Optical Transmitter Encoding for PAM4 Lane Sensitivity Uniformity
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Solution Overview
Problem
In optical transmitters using multi-level modulation like PAM4, the reception sensitivity between lanes is non-uniform due to varying error occurrence probabilities, leading to differences in bit error rates and lane errors.
Innovation Solution
An optical transmitter that selects between two encoding methods (gray code and binary code) to set voltage values for 2M logic levels based on error occurrence probabilities, ensuring equal bit error rates across lanes and reducing reception sensitivity differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a multi-level modulation method such as PAM4 is adopted to increase communication capacity, then the communication capacity increases, but the reception sensitivity becomes non-uniform between lanes due to varying error occurrence probabilities
Solution Approach 1:
The patent applies local quality by making different parts of the signal (different logic levels) have different voltage intervals. Specifically, the voltage interval between logic levels is adjusted based on the error occurrence probability of each level, with higher voltage intervals assigned to levels with higher error probabilities. This non-uniform voltage distribution compensates for the non-uniform error characteristics across different lanes, thereby improving reception sensitivity uniformity while maintaining high communication capacity through multi-level modulation
2Measurement precision
If the interval between two levels adjacent to each other is increased to make the S/N ratio at each level equal, then the S/N ratio uniformity improves, but the reception sensitivity difference between lanes increases due to varying error occurrence probabilities
Solution Approach 1:
The patent applies parameter changes by dynamically adjusting the voltage interval parameter based on the error occurrence probability of each logic level. Instead of using a uniform voltage interval for all levels, the system varies the voltage interval parameter to compensate for error probability differences. This allows the S/N ratio to be optimized for each specific level's error characteristics, thereby improving overall reception sensitivity uniformity across lanes
Data Source
AI summary
An optical transmitter includes: a controller that generates a multi-level amplitude modulated signal based on transmission data that is binary data; a driver that generates a drive signal in accordance with the multi-level amplitude modulated signal; and a light emitter that generates an optical signal in accordance with the drive signal. The controller selects one of a first encoding method and a second encoding method in accordance with a switching signal. The controller generates the multi-level amplitude modulated signal by converting a bit string of M (M is an integer of 2 or more) bits included in the transmission data into a pulse signal having 2M logic levels using a selected encoding method. The controller sets voltage values of the 2M logic levels depending on the selected encoding method.