Full Duplex Self-Interference Cancellation in Cable Networks
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Solution Overview
Problem
In cable networks operating in full duplex mode, the overlap of upstream and downstream signals leads to significant interference, particularly adjacent channel interference (ACI) and adjacent leakage interference (ALI), which affect the dynamic range of downstream converters and modulation error ratio, respectively, necessitating effective echo cancellation methods.
Innovation Solution
The implementation of a frequency domain echo cancellation system that allows for synchronized or unsynchronized training, using a full-band analog echo canceller to cancel upstream echoes across the entire FDX band, combined with digital echo cancellation, to reduce self-interference and improve channel estimation accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If full duplex mode is implemented to increase spectral efficiency, then downstream and upstream transmissions can use the same RF spectrum simultaneously, but upstream and downstream signals interfere with each other
Solution Approach 1:
The patent extracts the harmful upstream signal component from the downstream received signal using echo cancellation techniques. The system models the echo path and subtracts the estimated upstream interference from the received downstream signal, effectively removing the harmful factor while preserving the useful downstream signal.
Solution Approach 2:
The patent converts the harmful upstream signal into a useful cancellation reference. By capturing the upstream signal and modeling its effect on the downstream path, the system uses this previously harmful interference to generate a cancellation signal that eliminates the interference effect, turning the problem into a solution.
2Object-affected harmful factors
If frequency division duplex mode is used to avoid upstream interference to downstream, then cable modems can operate on separate frequency bands, but dedicated filters or duplexers are required which are expensive and require guard bands
Solution Approach 1:
The patent replaces the mechanical/analog filter-based interference rejection (physical separation of frequency bands) with a digital signal processing approach. Instead of using expensive hardware filters and duplexers, the system uses digital echo cancellation algorithms to reject upstream interference from the downstream signal path.
Solution Approach 2:
The patent changes the approach from physical frequency separation to temporal/digital signal separation. By using echo cancellation, the system allows frequency overlap while managing interference through signal processing parameter adjustment rather than physical hardware constraints.
3Power
If upstream transmit power levels are high to maintain signal strength, then transmission range is improved, but adjacent channel interference and adjacent leakage interference increase affecting downstream channels
Solution Approach 1:
The patent employs feedback mechanisms in the echo cancellation system. The system continuously monitors the downstream signal quality, estimates the upstream interference based on feedback information, and adjusts the cancellation signal accordingly. This feedback loop enables the system to handle high upstream power levels while maintaining downstream signal integrity.
Solution Approach 2:
The patent performs preliminary echo path characterization and modeling before actual communication begins. By training the echo cancellation system in advance and preparing cancellation filters, the system is ready to handle high upstream power levels without causing harmful interference to downstream channels.
4Reliability
If echo cancellation is performed to reduce self-interference, then modulation error ratio and dynamic range are improved, but training opportunities and system complexity increase
Solution Approach 1:
The patent implements continuous echo path tracking and cancellation updates during normal operation. Rather than performing training only at initialization, the system continuously adapts the echo cancellation filters to track changes in the echo path, making the training process ongoing and integrated with normal communication rather than a separate time-consuming phase.
Data Source
AI summary
A method and apparatus for cancelling self-interference in a cable network implementing full duplex (FDX). A processor in a cable modem may obtain a frequency response of an echo channel for at least one sub-channel in an FDX band and calculate filter coefficients based on the frequency response, and an interference canceller cancels the self-interference in the downstream signal. The interference canceller includes filters in a digital domain to generate an interference cancellation signal from a reference signal. In another example, the processor may obtain frequency responses of an echo channel for all sub-channels in a FDX band, combine the frequency responses for the FDX band, and generate a channel impulse response for the FDX band from the combined frequency response. The interference canceller may then cancel the self-interference in the downstream signal in an analog domain based on the channel impulse response.