SDM Optical Fiber IQ Coding for PDL-Resilient Transmission
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Solution Overview
Problem
Existing SDM optical communication systems face interference and dispersion issues such as PDL, CDL, and MDL, which degrade transmission capacity and error rates, and existing coding techniques complicate transmitters and receivers.
Innovation Solution
A method and device for SDM transmission on optical fibers using polarization duality, involving the separation of real and imaginary parts of modulation symbols, followed by an invertible linear transformation and IQ combination to generate complex emission symbols that modulate different polarization states, averaging attenuation effects across channels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If space-time coding techniques are used to combat CDL degradation, then transmission capacity is improved, but transmitter and receiver complexity increases
Solution Approach 1:
The information symbols are segmented into real and imaginary parts, which are then processed through separate linear transformation paths. This segmentation allows the system to achieve diversity gain without requiring complex space-time coding across multiple time intervals, thereby reducing transmitter and receiver complexity while maintaining transmission capacity.
Solution Approach 2:
The real and imaginary parts of the symbols undergo preliminary linear transformation before modulation. This preliminary processing incorporates the diversity mechanism in advance, eliminating the need for complex post-processing or multi-interval coding at the receiver, thus reducing overall system complexity.
2Productivity
If the number of modes/cores is increased to overcome transmission limits, then transmission capacity is improved, but interference between elementary channels increases
Solution Approach 1:
The invention changes the parameter representation by separating symbols into real and imaginary parts and applying linear transformations. This parameter transformation creates orthogonalized signal components that can be transmitted over multiple modes/cores simultaneously with reduced mutual interference, enabling higher transmission capacity.
Solution Approach 2:
The signal is constructed as a composite of transformed real and imaginary parts that are modulated onto different polarization states. This composite structure allows the system to exploit multiple spatial channels while the orthogonal transformation properties reduce inter-channel interference.
3Device complexity
If PDL effects are not compensated, then system simplicity is maintained, but error rate increases due to non-unitary nature of PDL
Solution Approach 1:
The invention converts the harmful PDL effect into a benefit by designing the linear transformation such that the energy of the transmitted signal is distributed across multiple polarization states. The PDL, which differentially attenuates polarization states, actually helps to equalize the received signal energies through the orthogonal transformation, thereby reducing error rates without requiring active compensation.
Data Source
AI summary
The present invention relates to a method and a device for dual-polarisation, fiber-optic SDM transmission. The transmission method uses specific I/Q coding that makes it possible to combat the effects of PDL. The modulation symbols to be transmitted on the 2N polarisation states of the N basic spatial channels are broken down into real and imaginary values (220). A real vector composed by concatenating these real values and imaginary values is then constructed. A first invertible linear transformation, represented by a dense real matrix, is applied (230) to the resulting real vector to provide a transformed real vector. Complex transmission symbols are formed by I/Q combination (240) of the components of the transformed vector, the transmission symbols then modulating the different polarisation states of the basic spatial channels.