Symbol Interleaving for Polarization-Dependent Nonlinearities in Optical Transmitters
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Solution Overview
Problem
Current optical communication systems face challenges in maintaining bit-error rate (BER) below the forward-error correction (FEC) threshold due to polarization-dependent nonlinearities and loss, especially in mixed transmission systems with different bit rates, where soft-decision FEC performance is degraded due to non-Gaussian statistical distributions and inter-channel nonlinearities.
Innovation Solution
The solution involves interleaving symbols intended for x and y polarizations before modulation, and using a receiver that de-interleaves and processes these symbols to estimate log likelihood ratios based on real probability density functions, thereby improving soft-decision FEC performance by reducing BER below the FEC threshold.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If symbols are transmitted using conventional polarization-division multiplexing without interleaving, then the transmission structure is simple, but polarization-dependent nonlinearities and loss cause bit-error rate to exceed the forward-error correction threshold
Solution Approach 1:
The patent divides the symbol stream into multiple interleaved streams that are separately modulated onto different polarizations. This segmentation allows each stream to experience different polarization-dependent nonlinearities, averaging out the overall effect and bringing the bit-error rate below the FEC threshold while maintaining a relatively simple transmission structure.
Solution Approach 2:
The patent applies symbol interleaving before modulation onto polarizations. This preliminary action redistributes symbols across different polarizations in advance, ensuring that no single polarization bears the full burden of nonlinearities and loss, thereby improving reliability before the signal even enters the transmission medium.
2Adaptability or versatility
If soft-decision FEC is used in mixed transmission systems with different bit rates, then data transport flexibility is improved, but performance is degraded due to non-Gaussian statistical distributions and inter-channel nonlinearities
Solution Approach 1:
By segmenting the symbol stream into interleaved streams assigned to different polarizations, the patent creates independent transmission paths with different statistical characteristics. This segmentation allows soft-decision FEC to operate on more Gaussian-like distributions in each polarization, improving performance while maintaining the adaptability of mixed bit rate transmission.
Solution Approach 2:
The patent changes the statistical distribution parameter of the transmitted signals by applying interleaving, transforming the non-Gaussian distribution that degrades soft-decision FEC performance into a more Gaussian-like distribution that improves FEC performance, while preserving the flexibility of mixed bit rate systems.
3Productivity
If high-order modulation formats are used to increase capacity, then spectral efficiency is improved, but sensitivity to polarization distortions increases causing higher bit-error rates
Solution Approach 1:
The patent segments high-order modulation symbols into interleaved streams across multiple polarizations, distributing the vulnerability of high-order formats to polarization distortions across independent channels. This segmentation maintains the high spectral efficiency of high-order modulation while reducing the impact of any single polarization's distortions on overall performance.
Data Source
Figure 1A~1D
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AI summary
An apparatus comprises a coherent optical transmitter. The coherent optical transmitter comprises a first modulator for generating a first polarization, a second modulator for generating a second polarization, and a symbol interleaver configured to receive a first symbol stream intended to be transmitted on a first polarization and a second symbol stream intended to be transmitted on a second polarization, to direct one portion of symbols of the first symbol stream to the first modulator for modulation onto the first polarization and another portion of the symbols of the first symbol stream to the second modulator for modulation onto the second polarization, and to direct one portion of symbols of the second symbol stream to the first modulator for modulation onto the first polarization and another portion of the symbols of the second symbol stream to the second modulator for modulation onto the second polarization.