Abstract
Dense wavelength-division multiplexing (DWDM) is the solution of choice for high-capacity optical backbone networks. However, in long-haul DWDM systems with periodic dispersion compensation and amplification, all-optical communications give rise to severe physical impairments, due to fiber dispersion and nonlinearity, together with noise due to amplified spontaneous emission (ASE), that adversely degrade system performance. To mitigate these physical impairments and fully exploit the system capacity of long-haul DWDM systems, a mathematical model that describes the input-output relationship of these systems and characterizes various physical impairments is required to serve as the foundation of discrete-time signal processing for fiber-optic communication systems.
This dissertation develops a model-centric approach for discrete-time signal processing for long-haul DWDM systems and addresses the development, validation and applications of a 2D discrete-time model of physical impairments in long-haul DWDM systems with periodic dispersion compensation and amplification.
The model development is based on the third-order Volterra series transfer function (VSTF) method. The model overcomes the well-known triple integral problem inherent in the original VSTF method and simplifies it to a simple integral that is easy to evaluate. The model takes into account multichannel effects, fiber losses, frequency chirp, optical filtering, and photodetection, which are ignored in the current literature. The model is in discrete-time and facilitates its applications in discrete-time signal processing to improve the system performance of long-haul DWDM systems. The model characterizes each individual physical impairment by introducing the corresponding impairment coefficient. The model offers obvious advantages over the third-order VSTF method and the split-step Fourier (SSF) method. The model is in excellent agreement with results obtained from the SSF method.
The 2D discrete-time model is applied in system analysis and system performance improvement of long-haul DWDM systems. In system analysis, two applications are developed. Using the 2D discrete-time model, the effects of varying system parameters (symbol rate and channel spacing) and pulse shape on individual physical impairments in long-haul DWDM systems are analyzed. The concept of range of influence (RoI) of physical impairments is proposed and the RoI of each individual physical impairment is determined to guide the development of discrete-time signal processing. In system performance improvement, two applications are developed. Using the 2D discrete-time model, a novel constrained code based on the Total Impairment Extent Rank (TIER) is proposed to mitigate nonlinear physical impairments in long-haul fiber-optic communication systems; a TIER-LDPC concatenation scheme is proposed to combine the strength of the TIER code in effectively suppressing severe nonlinear physical impairments and that of the LDPC code in correcting memoryless errors due to ASE noise. A nonlinear equalizer based on the third-order inverse Volterra theory is also proposed. Different from backpropagation which is hard to implement in hardware, this equalizer features the most basic discrete-time signal processing device. The nonlinear equalizer is effective in suppressing linear and nonlinear physical impairments in a long-haul fiber-optic communication system, particularly for high launched power levels where fiber nonlinearity dominates.