Optical Transmission Coding for PAM4 Bandwidth and Dispersion Limits
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Solution Overview
Problem
Ultrahigh speed optical transmission systems face signal quality deterioration due to band limitations of electro-optical devices and wavelength dispersion in optical fibers, particularly when using PAM4 modulation, which is not effectively addressed by existing methods like nonlinear trellis coding designed for QAM signals.
Innovation Solution
An optical transmission system employing nonlinear trellis coding on a symbol sequence to generate a narrower bandwidth signal spectrum, using an optical transmitter with a signal coding unit for nonlinear trellis coding and a digital signal processing unit for Viterbi decoding to reconstruct the symbol sequence, thereby reducing high-frequency components and improving resistance to band limitations and wavelength dispersion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If PAM4 modulation is used to achieve ultrahigh speed optical transmission at 100 Gb/s, then the transmission speed is improved, but the signal quality deteriorates due to band limitation of electro-optical devices and wavelength dispersion in optical fibers
Solution Approach 1:
The patent applies nonlinear trellis coding to transform the symbol sequence, which changes the spectral characteristics of the signal. This coding technique redistributes the signal energy in the frequency domain, concentrating power in low-frequency regions and suppressing high-frequency components. As a result, the signal becomes more resistant to band limitation effects and wavelength dispersion, thereby improving signal quality while maintaining the high transmission speed enabled by PAM4 modulation
Solution Approach 2:
The patent performs nonlinear trellis coding on the symbol sequence before modulation and transmission. This preliminary processing of the signal prepares it in advance to withstand the adverse effects of band limitation and wavelength dispersion that will occur during transmission through optical fibers and electro-optical devices, thereby preventing signal quality deterioration before it happens
2Area of stationary object
If nonlinear trellis coding is applied to QAM signals, then the signal spectrum bandwidth is narrowed, but the method is not applicable to PAM4 signals used in direct wave detection methods
Solution Approach 1:
The patent successfully adapts nonlinear trellis coding, which was originally developed for QAM signals, to work with PAM4 signals used in direct wave detection methods. By modifying the coding approach to suit the characteristics of PAM4 modulation, the patent makes this bandwidth-reducing technique universally applicable to both coherent detection (QAM) and direct wave detection (PAM4) systems, thereby extending the versatility of the method across different optical transmission architectures
Data Source
AI summary
Provided is an optical transmission system that includes an optical transmitter, and an optical receiver. The optical transmitter includes: a signal coding unit configured to perform nonlinear trellis coding, which corresponds to nonlinear calculation, on a symbol sequence; and a modulator configured to modulate the symbol sequence subjected to the nonlinear trellis coding by the signal coding unit, and transmit the modulated symbol sequence to the optical receiver. The optical receiver includes: a light receiving unit configured to receive an optical signal transmitted from the optical transmitter, and convert the received optical signal into an electrical signal; and a digital signal processing unit configured to perform digital signal processing on the electrical signal to reconstruct the symbol sequence.