Turbo Nonlinear Equalizer for Intermodulation Distortion Compensation
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Solution Overview
Problem
Multicarrier communication systems face significant interference due to intermodulation distortion (IMD) when multiple carriers share the same transponder high power amplifier (HPA), leading to unacceptable performance degradation, especially in systems with high-order modulation schemes and tight spectral spacing.
Innovation Solution
A turbo nonlinear equalizer is employed, utilizing a multicarrier Volterra series representation to model IMD and apply linear minimum mean-squared criterion compensation, which is adaptive and does not require prior knowledge of HPA characteristics, using pilot-based training for channel estimation and iterative equalization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the transponder HPA is operated near saturation output power level to transmit maximum signal strength, then the power efficiency is improved, but the non-linearities in the HPA increase leading to intermodulation distortion (IMD)
Solution Approach 1:
The system applies preliminary anti-action by computing predistortion coefficients that generate inverse non-linear characteristics before the signal enters the HPA. The predistorter modifies the input signal to counteract the expected non-linear distortion, effectively canceling out the IMD that would otherwise be generated when the HPA operates near saturation.
Solution Approach 2:
The system implements feedback by using pilot signals to estimate the actual non-linear characteristics of the HPA, then using these estimates to update and refine the predistortion coefficients. This adaptive feedback loop allows the system to continuously optimize the predistortion parameters to match the actual HPA behavior, improving compensation accuracy.
2Object-generated harmful factors
If output back-off is increased to reduce non-linearities and IMD, then the intermodulation distortion is reduced, but the power efficiency and output signal strength decrease
Solution Approach 1:
Instead of reducing output power to avoid non-linearities, the system applies preliminary anti-action by pre-distorting the signal to counteract the non-linear effects. This allows the HPA to operate at high power levels near saturation while the predistortion compensation prevents the generation of harmful IMD, achieving both high power efficiency and low distortion simultaneously.
3Object-generated harmful factors
If broadband filtering is applied to eliminate undesired distortion products, then the intermodulation products are reduced, but the in-band performance and signal quality are affected
Solution Approach 1:
The system applies preliminary action by compensating for non-linear distortions before the signal is transmitted through the HPA. By pre-distorting the signal with inverse characteristics, the system prevents the generation of distortion products in the first place, eliminating the need for subsequent filtering that would otherwise be required to remove these products.
Solution Approach 2:
The system converts the harmful non-linear characteristics of the HPA into a benefit by using these same characteristics to generate predistortion coefficients. The non-linearities that would normally create distortion are instead used to create the compensation signal, turning the problem into a solution.
4Productivity
If multiple carriers are tightly spaced to improve spectral efficiency, then the bandwidth utilization is improved, but the intermodulation interference between carriers increases
Solution Approach 1:
The system applies preliminary anti-action by computing predistortion coefficients that specifically address the intermodulation interactions between multiple tightly-spaced carriers. The predistorter pre-compensates for the carrier-specific non-linear interactions, allowing multiple carriers to be transmitted at high power levels with minimal mutual interference, thereby maintaining both spectral efficiency and signal quality.
Data Source
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AI summary
A receiver is provided that can receive a first signal transmitted on a first carrier and a second signal transmitted on a second carrier. The receiver includes a channel estimation portion (402), a multicarrier nonlinear equalizer (404), a first log likelihood computing portion (410) and a second log likelihood computing portion. The channel estimation portion (402) can output a first estimation. The multicarrier nonlinear equalizer (404) can output a first equalized signal and a second equalized signal. The first log likelihood ratio computing portion (410) can output a first log likelihood ratio signal based on the first equalized signal. The second log likelihood ratio computing portion can output a second log likelihood ratio signal based on the second equalized signal. The multicarrier nonlinear equalizer (404) can further output a third equalized signal and a fourth equalized signal. The third equalized signal is based on the first signal, the second signal and the first estimation. The fourth equalized signal is based on the first signal, the second signal and the first estimation.