Advanced equalization and performance monitoring techniques in single-carrier coherent optical systems (2014)

Coherent optical communication systems have already been demonstrated as a promising technique for next-generation optical transport networks.Coherent reception not only supports advanced modulation formats and polarization division multiplexing (PDM) to enhance bandwidth efficiency, but also allows the use of digital signal processing (DSP) algorithms to compensate fibre channel impairments in the electrical domain.Single-carrier PDM quadrature-phase-shift-keying (QPSK) transmission scheme has already been commercialized as solution for 100-G Ethernet.In the meantime, the ever increasing bandwidth requirement is driving the research towards high spectrum-efficient transmission, which leads to the combination of higher-order quadrature-amplitude-modulation (QAM) formats and advanced spectrum shaping techniques.For commercial PDM-QPSK systems, the receiver's DSP process mainly involves linear fibre channel impairment compensation and carrier recovery, where chromatic dispersion (CD) compensation and polarization de-multiplexing (pol-demux) are realized with separate filters.In particular, the CD filter is with static coefficients and is identical for both polarizations, while pol-demux is usually a multiple-input-multipleoutput (MIMO) adaptive process using constant modulus algorithm (CMA).This typical structure has two main issues: (i) the two-stage equalization process is computationally expensive, since in principle these two linear filters can be combined in one step to save the computational circuits; (ii) the CMA algorithm cannot achieve optimal adaptation with high-order QAM formats with non-constant modulus, which requires feedback process or otherwise will cause performance penalty.We first consider optimizing the system performance for the conventional two-stage system with high-order QAM format (especially 16-QAM format).The mis-adjustment between CMA and 16-QAM is derived, and we propose to use multi-modulus algorithm (MMA) in 16-QAM system with fewer mis-adjustment errors.We experimentally verify that with pre-frequency offset compensation, MMA achieves more than 1-dB Q improvement compared to the CMA algorithm.Furthermore, in order to cope with the ii residual adaptive error, we propose a phase insensitive method namely decision-directed MMA for steady state optimization.Experimental results show that DD-MMA achieves the best performance among all the steady state algorithms.We then present the work to design the DSP circuits with feed-forward, modulation format flexible channel estimation (CE) technique based on training-aided technique (TA-CE).We first derive the optimal TS criterion to achieve minimum mean square error (MMSE) CE performance, and then reliable and robust approaches for frame timing synchronization (FTS) and frequency offset compensation (FOC) are addressed.Then the linear noise removal technique is provided by windowing the time-domain CE taps while reducing the training overhead ratio effectively.Furthermore, we propose to use the polyphase equalizer to achieve significant complexity improvement compared to the two-stage system.Realizing that fibre nonlinearity is the main system performance limitation factor, we investigate the TA-CE system performance in the presence of fibre nonlinearity.The nonlinerity-tolerant TA-CE (NLT-TA-CE) method based on digital backpropagation (DBP) of just the TS is proposed.Experimental results show that NLT-TA-CE improves the received signal Q 2 factor at optimal launch power and achieves similar performance as blind equalization scheme in the highly-nonlinear region.Realizing that the main drawback for single-stage system is the long training overhead required for CE, we investigate the implicit training CE (ITA-CE) scheme for coherent optical systems to improve the spectral efficiency.We first provide a comprehensive analysis for the theory and implementation behind the ITA-CE solution.Then we report that by pre-distorting the transmitted signal before embedding with the TS, the converged CE can be achieved with smaller transmit power allocated to TS, which can largely improve the equalization performance.Optical performance monitoring (OPM) is a key issue for managing high capacity optical communication systems, and is essential in fault management applications and new channel establishment.Optical signal-to-noise ratio (OSNR) is an important parameter in OPM and is critical to network management.We present a novel statistical moments-based method for optical signal-to-noise ratio (OSNR) monitoring in polarization-multiplexed (pol-mux) coherent optical systems.This technique only requires the knowledge of the envelope of the equalized signal before phase correction, iii which can be achieved by using any two arbitrary statistical moments, and it is suitable for both constant and non-constant modulus modulation formats.The Nyquist-WDM system has received intensive research interest recently because of its ability to achieve high spectral efficiency transmission.However, the receiver Nyquist matched filter is computationally expensive with small roll-off factor.We propose to combine Nyquist-WDM transmission with Nyquist SS-TA-FDE (N-SS-TA-FDE), where a Nyquist-windowing process is applied to the coefficients of SS-TA-FDE instead of using a separate RRC matched filter.The proposed concept is experimentally demonstrated with a 1864-Gb/s Nyquist polarization-division-multiplexing (PDM) 16-QAM system with 800-km transmission, the modified frequency domain equalizer combines the matched filtering and linear equalization in a single step, achieves comparable performance to when the matched filter is implemented separately.The combined method is a significantly more computationally-efficient receiver DSP solution for Nyquist WDM systems.This thesis is conducted with the kind instruction, help and support from many people.