Research On Key Techniques Of Digital Signal Processing In High Speed Optical Communication Systems

High-speed optical telecommunication plays a significant role in modern telecommunication fields and it is the main scheme for large capacity transmission.In long-haul transmission applications,coherent detection and digital signal processing(DSP)techniques are combined.Multidimensional high level modulation formats are utilized for increasing transmission capacity,and all kinds of channel distortions can be compensated in digital domain,through which high spectral efficiency and sensitivity can be realized.While in short reach applications,especially in intra-datacenter,direct detection is attractive due to its low cost.In this thesis,high speed optical transmission systems,including coherent optical transmission systems for long-haul applications and single sideband direct detection systems for metro short reach applications are focused.The main research work and innovations of this dissertation are as follows.1.Research on coherent optical transmission systems for long-haul applications(1)Based on coherent optical orthogonal frequency division multiplexing simulation systems,a novel modulation format-4+12 amplitude phase shift keying(APSK)has been proposed to improve the tolerance towards nonlinearities.The theoretical calculation value and accumulated distribution function of peak to average power ratio(PAPR)of 16 quadrature amplitude modulation(QAM)modulated OFDM signals and 4+12 APSK modulated OFDM signals,reveals the advantage of 4+12 APSK.In back-to-back case,the required optical signal-noise ratio(OSNR)values(with bit error ratio(BER)threshold 1 × 10-3)of 16QAM-OFDM,8+8APSK-OFDM and 4+12 APSK are 13.3dB,14.6dB and 13.6dB respectively.The value for 16 QAM is smallest due to its largest Euclidean distance among constellation points resulting in the highest tolerance towards amplifier spontaneous emission(ASE)noise.And then is the 4+12 APSK.In single channel transmission case,the optimal launch power of 4+12 APSK-OFDM signals are 2 dB higher than that of 16 QAM-OFDM signals.Assessing the system performance with the maximum transmission distance when Q factor threshold is 8.2 dB(BER threshold equals 5 × 10-3),the performance of 4+12APSK modulated OFDM surpasses 8+8APSK and 16QAM by approximately 10.5%.And in WDM system,it surpasses 16QAM-OFDM by 18.75%.Simulation results above proves the effectiveness of 4+12 APSK on tolerance improvement towards nonlinearities and transmission performance optimization.(2)Coherent optical single carrier systems are simulated and weighed intra-symbol averaging(WISA)channel estimation has been proposed in this thesis.Based on 28 GBaud single polarization 16QAM system,two conventional channel equalization techniques,including digital back propagation(DBP)method and Volterra filters are realized and analyzed.And based on 28 GBaud polarization multiplexing 16QAM simulation system,WISA channel estimation is studied.In back-to-back case,the optimized averaging window and weigh factor are obtained.And in chromatic dispersion(CD)management case and non-CD management case,performance evaluated by maximum transmission distance when Q factor threshold is 8.5 dB(BER threshold equals 3.8 × 10-3)reveals that WISA outperfoms traditional intra-symbol averaging channel estimation by 13.3%.In addition,offline DSP algorithms,including IQ imbalance compensation,time recovery,polarization demultiplexing,frequency offset estimation and carrier phase recovery,are realized with 448 Gb/s dual-carnier polarization multiplexing 16QAM back-to-back experimental setup,achieving a BER of 3 × 10-5.2.Research on single sideband(SSB)direct detection systems for short reach metro applications(1)For short reach metro applications,a novel Twin-SSB-KK scheme,which combines the advantages of Twin-SSB and Kramer-Kronig receiver,has been proposed in this thesis.Twin-SSB scheme can fully use the transmitter bandwidth and Kramer-Kronig receiver can effectively mitigate the signal-signal beat interference.Focused on Twin-SSB-KK OFDM systems,time-interleaved frame(TIF)is applied to remove the crosstalk induced by non-ideal optical filters.140 Gb/s(the net bitrate is 88 Gb/s)signals over 80 km transmission is experimental demonstrated with hard-decision forward error correction(FEC)threshold.In addition,the impact of optical filter parameters,including roll-off,bandwidth and center frequency shift,are studied and analyzed in details by simulation.(2)Focused on Twin-SSB-KK single carrier systems,MIMO equalization is proposed to mitigate the crosstalk with higher spectral efficiency compared with TIF scheme.Experimental results show the effectiveness of MIMO processing.And 264 Gb/s(the net bitrate is 220 Gb/s)over 80km transmission is demonstrated with soft-decision FEC threshold.