| In recent years,optical fiber communication has become a research hotspot in the field of communication because of its advantages of large capacity and low loss.In this paper,several research works round the coding technology in digital coherent optical communication system have been done.The performance of LDPC under phase slip has been experimentally investigated.At the same time,the performance of combining trellis coded modulation with B/QPSK,mPPM and mPPM mixed modulation formats are simulated and analyzed.The main research works of this paper are as follows:.?1?Experimental research about LDPC coding performance under phase slip.In this paper,two commonly used anti-phase slip methods are researched.The first is improving the carrier phase recovery algorithm to reduce the probability of phase slip.The secend is improving the LDPC decoding algorithm to otabin high phase slip tolerance.For the first method,the influence of VVPE+PAPU on LDPC coding performance is investigated experimentally.The phase slip probability,PRE-FEC BER and POST-FEC BER of the system with VVPE+PAPU algorithm and VVPE algorithm are researched and compared under different carrier phase recovery filter lengths and different OSNR.In 56 Gbit/s,300 kHz,QPSK system,with carrier phase recovery filter lengths of 8,16 and 20,compared with VVPE scheme,the VVPE+PAPU scheme can respectively obtain about 3 dB,1 dB and 0.5 dB OSNR gains and can respectively increases FEC limit from 2×10-3 to 1.4×10-2,from 8.9×10-3 to 1.8×10-2,and from 1.6×10-2 to 1.9×10-2.The experimental results improve that VVPE+PAPU algorithm can effectively eliminate the phase slip caused by the inappropriate filter lengths.For the second method,the performance of LDPC decoding method based on Turbo demodulation affected by phase slip is experimentally studied.The influence of Turbo external iteration times,LDPC internal iteration times and carrier phase recovery filter length on the performance of Turbo-LDPC are studied when the laser linewidth is 100 kHz and 300 kHz.The experimental results show that the system has the best performance under the condition of LDPC iteration 5 times and turbo iteration 10 times when the total number of iterations is 50.At the same time,Turbo demodulation technology can effectively eliminate the phase slip caused by inappropriate filter lengths.In addition,the influence of the combination of Turbo demodulation and VVPE+PAPU on the system has also been investigated.The experimental results show that the performance of the combined system is not significantly improved compared with Turbo-LDPC.?2?Simulation research of TCM coded in PPM hybrid B/QPSK modulation system.The simulation system models of TCM-QPSK,TCM-8PSK,TCM-4PPM and TCM-8PPM are established and the performance analysis is carried out.The simulation results show that:compared with BPSK,TCM-QPSK achieves about 3.7 dB coding gain;TCM-8PSK achieves about 3 dB coding gain relative to QPSK;TCM-4PPM achieves 3 dB coding gain relative to 4PPM at the expense of 1/8 spectral efficiency;TCM-8PPM achieves 1 dB coding gain compared to 8PPM.In addition,TCM is implemented in three ways in mixed modulation formats such as mPPM-B/QPSK.Firstly,TCM is used only for mPPM part of mixed modulation format.Secondly,TCM is used only for B/QPSK part.Thirdly,TCM is used both on mPPM and B/QPSK.The performance of the three methods is simulated and compared.The simulation results show that,the coding gain of TCM-4PPM-BPSK is 1 dB compared with 4PPM-BPSK and 2PPM-BPSK at the cost of about 1/8 spectral efficiency in the first mode.The performance of 2PPM-TCM-QPSK is 3 dB improved compared with 2PPM-BPSK in the second mode under the same spectrum efficiency.For the 2PPM-BPSK,the performance of TCM-4PPM-.TCM-QPSK is 1 dB coding gain compared with 8PPM-BPSK under the same spectrum efficiency in the third mode.For 4PPM-QPSK,compared with 8PPM-8PSK,TCM-8PPM-TCM-8PSK can achieve 3.9 dB coding gain at the expense of 1/8 spectral efficiency. |
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