Complex-to-Unipolar Conversion for Optical Signal Embedding
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Solution Overview
Problem
Existing methods for generating real-valued signals from complex OFDM signals in optical transmission, such as DCO-OFDM and ACO-OFDM, suffer from reduced spectral efficiency due to the need for Hermitian symmetry and DC biasing, which limits their ability to convey phase and amplitude information effectively in intensity modulation systems.
Innovation Solution
A system and method that converts complex OFDM symbols from Cartesian to polar format, allowing for the generation of real-valued unipolar symbols with embedded phase and amplitude information, using a complex-to-unipolar conversion engine and pre-equalizer to optimize bit error rate and spectral efficiency, thereby avoiding the constraints of Hermitian symmetry and DC biasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Hermitian symmetry is used to generate real-valued signals from complex OFDM signals, then the signal becomes suitable for intensity modulation, but spectral efficiency is reduced by 50%
Solution Approach 1:
The patent applies asymmetry by abandoning the traditional Hermitian symmetry constraint. Instead of forcing symmetric relationships between positive and negative frequency components, the method allows asymmetric complex-to-unipolar conversion where each complex symbol is independently converted to a unipolar real-valued symbol through polar coordinate transformation and phase/amplitude embedding, thereby eliminating the 50% spectral efficiency loss.
Solution Approach 2:
The patent transitions from the complex plane (two-dimensional with real and imaginary parts) to a different representation space by converting to polar coordinates (magnitude and phase) and then embedding this information into unipolar real-valued signals. This dimensional transformation allows preservation of phase and amplitude information while generating signals suitable for intensity modulation without requiring Hermitian symmetry.
2Reliability
If DC biasing is applied to shift bipolar symbols to positive values, then the signal becomes unipolar for LED modulation, but power efficiency is reduced
Solution Approach 1:
The patent applies preliminary action by performing phase and amplitude pre-equalization on the complex OFDM symbols before the complex-to-unipolar conversion. This pre-processing optimizes the distribution of phase and amplitude values, ensuring that after conversion to unipolar real-valued symbols, the signal has reduced peak-to-average power ratio and improved power efficiency without requiring excessive DC biasing.
Solution Approach 2:
The patent changes the parameters of the signal representation by transforming from Cartesian coordinates (real and imaginary parts) to polar coordinates (magnitude and phase), and then embedding these parameters into unipolar real-valued symbols. This parameter transformation allows direct generation of unipolar signals without DC biasing, as the unipolar nature is achieved through the conversion process itself rather than through additive biasing.
3Productivity
If phase and amplitude information is embedded in unipolar symbols, then spectral efficiency is improved, but signal complexity increases
Solution Approach 1:
The patent replaces complex mechanical signal processing operations with mathematical transformations. Instead of using complex modulator hardware to encode phase and amplitude, the method uses software-based complex-to-unipolar conversion with polar coordinate transformation and phase/amplitude embedding, which can be efficiently implemented through digital signal processing algorithms without requiring additional hardware complexity.
Data Source
AI summary
A system for embedding phase and amplitude into a real valued unipolar signal suitable for intensity modulation (IM) by optical transmitters. The system includes a complex-to-unipolar conversion engine configured to receive complex symbols in Cartesian format and convert the complex symbols from the Cartesian format to a polar coordinate format and generate real valued unipolar symbols including embedded phase and amplitude information of complex symbols in the Cartesian format.