High Efficient Signal Processing In Fiber-optics Communication Systems

With the increasing of transmission distance and rate, the fiber chromatic-dispersion, polarization-mode-dispersion and nonlinear-effect will seriously affect the performance of transmission systems. To solve this problem, the paper focuses on the high efficient signal processing in high-rate and long-haul fiber-optics communication systems. We have explored the OFDM radio-over-fiber system which the advantage of high spectral efficiency, high-capacity and strong dispersion resistance of OFDM signal to realize seamless integration of the wireless and fiber communication system. Comparing to the direct-detection systems, the coherent-detection systems have higher receiving sensitivity and capacity, and can promote transmission distance. However, in coherent-detection systems, the two transceivers lasers’frequency is not possible the same, this will lead to the rotation and shifting of the constellation. Also, the phase noise of the lasers will damage the transmission system. Therefore, to promote the performance of the fiber transmission system, this paper involves some related key technical problems. The paper includes the following parts:First, we propose two methods including the Hadamard transforming with companding and the FE-Nyquist pulse sharping technique to reduce the PAPR of OFDM signal to diminish the nonlinear effect of electrical and optical device. The Hadamard transforming technique with companding takes the advantage of the nonsinusoidal orthogonal transforming without complex multiply operation, so as to reduce the signal correlation. The companding transforming which amplify the small amplitude signals and compress the large amplitude signal is used at the OOFDM transmitter to reduce the signal PAPR. The Hadamard transforming technique also takes the advantage of orthogonality FE-Nyquist matrix. The product of OFDM signal and the FE-Nyquist matrix has certain correlation, and this enhance the correlation of the sub-carriers of the OFDM signal so as to reduce the OFDM signal’s PAPR.Second, we employ the LDPC coding in catenation with TCM coding to promote the performance of OFDM-ROF system performance. We analyze the function and advantage of ROF system. And also we make an introduction of the LDPC and TCM coding, theoretically analyze the LDPC and TCM coding and decoding principle and structure. The TCM coding can get obvious coding gain without decrease system bandwidth and power utilization. We build a60GHz OFDM-ROF experimental system and verify that the LDPC-TCM coding can promote its performance.Third, to overcome the shortcoming of the traditional equalization and decoding technology, we employ the Turbo iterative equalization technology. The technology enables the Turbo equalizer input a priori information and output posterior information, exchange of information with the Turbo decoder and reduce Turbo coding algorithm complexity. To reduce interference of the polarization mode disper-sion and polarization crosstalk in polarization division multiplexing transmission system, we propose a low-complexity MIMO-CMA iterative equalization technology which is experimentally verified.Finally, to solve the problem of signal phase rotation and phase noise due to the frequency out-of-sync lasers and the laser linewidth in coherent detection systems, we propose an8-order blind phase estimation algorithm which is combined with max-likelihood method to enhance the accuracy of single-carrier phase estimation and reduce the algorithm complexity. We make a theoretical analysis of the estimation principle of8-order statistical blind phase estimation and the max-likelihood method, also with their merits and drawbacks. The proposed algorithm is also experimentally verified. The experiment shows that this method can reduce the influence of spectral linewidth of laser signal. And the method is simple, has the merit of real-time coherent detection of single carrier system.