QC-LDPC Coded Modulation With Two-Phase Decoding for Optical Links
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current optical communication systems face challenges in achieving high spectral efficiency and reliability for transoceanic data transport due to high signal-to-noise ratios and error floors, especially when using conventional forward error correction methods like LDPC codes, which result in limited net coding gain and vulnerability to fiber loss and aging.
Innovation Solution
The implementation of quasi-cyclic Low Density Parity Check (QC-LDPC) coded modulation with serial turbo decoding, involving the construction of structured QC-LDPC codes and two-phase decoding methods, including inner/outer and outer/inner decoding, to achieve a high net coding gain and low error rates, specifically using a concatenated code structure with a cyclic projective geometry LDPC code as the outer code and a high-rate QC-LDPC code as the inner code.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional LDPC codes with 20%-30% overhead are used for forward error correction, then the FEC limit Q is improved to about 5 dB Q, but the net coding gain is limited to less than 12 dB and the system margin remains barely above the FEC limit
Solution Approach 1:
The patent applies segmentation by dividing the single LDPC code into a concatenated structure of outer and inner LDPC codes. The outer code handles bulk error correction while the inner code provides additional protection, effectively segmenting the error correction function to achieve both improved FEC limit and increased system margin without excessive complexity.
Solution Approach 2:
The patent uses composite materials principle by combining two different LDPC codes (outer and inner codes with different parameters) into a concatenated structure. This composite coding approach leverages the strengths of both codes to achieve superior performance compared to using a single conventional LDPC code.
2Productivity
If 8QAM modulation is used for transpacific cables, then spectral efficiency is improved, but the system margin becomes barely above the FEC limit making it unreliable and risky due to higher fiber loss from aging and cable cut/repair
Solution Approach 1:
The patent applies beforehand cushioning by implementing a concatenated coding structure with both outer and inner LDPC codes that provides additional error protection margin. This pre-established cushioning allows the system to operate at higher spectral efficiency with 8QAM while maintaining sufficient margin to withstand fiber loss variations from aging and cable issues.
3Reliability
If concatenated codes with high redundancy are used to achieve better error-floor performance, then the net coding gain increases to 12.0 dB at BER of 10^-15, but the decoding complexity and hardware requirements increase
Solution Approach 1:
The patent segments the decoding process into two distinct phases: outer code decoding and inner code decoding. This segmentation allows each decoder to be optimized independently and operate with manageable complexity, while the iterative exchange of information between the two decoders achieves the target BER of 10^-15 without requiring excessively complex monolithic decoding hardware.
Solution Approach 2:
The patent implements dynamic decoding by allowing the outer and inner decoders to iteratively exchange information and adjust their decoding efforts based on the current error conditions. This dynamic approach enables the system to achieve low error floors adaptively without requiring static over-provisioning of decoding complexity.
Data Source
AI summary
Systems and methods for data transport in optical communications systems, including a transmitter for encoding a received information sequence by constructing an outer and inner quasi cyclic-low-density parity check (QC-LDPC) code. The encoding includes dividing the received information sequence into a plurality of messages of equal length, encoding each of the messages into a codeword to generate a plurality of outer codewords, cascading the plurality of outer codewords to generate a bit sequence, and executing inner encoding to encode each of the plurality of outer codewords into codewords in QC-LDPC inner code. A receiver decodes a received data stream based on the QC-LDPC inner code using two-phase decoding including iteratively performing at least one of inner/outer and outer/inner decoding until a threshold condition is reached.