Adaptive Filter Autocorrelation Preservation
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Solution Overview
Problem
Adaptive filters in on-channel repeaters face instability and spurious frequency components due to autocorrelation of the input signal, which can lead to numerical overflows and non-linear distortion, especially when the coupling between antennas changes rapidly.
Innovation Solution
A digital adaptive filter is modified to remove the part of the reference signal that is linearly dependent on the retransmitted signal by multiplying the impulse response function with a matrix containing shifted versions of the autocorrelation function of the recovered signal, and adjusting the cross-correlation between the reference and recovered signals to avoid adverse effects of autocorrelation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the adaptive filter removes all correlation between reference and input signals, then unwanted feedback is eliminated, but desirable autocorrelation of the recovered signal is also removed causing spurious frequency components
Solution Approach 1:
The patent segments the correlation removal process into two distinct parts: (1) removal of unwanted feedback correlation through adaptive filtering, and (2) preservation of desirable autocorrelation through signal regeneration. The recovered signal is separated into correlated components (unwanted feedback) and autocorrelation components (desirable signal characteristics), allowing selective processing of each.
Solution Approach 2:
The patent applies preliminary action by regenerating the autocorrelation signal before final output. The filter estimator first removes unwanted correlation, then the autocorrelation signal is regenerated from the recovered signal and added back to prevent spurious frequency components. This preliminary regeneration prevents the harmful effect before it manifests.
2Measurement precision
If the filter estimator minimizes power of the recovered signal, then unwanted correlation is reduced, but spurious out-of-band components appear due to autocorrelation removal
Solution Approach 1:
The patent converts the harmful removal of autocorrelation into a beneficial process by intentionally regenerating it. The autocorrelation that was mistakenly removed by the filter estimator is now deliberately recreated from the recovered signal and added back to the output, transforming a harmful side effect into a controlled, beneficial restoration of signal integrity.
3Object-generated harmful factors
If adaptive filtering is applied in on-channel repeaters, then feedback cancellation is achieved, but numerical overflows and non-linear distortion occur under rapid antenna coupling changes
Solution Approach 1:
The patent introduces a feedback mechanism where the recovered signal is used to regenerate autocorrelation that is then fed back into the output signal. This feedback loop ensures that desirable signal characteristics are restored after unwanted feedback is removed, preventing numerical overflows and non-linear distortion by maintaining proper signal statistics.
Data Source
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AI summary
An adaptive filter arrangement for removing undesired autocorrelation or cross-correlation from target signals containing desirable autocorrelation or cross-correlation, comprises a coefficient generator (24; 174) for generating one or a plurality of filter coefficients, an adaptive filter (26; 172) coupled to the coefficient generator to receive the filter coefficient(s) and apply a filter function dependent thereon; and compensating means (70, 72; 180) for modifying the coefficient(s) such as to prevent the adaptive filter from removing desirable autocorrelation or cross-correlation that exists in the target signal or signals. The arrangement can be used in an on-channel repeater (Fig. 7) or in an IQ crosstalk removal system (Fig. 10).