modulation formats,which means that the proposed CFO estimation and compensation algorithm is better.At the same time,under different optical signal-to-noise ratios(OSNR),the PN sequence maintain the same good correlation,that is,CO-TDS-OFDM can obtain accurate frequency offset estimation under various transmission conditions.Last,an improved channel estimation algorithm based on autocorrelation characteristics of PN sequence is proposed for eliminating noise interference in CO-TDS-OFDM system.Based on the MATLAB simulation platform,CO-TDS-OFDM and CO-CP-OFDM systems are constructed for comparison.The simulation results show that in the QPSK modulation format,the bit error rate(BER)of CO-TDS-OFDM system is less than 3.8e-3 when the OSNR is about 13dB,whose performance is slightly better than that of the CO-CP-OFDM system.In the 16QAM modulation format,the BER of the CO-TDS-OFDM system is less than 3.8e-3 when the OSNR is about 20dB,whose OSNR is 1.5dB lower than that of the CO-CP-OFDM system.In addition,under the above conditions,the spectrum efficiency of the CO-TDS-OFDM system is 11%higher than that of the CO-CP-OFDM system.Therefore,the CO-TDS-OFDM transmission system using the PN sequence as the guard interval has higher communication capacity,more stable anti-noise performance;thereby it can provide high-speed,large-capacity reliable transmission.