64-QAM Constellation Shaping for Higher Capacity at Lower SNR
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Solution Overview
Problem
Existing digital communication systems face limitations in achieving maximum capacity due to the use of constellations that do not approach the Gaussian channel capacity, resulting in inefficiencies in bandwidth and power usage, particularly at high signal-to-noise ratios.
Innovation Solution
The development of geometrically shaped symbol constellations that optimize capacity by iteratively optimizing the location of constellation points to maximize capacity measures such as parallel decode and joint capacity, allowing for reduced signal-to-noise ratios and increased data transmission rates with reduced power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional constellations maximizing minimum distance are used, then error rate performance is improved, but capacity approaches the Shannon limit poorly
Solution Approach 1:
The patent applies local quality by creating unequally spaced constellation points where different regions of the constellation have different spacing characteristics. Specifically, points are positioned with varying distances from the origin and from each other, optimizing local error performance while improving overall capacity. This non-uniform spacing allows the system to achieve both good error rate performance and higher capacity by adapting the local geometry to capacity optimization criteria rather than uniform minimum distance criteria.
2Productivity
If Gaussian distribution constellation is used, then capacity approaches Shannon limit, but practical implementation is difficult
Solution Approach 1:
The patent employs a practical approximation approach by using a finite set of discrete constellation points that mimic the properties of a continuous Gaussian distribution. Instead of implementing the theoretically optimal but complex Gaussian constellation, the invention uses a simplified discrete constellation with unequally spaced points that achieves near-Gaussian capacity performance. This practical approximation makes the system implementable while retaining most of the theoretical capacity benefits.
3Ease of manufacture
If uniformly spaced constellations are used, then implementation is simple, but capacity is limited below Shannon limit
Solution Approach 1:
The patent applies asymmetry by transitioning from uniform constellation spacing to non-uniform spacing. The constellation points are positioned asymmetrically with respect to the origin and each other, with different radial distances and angular separations. This asymmetric positioning optimizes the constellation for capacity while maintaining reasonable implementation complexity, breaking the symmetry of traditional uniform constellations to achieve superior performance.
Data Source
AI summary
Communication systems are described that use unequally spaced constellations that have increased capacity compared to conventional constellations operating within a similar SNR band. One embodiment is a digital communications system including a transmitter transmitting signals via a communication channel, the transmitter including a coder capable of receiving user bits and outputting encoded bits at a rate, a mapper capable of mapping encoded bits to symbols in a constellation, and a modulator capable of generating a modulated signal for transmission via the communication channel using symbols generated by the mapper, wherein the constellation is unequally spaced and characterizable by assignment of locations and labels of constellation points to maximize parallel decode capacity of the constellation at a given signal-to-noise ratio so that the constellation provides a given capacity at a reduced signal-to-noise ratio compared to a uniform constellation that maximizes the minimum distance between constellation points of the uniform constellation.