Coherent Optical Receiver Frequency Error Estimation
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Solution Overview
Problem
Existing frequency offset estimation methods in coherent optical receivers, such as the N-th power method and PADE technique, face challenges in accurately estimating frequency errors, especially for modulation schemes like 16QAM, leading to unstable frequency offset estimation and deteriorated tracking performance.
Innovation Solution
A frequency error estimating apparatus and method that determines the amplitude and modulated phase component of a baseband digital electrical signal using phase noise and frequency error estimation values from previous symbols, calculating frequency errors based on inter-symbol phase differences, and compensating these errors to improve estimation accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the N-th power method is used for frequency offset estimation, then the method can support PSK modulation schemes, but the frequency offset quadruples requiring additional argument operation and the estimation becomes unstable for 16QAM
Solution Approach 1:
The patent changes the estimation parameter from direct frequency offset to inter-symbol phase difference, which linearly reflects frequency offset without multiplication effects. This avoids the quadrupling issue in N-th power method and provides stable estimation for 16QAM by using phase difference calculation between consecutive symbols after data phase cancellation.
2Adaptability or versatility
If the PADE technique is used to increase estimatable range, then the range becomes larger than N-th power method, but the provisional determination becomes erroneous when frequency offset estimation value is significantly different from actual frequency offset
Solution Approach 1:
The patent performs preliminary cancellation of data phase using estimated values from previous symbols before calculating frequency offset. This preliminary action removes the modulated phase component that causes erroneous provisional determination in PADE, enabling accurate frequency offset estimation even when initial estimates are significantly off from actual values.
3Measurement precision
If frequency error estimation value is significantly different from actual frequency offset, then tracking performance deteriorates and erroneous determination occurs, but accurate estimation is needed for stable operation
Solution Approach 1:
The patent uses feedback by incorporating frequency error estimation values from previous symbols (1 to N previous symbols) into the current estimation process. This feedback mechanism allows the system to converge to accurate frequency offset values even when initial estimates are significantly different, improving both precision and reliability of tracking performance.
Data Source
AI summary
Provided is a frequency error estimating apparatus used for a coherent optical receiver, which determines an amplitude of a baseband digital electrical signal converted from a received light signal modulated with a phase and amplitude shift keying, determines, with respect to each determined amplitude, a modulated phase component of the baseband digital electrical signal based on phase noise estimation values and frequency error estimation values of N previous symbols (N is a positive integer), and calculates a frequency error based on an inter-symbol phase difference of a signal obtained by cancelling the modulated phase component from the baseband digital electrical signal.