Probabilistic QAM Shaping With LDPC Rate Matching
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Solution Overview
Problem
Existing communication systems using uniformly distributed QAM constellations suffer from a 1.53 dB gap in achievable transmission rate compared to the Shannon limit due to inefficient energy distribution, leading to suboptimal spectral efficiency.
Innovation Solution
Implementing probabilistic constellation shaping with LDPC-coded modulation systems, utilizing a shaping encoder to apply non-uniform probability distributions to QAM symbols, combined with an LDPC encoder to achieve a target code rate and spectral efficiency, thereby reducing the gap and enhancing energy efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If uniformly distributed QAM constellations are used, then the modulation scheme is simple to implement, but spectral efficiency is significantly reduced with a loss of up to 1.53 dB from the Shannon limit
Solution Approach 1:
The patent changes the probability distribution parameter of the QAM constellation from uniform to non-uniform distribution. By assigning different probabilities to different constellation points based on their distance from the origin, the system achieves probabilistic constellation shaping that improves spectral efficiency and reduces the gap to the Shannon limit while maintaining implementation feasibility through lookup tables and probability mapping
2Productivity
If probabilistic constellation shaping is implemented, then spectral efficiency is improved by achieving shaping gain, but the encoding process complexity increases
Solution Approach 1:
The patent introduces a shaping encoder as an intermediary component between the data source and the modulator. This shaping encoder uses lookup tables and probability mapping to transform uniformly distributed input bits into non-uniformly distributed constellation symbols, achieving probabilistic constellation shaping without requiring complex real-time calculations, thus balancing performance improvement with implementation complexity
Solution Approach 2:
The patent performs probability mapping and constellation shaping in advance through pre-computed lookup tables. By preparing the probability distribution mappings beforehand rather than calculating them in real-time during transmission, the system reduces the computational complexity of the encoding process while still achieving the desired spectral efficiency improvements
Data Source
AI summary
An apparatus may include a transmitter and one or more processors. The one or more processors may identify, by a low-density parity-check (LDPC) encoder, a target code rate for which to encode data. The one or more processors may receive, by the LDPC encoder, a first set of information bits. The one or more processors may receive, by the LDPC encoder from an output of a shaping encoder, a second set of information bits. The one or more processors may adjust a code rate of an LDPC code to a second code rate higher than the target code rate to cause the LDPC encoder to encode the data at the target code rate. The one or more processors may encode the data using the LDPC code. The transmitter may transmit the encoded data.