On-Channel Repeater Feedback Cancellation for Adjacent-Channel Interference

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

On-channel repeaters face instability due to unwanted feedback between receiving and transmitting antennas, which existing solutions fail to fully address, especially when dealing with strong adjacent channel signals and intermodulation products.

Innovation Solution

An adaptive filter in the on-channel repeater uses external feedback from the transmitting antenna, including distortion and intermodulation products, to cancel parasitic signals, rather than relying solely on internal references, thereby mitigating the effects of power amplifier intermodulation and improving feedback cancellation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If an on-channel repeater is used to extend coverage, then the received signal strength is improved in areas with insufficient signal, but unwanted feedback between the receiving and transmitting antennas causes instability and relaxation oscillations

Engineering Contradiction:

Improvesignal coverageVSAvoidsystem stability

Core Design Contradiction:

ReliabilityVSStability of the object's composition

Solution Approach 1:

The patent implements a feedback cancellation mechanism where the output signal is delayed and fed back to the input, creating an artificial feedback path that counteracts the unwanted natural feedback between antennas. This controlled feedback loop stabilizes the system by canceling the parasitic oscillations through destructive interference

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

An intermediary delay element is introduced into the feedback path to decorrelate the transmitted and received signals. This delay component acts as a mediator that transforms the harmful direct feedback into a controlled, time-shifted signal that can be subtracted from the original input, eliminating instability without affecting the primary coverage extension function

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If the repeater amplifies and retransmits signals, then coverage extension is achieved, but intermodulation products from adjacent channels interfere with the wanted signal

Engineering Contradiction:

Improvesignal coverageVSAvoidintermodulation interference

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The feedback cancellation mechanism captures and subtracts not only the direct transmitted signal but also the intermodulation products generated by the power amplifier. By feeding back the complete transmitted signal including all distortion products and subtracting it from the received signal, the system removes intermodulation interference while preserving the wanted signal

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent converts the harmful intermodulation products into useful information by including them in the feedback signal. These distortion products, which would normally corrupt the received signal, are now captured by the feedback path and subtracted along with the main transmitted signal, transforming a harmful effect into a cancellation opportunity

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Stability of the object's composition

If a feedback cancellation mechanism is implemented, then unwanted feedback is reduced, but the system complexity increases due to additional components

Engineering Contradiction:

Improvefeedback cancellationVSAvoidsystem complexity

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

The delay element serves multiple functions simultaneously: it decorrelates the feedback signal to enable cancellation, provides timing alignment for the subtraction operation, and prevents direct oscillation by breaking the immediate feedback loop. This multi-functionality reduces the need for additional dedicated components, managing system complexity

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Stability of the object's composition

If the adaptive filter processes the retransmitted signal internally, then feedback cancellation is achieved, but uncorrelated parasitic signals arriving at the receiving antenna are not fully canceled

Engineering Contradiction:

Improvefeedback cancellationVSAvoidcancellation accuracy

Core Design Contradiction:

Stability of the object's compositionVSReliability

Solution Approach 1:

The patent extracts the reference signal directly from the transmitting antenna output, separating it from the internal processing path. This extraction ensures that the feedback cancellation mechanism works with the actual transmitted signal including all real-world distortions and interference, rather than an idealized internal representation, thereby improving cancellation accuracy for parasitic signals

Inventive Principle:

Principle #2Taking out (Extraction)

Data Source

PatentUS7627287B2On-channel repeater

Publication Date: 2009.12.01 BRITISH BROADCASTING CORP

AI summary

An on-channel repeater has a receiving antenna for receiving an RF signal and a transmitting antenna for transmitting on the same frequency as the input signal. An amplification path between the antennas provides substantially linear processing and includes a combiner, a decorrelating delay and a power amplifier. A filter estimator receives a reference signal and the combiner output and generates a plurality of control coefficients which are applied to an adaptive filter which filters the reference signal accordingly and applies its output to the combiner. The reference signal is derived not from the output of the decorrelating delay but rather from the output antenna after the power amplifier. Where there is an adjacent transmitter generating potentially interfering signals, the transmitter output is coupled to the same transmitter antenna.