Dual-Polarization I/Q Coding for PDL-Resilient WDM Links
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Solution Overview
Problem
WDM communication systems over optical fibers face limitations due to inter-channel interference and polarization-dependent loss (PDL), which degrade transmission performance and capacity, particularly in high-density channel configurations.
Innovation Solution
A WDM transmission method over optical fiber utilizing polarization duality, where modulation symbols are separated into real and imaginary parts, subjected to distinct orthogonal linear transformations, and combined to modulate multiple polarization states, effectively averaging PDL attenuation across channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the density of WDM transmission channels is increased to raise transmission capacity, then the transmission capacity is improved, but the inter-channel interference level increases
Solution Approach 1:
The transmission system segments the signal into multiple polarizations (horizontal and vertical) and processes each polarization independently through separate modulation and transmission paths, allowing interference management on a per-polarization basis while maintaining high overall capacity
Solution Approach 2:
The system transitions from single-polarization transmission to dual-polarization transmission, adding the polarization dimension to the transmission space. This allows doubling the capacity per wavelength without increasing spectral density, thereby avoiding inter-channel interference while maintaining high transmission capacity
2Productivity
If polarization-dependent loss is present in the optical fiber, then transmission performance is improved through polarization multiplexing, but the error rate increases due to non-unitary PDL effects
Solution Approach 1:
The system applies preliminary polarization scrambling and precoding transformations before transmission to distribute PDL effects uniformly across all transmitted symbols. This preliminary action ensures that PDL does not preferentially affect any particular polarization state, maintaining low error rates while enabling polarization multiplexing for high capacity
Solution Approach 2:
The system dynamically adjusts modulation parameters and polarization transformation matrices to compensate for PDL effects. By changing the parameters of the precoding operations based on channel conditions, the system maintains optimal performance and low error rates despite the presence of PDL
Data Source
AI summary
The present invention relates to a method for dual-polarisation WDM transmission over optical fibre. The transmission method uses specific I/Q coding for combating the effects of PDL. The modulation symbols to be transmitted on the 2N polarisation states of the N wavelengths are broken down into real values and imaginary values (220). A first orthogonal linear transformation (230-1) is applied to the vector of the real values thus obtained and a second orthogonal linear transformation (230-2), separate from the first, is applied to the vector of the imaginary values thus obtained. A complex scalar solving an irreducible polynome of [X] in is multiplied with the first or second transformed vector before the two transformed vectors are summed (240) in order to provide a vector of transmission symbols for modulating the different states of polarisation of the WDM channels.