Optical Receiver Concatenated FEC for Lower BER Overhead
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Solution Overview
Problem
Optical communication systems face challenges in maintaining low Bit Error Rate (BER) due to signal degradation caused by noise and distortion in optical fibers and amplifiers, which conventional Forward Error Correction (FEC) techniques attempt to mitigate but at the cost of increased overhead and complexity.
Innovation Solution
The implementation of a dual FEC scheme with hard decision and soft decision encoding, combined with probabilistic constellation shaping, where bits are mapped to symbols using a labeling scheme that enhances the minimum Euclidean distance between constellation points, allowing for unequal visitation probabilities and improved noise tolerance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional FEC techniques are used to reduce BER, then bit error rate is reduced, but overhead and system complexity increase
Solution Approach 1:
The patent applies segmentation by dividing the single FEC encoding process into two distinct stages: first FEC encoding (hard decision) and second FEC encoding (soft decision). Each stage processes different portions of the data with different encoding strengths, allowing the system to achieve comprehensive error protection while optimizing the complexity overhead distribution across stages.
Solution Approach 2:
The patent implements local quality by applying different FEC encoding schemes to different data portions. The first FEC encoding (hard decision) is applied to all data bits, while the second FEC encoding (soft decision) is applied selectively to specific portions requiring stronger protection. This differentiated approach optimizes the balance between reliability and complexity by concentrating computational resources where most needed.
2Reliability
If FEC encoding adds redundancy to reduce BER, then error correction capability is improved, but information rate decreases
Solution Approach 1:
The patent segments the redundancy addition process into two FEC encoding stages, where the first stage adds a certain level of redundancy and the second stage adds additional redundancy selectively. This segmented approach allows the system to achieve the required error correction capability while optimizing the total overhead compared to applying a single strong FEC scheme to all data.
Solution Approach 2:
The patent applies different levels of redundancy to different portions of data through the two-stage FEC process. The first FEC encoding provides baseline redundancy for all data, while the second FEC encoding provides enhanced redundancy only for specific data portions that require stronger protection, thereby optimizing the overall information rate while maintaining reliability.
3Reliability
If minimum Euclidean distance between constellation points is increased, then noise tolerance is improved, but constellation mapping complexity increases
Solution Approach 1:
The patent applies local quality in constellation mapping by assigning different subsets of encoded bits to different positions within constellation points. Specifically, the mapping scheme assigns bits from the first FEC encoding to certain positions and bits from the second FEC encoding to other positions, creating a structured labeling scheme that enhances minimum Euclidean distance for critical bit positions while maintaining overall mapping efficiency.
Solution Approach 2:
The patent utilizes another dimension by employing a multi-dimensional constellation structure (e.g., QAM with multiple in-phase and quadrature components). The labeling scheme distributes encoded bits across multiple dimensional positions within each constellation point, where certain positions contribute more significantly to the minimum Euclidean distance. This dimensional distribution allows the system to achieve enhanced noise tolerance through optimized bit-to-position mapping without requiring an excessive increase in overall constellation size.
Data Source
AI summary
An optical receiver is configured to receive optical signals representative of digital information over an optical communication link. The optical receiver is further configured to decode symbol estimates from an optical signal received over the optical communication link; to demap first bit estimates and second bit estimates from the symbol estimates; to decode third bit estimates from the first bit estimates using second FEC decoding of a second FEC scheme; and to decode fourth bit estimates from both the second bit estimates and the third bit estimates using first FEC decoding of a first FEC scheme. The optical receiver is further configured to use one or more of the third bit estimates to demap one or more of the second bit estimates. Concatenation of the first and second FEC schemes as described herein may relax design constraints on the second FEC scheme, which may reduce power consumption and design complexity.