Coherent Optical Receiver Phase Error Compensation
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Solution Overview
Problem
Coherent optical receivers for multicarrier offset-quadrature-amplitude-modulation (MC-OQAM) signals face challenges in phase error compensation due to crosstalk between in-phase and quadrature components and neighboring subcarriers, which existing methods, developed for conventional QAM, fail to address effectively, leading to signal impairments and decoding issues.
Innovation Solution
A coherent optical receiver that tracks and compensates phase errors in real-time using a cost function sensitive to crosstalk between subcarriers, operating without a PLL circuit and relying on pure feed-forward processing, allowing for the use of inexpensive laser sources and compensating for phase errors across multiple subcarriers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional laser source with large linewidth is used as local oscillator, then the receiver can use inexpensive laser sources, but phase uncertainty and phase noise increase making optical phase-lock difficult to achieve and maintain
Solution Approach 1:
The patent changes the approach from maintaining phase-lock in the optical domain to tracking phase error in the electrical domain after opto-electrical conversion. This parameter change allows using inexpensive laser sources with larger linewidths while still achieving reliable phase compensation through digital processing of the electrical signal.
2Reliability
If an optical phase-lock loop (PLL) is used to track carrier frequency and maintain phase lock, then phase-lock stability is improved, but the PLL circuit becomes complex and expensive
Solution Approach 1:
The patent extracts the phase-tracking function from the optical domain to the electrical domain. Instead of using a complex optical PLL, the system converts the optical signal to electrical signals and performs phase error tracking digitally, thereby simplifying the hardware while maintaining phase-lock stability.
Solution Approach 2:
The patent replaces the mechanical/optical PLL system with a digital signal processing approach. The phase error tracking is performed through digital computation of cost functions rather than through optical feedback mechanisms, reducing hardware complexity while maintaining functionality.
3Device complexity
If existing phase-recovery methods for conventional QAM are used, then the receiver design is simplified, but they fail to address crosstalk between in-phase and quadrature components and neighboring subcarriers in MC-OQAM signals
Solution Approach 1:
The patent applies local quality by creating different processing paths for different signal components. The cost function is specifically designed to evaluate phase error based on characteristics unique to MC-OQAM signals, including sensitivity to crosstalk between in-phase and quadrature components and between neighboring subcarriers, rather than using a generic phase-recovery method.
4Reliability
If pilot symbols are used for phase recovery, then phase error compensation can be achieved, but the receiver requires complex processing and the phase compensation cannot be performed in real-time without delay
Solution Approach 1:
The patent implements self-service by enabling the system to track and compensate phase error continuously using the data signal itself, without requiring external pilot symbols. The cost function is computed from the received signal and used to adjust the phase compensation in real-time, making the system self-sufficient and eliminating processing delays associated with pilot symbol insertion and detection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate phase error compensation and improved signal decoding for MC-OQAM signals, reducing crosstalk and maintaining data integrity even with phase errors near 90 degrees, thus overcoming limitations of prior-art phase-recovery methods.
Implementation Method 1
an opto-electrical frontend circuit configured to mix the optical input signal with an optical LO signal to produce an electrical digital measure of the optical input signal
Data Source
AI summary
In a representative embodiment, a disclosed receiver of an optical multicarrier offset-quadrature-amplitude-modulated (MC-OQAM) signal is configured to track and compensate for the phase error in the local-oscillator (LO) signal with respect to a carrier wave of a modulated subcarrier of the optical MC-OQAM signal by tracking a minimum of a cost function that is sensitive to crosstalk between in-phase and quadrature components of the modulated subcarrier and/or crosstalk between the modulated subcarrier and at least one other modulated subcarrier of the optical MC-OQAM signal. The receiver can operate based on pure feed-forward processing and compensate the phase error in real time and without relying on pilot symbols or a PLL circuit coupled to the LO source.