Adaptive Digital Pre-emphasis for Optical Transmitters
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Solution Overview
Problem
High baud rate optical communication systems face bandwidth limitations due to opto-electronic components, which digital pre-emphasis techniques struggle to fully compensate for, especially at high symbol rates.
Innovation Solution
A coherent optical transceiver with an optical transmitter that shapes the optical spectrum of a data-modulated signal based on signal-to-noise ratio (SNR) information from a remote receiver, using a digital signal processor to provide frequency-selective pre-emphasis and partially pre-compensate for high-frequency roll-off, with stronger pre-compensation for lower SNR values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If digital pre-emphasis is applied to compensate for bandwidth limitations, then transmission quality improves, but the compensation becomes less effective at higher symbol rates
Solution Approach 1:
The patent implements adaptive digital pre-emphasis where the pre-emphasis filter coefficients are dynamically adjusted based on the actual bandwidth characteristics measured from transmitted test signals. This allows the system to adapt to varying symbol rates and bandwidth conditions, maintaining effectiveness across different operating points rather than using fixed compensation parameters
Solution Approach 2:
The system employs feedback mechanisms where the transmitter analyzes the actual bandwidth response by transmitting test signals and measuring the received signal characteristics. This feedback information is used to adjust the pre-emphasis filter settings, creating a closed-loop system that continuously optimizes compensation for the current operating conditions
2Reliability
If aggressive pre-emphasis is applied to flatten the optical spectrum, then bandwidth limitations are compensated, but noise contributions from the fiber-optic link increase
Solution Approach 1:
The patent applies partial pre-emphasis rather than aggressive full compensation. The pre-emphasis filter is designed to provide just enough compensation to flatten the spectrum within acceptable limits, avoiding excessive amplification of high-frequency components that would otherwise be over-compensated. This partial action approach balances bandwidth compensation with noise control
Solution Approach 2:
The system dynamically adjusts the pre-emphasis filter parameters based on measured bandwidth characteristics and noise conditions. By changing the filter coefficients adaptively, the system optimizes the trade-off between spectrum flattening and noise amplification for each specific transmission scenario
3Device complexity
If fixed pre-emphasis filters are used, then device complexity is reduced, but adaptability to different bandwidth conditions deteriorates
Solution Approach 1:
The system implements self-service through automatic bandwidth measurement and adaptive filter configuration. The transmitter automatically transmits test signals, measures the received signal bandwidth characteristics, and adjusts its pre-emphasis filter settings without requiring manual configuration or complex external calibration equipment
Data Source
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AI summary
An optical transmitter for an fiber-optic communication system includes an adaptive digital pre-compensating filter configured to pre-compensate a data signal to be transmitted by an optical transmitter over a fiber-optic link based on a known transmitter response, a transmitter noise information, and an optical noise information for the fiber-optic link, e.g. an OSNR estimate at the receiver end of the fiber-optic link. The optical noise information may be provided with a feedback from the receiver, computationally using a link model, or by selecting from an OSNR or similar database. The link-adaptive pre-compensation may provide SNR gain compared to transmitter-only pre-compensation.