Hybrid Modulation Format Clock Tone Generation
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Solution Overview
Problem
Existing optical transmission networks face challenges in achieving high spectral efficiency due to intersymbol interferences and signal degradation caused by filtering, which can lead to loss of timing recovery and reduced data capacity when using small roll-off factors in Nyquist pulse shaping.
Innovation Solution
A digital modulation method employing a hybrid modulation format that combines different modulation formats, such as 4-QAM and 16-QAM, to produce clock tones at lower frequencies, allowing for timing recovery while using small roll-off factors, and dynamically allocating transmission power to maintain equivalent bit error rates across formats.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If additional filtering with small roll-off factors is applied to reduce spectral bandwidth, then spectral efficiency is improved, but timing recovery fails due to loss of clock tones
Solution Approach 1:
The invention changes the parameter of modulation format from uniform to hybrid (combining different formats like QPSK and 16-QAM), which shifts the clock tone frequencies to lower values that fall within the passband of small roll-off filters, thereby preserving timing recovery capability while maintaining spectral efficiency
Solution Approach 2:
The bitstream is segmented and mapped to modulation symbols using different modulation formats in a hybrid scheme, where some symbols use robust formats (QPSK) and others use higher-order formats (16-QAM), allowing clock tones to be preserved at specific positions while maximizing data throughput
2Reliability
If pilot tones are transmitted for timing recovery, then timing recovery is enabled, but data capacity is reduced due to reserved spectral bandwidth
Solution Approach 1:
The data symbols themselves serve the dual purpose of carrying information and providing timing recovery functionality through the hybrid modulation format structure, eliminating the need for separate dedicated pilot tones and maximizing spectral utilization for data transmission
3Productivity
If high order modulation formats are used to increase throughput, then data capacity is improved, but signal degradation increases due to intersymbol interferences
Solution Approach 1:
Different parts of the modulation symbol sequence are assigned different modulation formats based on their local requirements: positions requiring robustness against ISI use lower-order formats (QPSK), while positions prioritizing throughput use higher-order formats (16-QAM), achieving local optimization of signal quality and throughput
Data Source
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Figure 5a~5c
AI summary
The present invention refers to a digital modulation method for signal transmission through an optical network comprising an encoding and mapping step (101) of producing a series of modulation symbols from a bitstream, wherein the series of produced modulation symbols can be divided into successive hybrid symbol sequences, a hybrid symbol sequence comprising symbols of at least two different modulation formats, such hybrid symbol sequences producing clock tones at frequencies corresponding to a shift of a fraction of the symbol rate on both sides of the carrier frequency, the said fraction being smaller than the fraction obtained with symbol sequences comprising a single modulation format, the said clock tones enabling timing recovery after transmission of the signal.