Frequency Domain Echo Cancellation for DMT Modems
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Solution Overview
Problem
In digital communication systems, especially those using DSL and OFDM, echo signals caused by impedance mismatches and temperature variations lead to reduced signal-to-noise ratios due to intersymbol and interchannel interference, which existing echo cancellers struggle to effectively address.
Innovation Solution
A frequency domain echo canceller is introduced that models the echo path channel, including effects of intersymbol and interchannel interference, using matrices to estimate and subtract echo signals from received signals, reducing computational complexity through various embodiments that optimize matrix operations and sampling rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional echo canceller is used to reduce echo signals, then the echo signal is subtracted from the received signal, but the computational complexity increases and the ability to handle intersymbol and interchannel interference is insufficient
Solution Approach 1:
The patent segments the echo cancellation process into frequency domain operations using FFT, dividing the time domain convolution into multiple frequency domain multiplications. This segmentation reduces computational complexity by transforming O(N²) time domain operations into O(N log N) frequency domain operations while maintaining echo cancellation effectiveness.
Solution Approach 2:
The patent introduces frequency domain transformation (FFT) as an intermediary mechanism between the transmitted signal and the echo estimation process. This intermediary allows the system to model and cancel echo signals more efficiently by operating in the frequency domain where convolution becomes multiplication, reducing computational burden.
2Productivity
If the hybrid circuit is designed to match impedance, then signal transmission is optimized, but impedance mismatches due to temperature variations and line differences cause echo signals to leak into the receiver
Solution Approach 1:
The patent implements an adaptive echo canceller that continuously monitors the received signal, estimates the echo component, and subtracts it from the total received signal. This feedback mechanism dynamically compensates for impedance mismatches caused by temperature variations and line differences, preventing echo signals from degrading receiver performance.
Solution Approach 2:
The patent uses adaptive filtering techniques that dynamically adjust filter coefficients based on changing line conditions. By continuously updating the echo path model parameters, the system adapts to temperature variations and impedance changes, effectively canceling echo signals without requiring perfect initial impedance matching.
3Productivity
If DMT modulation is used to partition the channel into subchannels, then data transmission efficiency is improved, but intersymbol and interchannel interference occur due to echo signals
Solution Approach 1:
The patent applies frequency domain equalization that processes each DMT subchannel independently after FFT decomposition. By segmenting the broadband channel into multiple narrowband subchannels and applying echo cancellation to each separately, the system eliminates intersymbol and interchannel interference while maintaining the high data transmission efficiency of DMT modulation.
Data Source
AI summary
An echo canceller reduces an echo signal produced when the transmitted signal leaks back into the receiver via a hybrid. The echo canceller estimates the echo signal from the transmitted signal, and then subtracting the estimated echo signal from the received signal. In practice, the echo path channel in a DMT-modem is much longer than the cyclic prefix, and therefore, the received echo signal will be subjected to both ISI (inter-symbol-interference) and ICI (inter-carrier-interference). A traditional echo canceller, designed for a xDSL-modem, uses either a time domain adaptive FIR-filter or a combined echo canceller implemented in both time and frequency domain. A matrix-based adaptive echo canceller is implemented in the frequency domain. Various example embodiments are disclosed.