Frequency Domain Linear Equalizer for Coherent Optical Receivers
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Solution Overview
Problem
Current coherent optical receivers face challenges in tracking fast SOP rotation, high complexity, and power consumption due to large FFTs and IFFTs, and lack flexibility in handling varying oversampling ratios, which affects their performance in long-distance optical links.
Innovation Solution
The proposed solution involves a digital signal processing approach that includes a fast Fourier transformer (FFT) for transforming input signals, a splitter for separating positive and negative frequency components, digital filters for processing these components, a combiner for down-sampling, and an inverse FFT (IFFT) for converting back to the time domain, allowing for reduced FFT size and discarding non-valuable frequency components, thereby reducing complexity and power consumption while maintaining performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If large FFTs and IFFTs are used for CD compensation and MIMO equalization in frequency domain, then equalization performance is improved, but device complexity and power consumption increase
Solution Approach 1:
The patent divides the frequency domain processing into separate positive and negative frequency components, processing them independently through parallel digital filters. This segmentation allows for reduced FFT sizes while maintaining equalization performance, as each component can be processed with fewer computational resources.
Solution Approach 2:
The patent extracts and processes only the necessary frequency components for equalization, discarding non-valuable frequency components. By taking out only the essential positive and negative frequency components required for CD compensation and MIMO equalization, the system reduces the FFT size and associated complexity while preserving equalization effectiveness.
2Reliability
If large FFTs and IFFTs are used for CD compensation and MIMO equalization in frequency domain, then equalization performance is improved, but power consumption increases
Solution Approach 1:
The patent segments the frequency domain processing into independent positive and negative frequency components that can be processed in parallel with reduced computational load. This segmentation enables the use of smaller FFT sizes, directly reducing power consumption while maintaining equalization performance through efficient processing of essential frequency components.
Solution Approach 2:
The patent discards non-valuable frequency components that do not contribute significantly to equalization performance. By eliminating these redundant components from processing, the system reduces the computational burden and power consumption of FFT and IFFT operations while recovering and preserving only the essential frequency information needed for effective equalization.
3Reliability
If fixed equalizer with long impulse response is used for CD compensation, then CD compensation performance is improved, but device complexity increases
Solution Approach 1:
The patent transitions from time domain processing with long impulse responses to frequency domain processing. By moving to another dimension (frequency domain), the system achieves equivalent or superior CD compensation performance with significantly reduced computational complexity, as frequency domain operations can handle long dispersion effects more efficiently through compact spectral representations.
Solution Approach 2:
The patent uses frequency domain copies of the signal spectrum to represent and process the long impulse response effects. Instead of directly implementing a long time-domain filter, the system creates and manipulates frequency domain representations that capture the same dispersion characteristics, enabling efficient CD compensation with reduced complexity through spectral processing.
4Speed
If adaptive MIMO equalizer operates in time domain for fast SOP tracking, then tracking speed is improved, but device complexity increases
Solution Approach 1:
The patent implements a dynamic hybrid architecture where the fixed CD equalizer operates in frequency domain and the adaptive MIMO equalizer operates in time domain. This dynamic configuration allows fast SOP tracking through time-domain adaptation while managing overall system complexity by keeping the frequency-domain CD compensation fixed and computationally efficient.
Solution Approach 2:
The patent segments the equalization function into two distinct parts: frequency domain CD compensation with a fixed equalizer and time domain polarization tracking with an adaptive MIMO equalizer. This segmentation allows each component to be optimized independently, enabling fast SOP tracking where needed while maintaining overall system complexity at acceptable levels through efficient frequency domain processing.
Data Source
AI summary
The present disclosure relates to an apparatus and method of processing a digital signal, wherein an input signal is transformed into the frequency domain by applying a fast Fourier transformation (FFT) processing to obtain a transformed input signal. Positive and negative frequency components of the transformed input signal are separated and respective ones of the separated positive and negative frequency components are separately processed by respective digital filtering to obtain filtered frequency components. The filtered frequency components are combined in the frequency domain using a down-sampling operation for down-sampling the filtered frequency components from an input number of samples per symbol to a different output number of samples per symbol, and the combined output components are converted into the time domain by applying an IFFT processing.