Coherent Optical OFDM Communication Systems PAPR Reduced Method

Frequency Division Multiplexing (OFDM) technology used in optical communication can constructing a high-speed and large capacity optical orthogonal frequency division multiplexing (O-OFDM) transmission system. CO-OFDM which has the dual advantages of the coherent detection and OFDM technology, has been widely applied. CO-OFDM system has a series of advantages of high spectrum efficiency and effective against the dispersion, etc., but because of its use of the OFDM technology, the inherent drawbacks of the OFDM technology, a high peak to average power ratio (PAPR) still exists, and when those signal through the optical channel transport, it can easily cause serious nonlinear effect, affect the transmission performance. High peak to average power ratio has become one of the major bottlenecks for CO-OFDM technology applications, so make a study of how to reduce the CO-OFDM system PAPR has important significance.This paper focuses on research of PAPR decreasing schemes for CO-OFDM system. Firstly, introduce the basic principle and key technology of the CO-OFDM system, and analyzing why signals which has a high PAPR can caused nonlinear effects in the optical channel transport, the modulator and fiber nonlinear effects was mainly to discussed, the necessity of reducing the PAPR of the CO-OFDM system was Pointed out.In order to reduce the PAPR of the wireless OFDM system, the signal pre-distortion method, the coding method and scrambling code method has been adopted. So Amplitude limiting, complementary Golay sequences encoding method, select mapping method(SLM) and part transmission sequence method(PTS) as the classic representative method, was specific analysis. Then, the feature of above three methods was compared and introduced. According the requirement of the reduce PAPR, PTS method was applied in the CO-OFDM system was reasonable.The real part and the imaginary part of the sequence use the PTS optimization respectively can further more reduce the PAPR. In order to reduce the complexity of the calculation, the iteratively arithmetic and Searching the Hadarmard matrix was applied in the process of the calculation. So two improved PTS method were proposed. It is shown by theory analysis and simulation that these two method can improve the PAPR performance and reduce the amount of calculation.at the same time, the performance of those two methods was simulation, it is proved that those improved PTS method can reduce the influence of the nonlinear effect in the CO-OFDM system.