DSP Lock-in DGD Removal for Accurate PMD Calculation
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Solution Overview
Problem
Existing optical communication systems face inaccuracies in determining polarization mode dispersion (PMD) due to the phenomenon of lock-in differential group delay (DGD) when using blind equalization algorithms like the constant modulus algorithm (CMA), which can lead to arbitrary output delays and affect the accuracy of PMD calculations.
Innovation Solution
A digital signal processor (DSP) in the optical receiver equalizes x-polarization and y-polarization signals using a CMA, identifies frame header patterns to determine lock-in DGD, and adjusts filter coefficients to remove this delay, allowing for accurate PMD determination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If blind equalization using CMA is used to equalize signals, then signal equalization is achieved, but lock-in DGD is introduced causing inaccurate PMD determination
Solution Approach 1:
The patent applies preliminary action by measuring lock-in DGD before PMD determination. The system first equalizes signals using CMA, then measures the resulting lock-in DGD, and compensates for it before performing PMD measurements. This ensures that the harmful lock-in DGD effect is accounted for and removed from subsequent PMD calculations, resolving the contradiction between achieving signal equalization and maintaining accurate PMD determination.
2Measurement precision
If filter coefficients are adjusted to remove lock-in DGD, then PMD determination accuracy is improved, but system complexity increases
Solution Approach 1:
The patent implements self-service by having the system automatically measure and compensate for its own lock-in DGD. The DSP autonomously performs lock-in DGD measurement, calculates the compensation amount, and adjusts filter coefficients without external intervention. This self-correcting mechanism improves PMD determination accuracy while managing complexity through automation rather than requiring additional complex hardware or manual calibration procedures.
Data Source
AI summary
A digital signal processor (DSP) may receive a signal that has an x-polarization (x-pol) and a y-polarization (y-pol). The DSP may equalize the x-pol of the signal and the y-pol of the signal based on filter coefficients determined using a constant modulus algorithm (CMA). The DSP may perform phase correction on the equalized x-pol signal and the equalized y-pol signal. The DSP may identify a first frame header pattern within the phase-corrected x-pol signal, and may identify a second frame header pattern within the phase-corrected y-pol signal. The DSP may determine, based on the first frame header pattern and the second frame header pattern, a quantity of lock-in differential group delay (DGD). The device may adjust one or more of the filter coefficients to remove the quantity of lock-in DGD and to permit an amount of polarization mode dispersion to be determined based on the filter coefficients.