Adaptive Filter Feedback for Receiver Path IMD2 Cancellation
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Solution Overview
Problem
Conventional frequency division duplex (FDD) microwave communication systems suffer from even-order intermodulation distortion due to transmitter signal leakage, which degrades performance, especially in higher-order modulation schemes like 8 k-QAM, as the distortion falls within the bandwidth of the desired baseband signal.
Innovation Solution
An adaptive filtering module and an intermodulation distortion (IMD) compensation module work together to cancel IMD by using a reference signal from the transmitter part, and the adaptive filtering module adjusts the DC bias of the demodulator to minimize even-order intermodulation distortion, specifically targeting second-order intermodulation distortion (IMD2) in the receiver path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the duplexer is used to share the antenna between transmitter and receiver, then the system can simultaneously transmit and receive data, but even-order intermodulation distortion is generated due to transmitter signal leakage through the duplexer
Solution Approach 1:
The patent captures the harmful transmitter leakage signal that leaks through the duplexer and converts it into a useful reference signal. This reference signal is then used by the adaptive filter to generate cancellation signals that eliminate the even-order intermodulation distortion. The harmful leakage is thus transformed into a beneficial resource for distortion cancellation.
Solution Approach 2:
The patent introduces an adaptive filter as an intermediary component between the receiver mixer and the baseband signal processing. This adaptive filter receives the leaked transmitter signal as input and generates a cancellation signal that mediates the harmful intermodulation distortion, subtracting it from the received signal to produce a clean baseband output.
2Reliability
If conventional filtering is used to remove transmitter leakage, then the duplexer can separate frequencies, but even-order intermodulation products fall within the desired signal bandwidth and cannot be filtered out
Solution Approach 1:
Instead of attempting to filter out the intermodulation distortion products that fall within the signal bandwidth, the patent converts the harmful transmitter leakage into a beneficial reference signal. This reference signal enables the adaptive filter to generate precise cancellation signals that remove the distortion without affecting the desired signal.
Solution Approach 2:
The patent changes the approach from frequency-based filtering to adaptive signal processing. By using the adaptive filter to dynamically adjust cancellation signals based on the actual leaked transmitter signal, the system can remove intermodulation distortion products regardless of their frequency position relative to the desired signal bandwidth.
3Object-generated harmful factors
If the adaptive filtering module uses the leaked transmitter signal as a reference signal to cancel IMD, then distortion is reduced, but additional signal processing complexity is introduced
Solution Approach 1:
The adaptive filter serves multiple functions: it processes the leaked transmitter signal to generate cancellation signals, adapts its coefficients to changing channel conditions, and removes even-order intermodulation distortion from the received signal. This multi-functionality reduces the need for separate dedicated components for each function.
Solution Approach 2:
The system uses the leaked transmitter signal itself as the reference signal for the adaptive filter, eliminating the need for a separate pilot signal or external reference source. The leaked signal serves its harmful purpose of creating distortion while simultaneously providing the reference needed to cancel that same distortion.
Data Source
AI summary
A duplex communication system includes a duplexer for transmitting and receiving signals, a transmitter part connected to the duplexer, and a receiver part connected to the duplexer, the receiver part comprising an amplifier for amplifying a received signal to provide an amplified received signal and a demodulator for downconverting the amplified received signal, and a second-order intermodulation distortion (IMD2) compensation module for compensating for second-order intermodulation distortion. The system also includes an adaptive filtering module that obtains a transmitter reference signal, generates filter coefficients for the IMD2compensation module and also adjusts a direct current (DC) bias of the demodulator based on the reference signal.