Star-8QAM Constellation Asymmetry for Single-Bit Error Reduction
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Solution Overview
Problem
The star-8QAM constellation in optical communications experiences high bit error rates due to two-bit errors, which are equally likely to occur when an erroneous transition happens to either of the nearest inner points, increasing the bit error rate (BER) of optical signals.
Innovation Solution
The constellation points are modified by adjusting their Euclidean distances and phase rotation, where the first and second Euclidean distances are made less than those to other points, ensuring that erroneous transitions result in single-bit errors, allowing for more efficient binary forward error correction and reduced noise tolerance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the star-8QAM constellation uses equal Euclidean distances to inner points, then the modulation format is simple to implement, but the bit error rate increases due to two-bit errors
Solution Approach 1:
The patent applies asymmetry by making the Euclidean distances from outer constellation points to inner points unequal. Specifically, one distance is made shorter than the other, which causes erroneous transitions to preferentially result in single-bit errors rather than two-bit errors. This asymmetric distance configuration reduces the bit error rate while maintaining the same constellation structure, directly resolving the contradiction between implementation simplicity and reliability.
2Reliability
If the constellation points are configured to minimize Euclidean distances to nearest points, then single-bit errors become more likely, but the system becomes more sensitive to noise
Solution Approach 1:
The patent applies local quality by differentiating the Euclidean distances for different transitions rather than using a uniform distance configuration. By locally optimizing the distances from each outer point to its neighboring inner points, the system creates a configuration where erroneous transitions predominantly result in single-bit errors. This local differentiation improves reliability without requiring a complete redesign of the entire constellation, thereby balancing noise tolerance with error correction efficiency.
Data Source
AI summary
Consistent with an aspect of the present disclosure, optical signals are transmitted that are modulated in accordance with an 8QAM modulation format. The optical signals carry symbols of data and may be represented by a constellation in the IQ plane that includes four inner points that are symmetrically arranged about the origin, and four outer points that are uniformly distributed about the origin, but rotated relative to the inner points. The rotation is toward inner points that represent symbols for which an erroneous transition between the outer points and such inner points is more likely to result in a single bit error, instead of two bit errors, because the symbol corresponding to the outer point and the symbol corresponding to such inner point differ by just one bit. Accordingly, a binary forward error correction algorithm may be employed to correct the errored bit. Such binary forward error correction operates with greater efficiency compared to symbol-wise error correction and thus additional noise can be tolerated by the optical receiver.