Non-Linear Compensation Using Frequency Mixing for Photodetector Distortion
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Solution Overview
Problem
Microwave photonic beamforming systems experience non-linear distortion due to the non-linearity of photodetectors, which cannot be accurately compensated by conventional digital predistortion methods, especially in hybrid beamforming architectures with a large number of analog channels.
Innovation Solution
A non-linear compensation apparatus using frequency mixing, optical-to-electrical conversion, and processing modules to generate and superimpose even-order intermodulation products on the direct current bias voltage, canceling out odd-order intermodulation products in the signal, thereby reducing distortion and improving linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital predistortion method is used for electrical-domain beamforming, then non-linear distortion of digital channel output-stage device is compensated, but it cannot accurately pre-compensate analog channel output-stage devices in hybrid beamforming architecture
Solution Approach 1:
The patent divides the beamforming system into digital channels and analog channels, and applies different compensation methods to each segment. Digital predistortion is applied to digital channel output-stage devices, while optical domain predistortion is applied to analog channel output-stage devices, allowing each segment to be optimized independently for its specific characteristics
Solution Approach 2:
The patent introduces an optical domain as an intermediary layer between digital baseband signals and analog RF signals. By converting digital signals to optical signals for processing and then back to electrical signals, the system can apply predistortion compensation to analog channels that was previously only available in the digital domain
2Power
If incident light power is increased to improve signal strength, then output current increases, but non-linear distortion increases due to electron concentration in depletion region
Solution Approach 1:
The patent applies predistortion processing to the optical signals before they reach the photodetector. By pre-modulating the optical carrier with the inverse of the expected non-linear distortion characteristics, the system compensates for the non-linear effects that will occur when the photodetector processes the high-power incident light, allowing high output power without excessive distortion
Solution Approach 2:
The patent converts the inherent non-linear characteristics of the photodetector into a beneficial effect by using optical domain predistortion. The non-linear transfer function of the photodetector, which normally causes distortion, is counteracted by pre-applying the inverse non-linearity in the optical domain, effectively transforming the harmful non-linear effect into a predictable and compensatable characteristic
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
The proposed solution effectively reduces in-band and out-of-band distortion and enhances the linearity of the system by canceling odd-order intermodulation products, utilizing low-cost, easily implemented components like 3-dB couplers and photodetectors.
Implementation Method 1
a frequency mixing module, configured to perform frequency mixing on a first light beam and a second light beam to obtain a first frequency-mixed signal and a second frequency-mixed signal
Implementation Method 2
an optical-to-electrical conversion module connected to the frequency mixing module, configured to: convert the first frequency-mixed signal into a first electrical signal, and convert the second frequency-mixed signal into a second electrical signal
Data Source
AI summary
A non-linear compensation apparatus includes a frequency mixing module, an optical-to-electrical conversion module, and a processing module. The frequency mixing module is configured to convert a first light beam and a second light beam into two frequency-mixed signals that are mutually reverse signals, and transmit the two frequency-mixed signals to the optical-to-electrical conversion module. The optical-to-electrical conversion module is configured to: convert a first frequency-mixed signal into a first electrical signal that includes a first third-order intermodulation product; and convert a second frequency-mixed signal into a second electrical signal, and transmit the second electrical signal to the processing module. The processing module extracts an even-order intermodulation product from the second electrical signal, and superimposes the even-order intermodulation product on a direct current bias voltage of the optical-to-electrical conversion module.