Frequency-Domain GVD Compensation With Segmented DFT Filtering
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Solution Overview
Problem
Existing digital filter arrangements for compensating group velocity dispersion (GVD) in optical transmission systems face increasing complexity with transmission distance and symbol rate, particularly in long-haul, high-rate applications, due to the need for very long finite impulse response (FIR) filters.
Innovation Solution
A digital filter arrangement (DFA) that transforms input signals from the time domain to the frequency domain using multiple discrete Fourier transform (DFT) filters, each of reduced size, followed by interpolation and compensation filtering, to reduce complexity by logarithmic growth in operations with filter size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If very long FIR filters are used to compensate GVD in long-haul, high-rate applications, then signal quality is maintained, but device complexity increases significantly
Solution Approach 1:
The patent divides the long FIR filter into multiple shorter sub-filters arranged in a cascade structure. Each sub-filter processes a portion of the frequency spectrum, and their combined effect achieves the same GVD compensation as a single long filter would provide, but with reduced individual filter lengths and lower overall complexity.
Solution Approach 2:
The patent transitions from time-domain filtering to frequency-domain filtering by applying DFT/IDFT transforms. This dimensional change allows the use of shorter filters in the frequency domain that achieve the same compensation effect, reducing the required filter length from being proportional to transmission distance to a much smaller fixed value.
2Measurement precision
If FIR filter length is increased to compensate for longer transmission distances, then GVD compensation accuracy improves, but computational operations increase linearly with filter size
Solution Approach 1:
The patent applies DFT to transform the filtering operation from the time domain to the frequency domain. In the frequency domain, filtering becomes a simple element-wise multiplication operation, reducing computational complexity from O(N*M) for time-domain convolution to O(N log N) for the DFT-based approach, where N is the signal length and M is the filter length.
Solution Approach 2:
The frequency domain is divided into multiple sub-bands, each processed by a separate sub-filter. This segmentation allows each sub-filter to be much shorter than the equivalent time-domain filter, reducing the number of computational operations while maintaining overall compensation accuracy through the combined effect of all sub-filters.
Data Source
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AI summary
The present disclosure relates to a digital filter arrangement, DFA, for compensating group velocity dispersion, GVD, in an optical transmission system, OTS, wherein the DFA is configured to receive a sequence of samples of a digital input signal in the time domain in the form of consecutive blocks of size L. The DFA is configured to generate M discrete Fourier transforms (X1, X2,..., XM) of a current overlap block (s1) of a size N greater than the size L and of M-1 delayed versions (s2,..., SM) of the current overlap block (s1). The DFA is configured to filter the entries (Χ1,1, X1,2,..., X1,N; X2,1, Χ2,2,..., Χ2,Ν;...; ΧΜ,1, XM,2,..., ΧΜ,Ν) of the generated M discrete Fourier transforms (X1, X2,..., XM) to generate an output discrete Fourier transform (Y) with N entries (Y1, Y2,..., YN), wherein the compensation filter is implemented by a delay network and a linear combination algorithm.