Optical Transmitter Precoding for Multi-Level Signals Within DAC Range
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Solution Overview
Problem
Existing optical transmitters face challenges in transmitting multi-level signals due to increased power consumption, error rates, and circuit limitations, particularly when performing Tomlinson-Harashima precoding on both main and pilot signals, which complicates polarization separation and phase noise compensation.
Innovation Solution
An optical transmitter modulates a first signal, inserts a second modulated signal, performs pre-equalization and modulo calculation to maintain signal amplitudes within the DAC range, and transmits a modulated optical signal, while ensuring the pilot signal is processed with a simpler QPSK scheme to facilitate easier recovery at the reception node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Tomlinson-Harashima precoding is performed on both main signal and pilot signal, then frequency distortion compensation is improved, but reception circuit complexity increases
Solution Approach 1:
The patent divides the signal processing into two segments: the main signal undergoes THP processing for distortion compensation, while the pilot signal uses simple QPSK modulation without THP. This segmentation allows the reception circuit to process the pilot signal simply while still obtaining accurate frequency distortion characteristics from the main signal path.
Solution Approach 2:
The patent uses the pilot signal as an intermediary carrier that does not require complex processing. By keeping the pilot signal processing simple (QPSK only), it serves as an effective mediator for transmitting frequency distortion information without adding reception complexity, while the main signal carries the actual data with full THP processing.
2Measurement precision
If the number of signal points for pilot signal is increased, then frequency distortion compensation accuracy is improved, but polarization separation and phase noise compensation become more difficult
Solution Approach 1:
The patent applies different quality levels to different signal components: the main signal receives high-quality processing with THP and multiple signal points for accurate distortion compensation, while the pilot signal maintains simple QPSK with four signal points for easy polarization separation and phase noise compensation. This local differentiation optimizes both accuracy and ease of operation.
3Productivity
If baud rate is increased to achieve large-capacity communication, then transmission capacity is improved, but DAC bandwidth requirements become unattainable
Solution Approach 1:
The patent performs pre-equalization through THP processing before DAC conversion. By compensating for frequency distortion and limiting the signal amplitude in advance (pre-action), the system can handle high baud rates without requiring the DAC to have excessively high bandwidth, making the system manufacturable with current technology.
Data Source
AI summary
An optical transmitter includes a signal processor and transmission circuit. The signal processor modulates a first signal to generate a first modulated signal, determines a modulo amplitude that is larger than an amplitude of the first modulated signal, inserts a second modulated signal into the first modulated signal to generate a transmission signal, corrects a symbol of the transmission signal by using an amplitude of one or a plurality of previous symbols to generate a pre-equalized signal, and performs modulo calculation based on the modulo amplitude on the pre-equalized signal. The transmission circuit generates a modulated optical signal based on an output signal of the signal processor and transmits the modulated optical signal to a reception node. An amplitude of the second modulated signal is equal to the modulo amplitude.