Blind LOFO Estimation in Coherent Optical Receivers
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Solution Overview
Problem
Coherent optical communication systems face challenges in estimating local oscillator frequency offset (LOFO) without relying on transmit frame headers or unique words, especially when commercial lasers have significant frequency offsets, which can lead to failure of carrier recovery algorithms and reduced spectral efficiency.
Innovation Solution
A method and apparatus for blind LOFO estimation using digital signal processing, involving digital frequency shifting, hybrid shifted filter responses, and power/slope difference calculations based on the baud rate of the received signal, to estimate LOFO independently of transmit frame headers and unique words.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If frame header or unique word is used for frequency offset estimation, then LOFO estimation accuracy is improved, but spectral efficiency deteriorates and device complexity increases
Solution Approach 1:
The patent extracts only the necessary spectral information (power spectral density at specific frequency offsets) from the received signal without requiring frame headers or unique words. By taking out only the essential spectral characteristics at frequencies corresponding to half the baud rate, the method achieves LOFO estimation without the overhead of special training sequences, thus maintaining spectral efficiency while obtaining sufficient measurement precision for carrier recovery
Solution Approach 2:
The patent makes the LOFO estimation method universal by eliminating dependency on specific frame structures, headers, or unique words that vary between manufacturers. The method works with any coherent optical signal regardless of its frame format, making it compatible with commercial lasers and different transmit configurations. This multi-functionality allows the same estimation approach to work across diverse system implementations without requiring system-specific modifications
2Measurement precision
If frame header or unique word is included in transmit data frame, then LOFO estimation capability is improved, but device complexity and proprietary knowledge requirements increase
Solution Approach 1:
The patent extracts LOFO information directly from the spectral characteristics of the modulated data signal itself, without extracting or requiring any special frame headers or unique words. By focusing on the power spectral density at specific frequency offsets (half baud rate) of the regular data signal, the method obtains estimation capability while avoiding any additional frame structure elements that would increase device complexity
Solution Approach 2:
The received signal itself provides all necessary information for LOFO estimation through its inherent spectral properties. The method uses the signal's own power spectral density distribution, particularly at frequencies corresponding to half the baud rate, to self-determine the frequency offset. This self-service approach eliminates the need for external training sequences or manufacturer-specific frame knowledge, reducing device complexity while maintaining estimation capability
3Adaptability or versatility
If commercial tunable lasers with large frequency offset range are used, then system adaptability is improved, but carrier recovery reliability deteriorates
Solution Approach 1:
The patent performs preliminary LOFO estimation using the spectral method before initiating carrier recovery algorithms. By estimating the frequency offset in advance from the power spectral density at half baud rate frequencies, the system can pre-compensate for large frequency offsets (up to ±5 GHz) from commercial tunable lasers. This preliminary action enables subsequent carrier recovery algorithms to operate reliably within their acceptable offset ranges, thus maintaining reliability while supporting wide laser frequency ranges
Solution Approach 2:
The patent changes the approach from direct carrier recovery to a two-stage process: first estimating LOFO from spectral characteristics, then using this estimated parameter to correct the frequency offset before carrier recovery. By transforming the problem from direct recovery to estimation-plus-correction, the system can handle large frequency offsets from adaptable commercial lasers while maintaining reliable carrier recovery
Data Source
AI summary
A method and apparatus is described for estimating a local oscillator frequency offset (LOFO) of a received optical signal in a coherent optical receiver. The method includes receiving a signal by the coherent optical receiver; digitally shifting a spectrum of the received signal according to one of a plurality of frequency shifts within a first range; equalizing each shifted signal based on a hybrid shifted filter response of a receiver driver, the hybrid shifted filter response is based on a combination of a linear shifted response and a circular shifted filter response of the receiver driver; calculating a difference of power between two sides of a spectrum of each equalized signal, the power for each side of the spectrum being calculated within a first area centered around a frequency corresponding to half of a baud rate of the received signal; determining a minimum power difference among the plurality of frequency shifts within the first range; and estimating the LOFO based on the minimum power difference.