Digital Self-Interference Cancellation for FDD Duplexer Isolation
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Solution Overview
Problem
In FDD wireless communication systems, high isolation requirements between uplink and downlink signal paths due to transmitter leakage and noise floor interference degrade receiver sensitivity, leading to increased costs and form factor of duplexers.
Innovation Solution
Implementing an active self-interference cancellation (SIC) system in the digital domain, which samples and processes the transmit signal to cancel interference, reducing the isolation burden on duplexers and allowing for smaller, less costly components while maintaining effective TX-RX isolation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high isolation duplexers are used to mitigate interference between UL and DL bands, then interference cancellation is improved, but device cost and form factor increase
Solution Approach 1:
The patent extracts the self-interference cancellation function from the duplexer component and implements it as a separate digital signal processing system. By coupling a sample of the transmit signal through a separate path with a second RFADC and processing it digitally to generate a cancellation signal, the system removes the burden of high isolation requirements from the analog duplexer, allowing use of lower isolation duplexers while achieving the same overall interference mitigation performance
Solution Approach 2:
The patent introduces a coupler as an intermediary component that samples the transmit signal and feeds it to a separate RFADC path. This intermediary sampling path enables digital processing of the transmit signal to create an accurate cancellation signal, which then mediates the interference problem by being subtracted from the received signal in the digital domain
2Object-affected harmful factors
If large frequency separation band is assigned between UL and DL bands, then interference is reduced, but spectral efficiency decreases
Solution Approach 1:
The patent replaces the mechanical/frequency-domain approach of using large guard bands with a digital signal processing approach. Instead of allocating excessive frequency separation to prevent interference, the system uses digital subtraction of the transmitted signal from the received signal, achieving interference cancellation without sacrificing spectral efficiency or requiring large frequency separation bands
3Device complexity
If conventional RX path with LNA and down-converter is used, then receive signal processing is simplified, but receiver sensitivity degrades due to transmitter leakage
Solution Approach 1:
The patent performs preliminary action by digitally canceling the self-interference component from the received signal before subsequent processing stages. By generating the cancellation signal from a coupled sample of the transmit signal and subtracting it in the digital domain, the system eliminates transmitter leakage effects early in the signal chain, protecting receiver sensitivity without requiring complex analog filtering or isolation components
Data Source
AI summary
A method and an apparatus for self-interference cancellation in a communication device. The communication device includes an antenna configured to transmit a transmit signal and receive a receive signal through a duplexer in FDD communications, a first analog to digital converter (ADC) configured to convert the receive signal from analog to digital, a coupler configured to couple a sample of the transmit signal to a second ADC, which is configured to convert the sample of the transmit signal from analog to digital, and self-interference cancellation circuitry configured to process the digital sample of the transmit signal to generate a self-interference cancellation signal and apply the self-interference cancellation signal to the digital receive signal to cancel an amount of interference induced in the receive signal by the transmit signal. The SIC process provides additional isolation between TX signal and RX signal. The additional isolation could be utilized to relief the isolation requirement for the duplexer, reducing the cost and form factor of the duplexer.