FPGA Predistortion for Laser Intermodulation Compensation
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Solution Overview
Problem
Cable television signal distribution systems face significant signal losses and distortion due to coaxial cable transmission, requiring frequent and costly repeater amplifiers, and optical communications lasers exhibit intermodulation distortion that current predistortion techniques do not adequately address.
Innovation Solution
The use of Field Programmable Gate Arrays (FPGAs) with digital-analog (DA) converters to digitally compute and apply predistortion signals, reducing distortion in RF signals and laser output, and incorporating analog predistortion circuits for large distortions, allowing for efficient distortion compensation in RF and fiber-optic transmitters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If coaxial cable is used for RF transmission, then signal distribution is achieved, but signal losses increase requiring frequent repeater amplifiers
Solution Approach 1:
The patent replaces coaxial cable RF transmission with optical fiber transmission. Electrical signals are converted to optical signals using laser diodes, transmitted through optical fiber with minimal loss, then converted back to electrical signals. This substitution eliminates the fundamental limitation of coaxial cable signal loss and the need for frequent repeaters.
Solution Approach 2:
The patent changes the transmission medium parameter from electrical conductors (coaxial cable) to optical waveguides (fiber optic cable). This parameter change fundamentally alters the transmission characteristics, enabling signals to travel much longer distances without degradation and eliminating the need for signal regeneration at frequent intervals.
2Loss of energy
If repeater amplifiers are installed frequently, then signal loss is compensated, but system cost and maintenance requirements increase
Solution Approach 1:
The patent replaces the mechanical/electrical repeater amplifier system with an optical transmission system. By converting signals to optical domain for transmission and then back to electrical domain, the system achieves long-distance transmission without needing frequent signal regeneration, thereby eliminating the cost and maintenance burden of numerous repeaters.
Solution Approach 2:
The patent extracts the signal conversion functions (electrical-to-optical and optical-to-electrical) and places them only at the endpoints of the transmission link. This eliminates the need for intermediate repeater amplifiers, reducing system complexity, cost, and maintenance requirements while maintaining signal integrity over long distances.
3Length of stationary object
If laser diodes are used for optical transmission, then transmission distance is extended, but intermodulation distortion increases
Solution Approach 1:
The patent applies predistortion techniques that introduce opposite distortion characteristics before the signal enters the laser diode. By pre-distorting the modulation signal in the electrical domain, the subsequent non-linear distortion introduced by the laser diode is compensated, reducing intermodulation distortion products in the final optical output.
Solution Approach 2:
The patent introduces digital signal processing algorithms as an intermediary between the electrical signal source and the laser diode. These algorithms compute predistortion correction factors and apply them to the modulation signal, serving as a mediator that compensates for the laser diode's non-linear characteristics before optical transmission occurs.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces distortion, lowers the need for repeater amplifiers, and achieves effective distortion compensation with lower-cost components, even at high bandwidths, enhancing the reliability and cost-effectiveness of cable television and optical communication systems.
Implementation Method 1
Predistortion techniques have been proposed as one method for improving distortion levels at the laser output
Implementation Method 2
The techniques digitally compute predistortion signals in field programmable gate arrays and forward them to radio frequency components using digital-analog converters
Implementation Method 3
Optical communications lasers, such as laser diodes and distributed feedback ('DFB') lasers
Data Source
AI summary
Predistortion logic for an optical communications laser or optical modulator, includes predistortion logic embodied in a field programmable gate array (FPGA). A first analog to digital converter (ADC) provides a representation of an RF signal at an input of the FPGA. A digital to analog converter provides a representation of an output of the FPGA to a laser or modulator.