Research On The Novel Constellation Design For Coherent Optical OFDM Communication System

With the rapid development of fiber optical communication and the dramatic capacity requirements of various new applications, coherent orthogonal frequency division multiplexing(CO-OFDM) which combines the advantages of both coherent detection technique and orthogonal frequency division multiplexing is believed to play a significant role in future long-distance and high-speed optical transmission system, due to its high spectral efficiency(SE) and immunity to chromatic dispersion(CD) and polarization mode dispersion(PMD). Recently, offset-quadrature amplitude modulation(OQAM) has been investigated during the implementation of CO-OFDM, in order to achieve higher SE and the sideband suppression ratio. However, there are two significant problems in both CO-OFDM and CO-OFDM/OQAM systems. One is the sensitivity to the phase noise, and the other is the nonlinear distortion caused by high peak to average power ratio(PAPR).In this thesis, in order to mitigate the effect of phase noise and nonlinear distortion, we carry out several novel constellation diagram optimizations to improve the transmission performance.First, the effect of nonlinear distortion and phase noise is analyzed based on the principle of CO-OFDM. Algorithms of coding/decoding and phase noise estimation are realized. In this way, the high speed(17.43Gb/s) and long distance(1000km) CO-OFDM simulation system is built by VPI Transmission Maker 9.0.Second, the impacts caused by the constellation design on the system performance are analyzed. By inducing spiral QAM constellation, the tolerance of CO-OFDM system to phase noise and nonlinear distortion is verified to improve by 75 kHz and 1 d B, respectively. Meanwhile, experimental demonstration of 16.72Gb/s 100km-SSMF-CO-OFDM transmission with spiral 16 QAM is carried out.At last, coding/decoding algorithm of OFDM/OQAM is realized and the advantage of OFDM/OQAM compared with conventional QAM is verified by simulation. Subsequently, inducing, we investigate the performance improvement of CO-OFDM/OQAM by using amplitude phase shift keying(APSK) constellation diagram scheme with different structures. The simulation result shows that the tolerance of fiber nonlinearity is improved by 1 dB by using “6+10” APSK constellation. Moreover, the signal PAPR is further decreased by 2.2 dB by using the method of active constellation extension(ACE).