QAM Constellation Shaping Using Forbidden Branch Flags
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Solution Overview
Problem
Higher order modulations in digital communication systems, such as 64 QAM and 256 QAM, are susceptible to noise and interference, leading to performance gaps and increased average transmit power requirements due to non-optimum transmission with uniform QAM signal points.
Innovation Solution
The implementation of forbidden branch flags in a transceiver device's constellation shaper module to restrict certain signal points in the trellis diagram, allowing the Viterbi algorithm to select a subset of signal points that minimize average output symbol power, thereby optimizing signal transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If higher order modulations (64 QAM, 256 QAM) are used to increase data capacity, then optical channel capacity is improved, but noise susceptibility increases and performance gaps occur
Solution Approach 1:
The patent applies probabilistic constellation shaping by changing the probability distribution parameters of signal points in the QAM constellation. Different signal points are assigned different probabilities of being selected, with inner points having higher probability and outer points having lower probability. This parameter change optimizes the signal distribution to better match the channel capacity while reducing noise susceptibility.
Solution Approach 2:
The patent implements local quality by assigning different properties to different regions of the constellation diagram. Inner signal points are used more frequently than outer points, creating a non-uniform distribution where each region contributes differently to the overall transmission. This local differentiation improves reliability by concentrating energy in more robust signal regions.
2Ease of manufacture
If uniform QAM signal points are used for transmission, then implementation is simple, but average transmit power requirements increase and performance is non-optimum
Solution Approach 1:
The patent changes the probability parameters of signal point selection from uniform distribution to non-uniform probabilistic distribution. This parameter change reduces average transmit power by preferentially selecting signal points with lower amplitude while maintaining data transmission requirements. The implementation remains relatively simple using lookup tables and random number generators.
3Productivity
If conventional modulation schemes are used to achieve high data rates, then data capacity increases, but the system becomes more susceptible to interference
Solution Approach 1:
The patent optimizes the probability distribution parameters of the constellation points to achieve better interference resistance. By carefully selecting which signal points are more or less likely to be transmitted, the system achieves higher effective data rates while the probabilistic nature provides inherent robustness against interference and noise.
Solution Approach 2:
The patent employs feedback mechanisms where the receiver estimates the channel conditions and feeds back information to the transmitter. This allows the probabilistic constellation shaping parameters to be adjusted dynamically based on actual channel quality, optimizing performance in the presence of interference while maintaining high data rates.
Data Source
AI summary
Techniques are presented for mapping a digital data sequence into a signal point sequence for transmission. The signal point sequence belongs to a set of possible signal point sequences. In one example, a digital data sequence is received. Forbidden branch flags that forbid certain signal points in the possible signal points sequences are applied. The signal point sequence is selected from a subset of all the possible signal point sequences based on the digital data sequence. The subset is defined by the forbidden branch flags.