Decision Feedback Equalizer for OFDM WLAN Interference
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Solution Overview
Problem
OFDM receivers face inter-OFDM-symbol interference due to long delay spreads, which degrades signal quality and increases packet error rates, especially at higher data rates, as existing equalizers are ineffective in removing distant delays without affecting short delays.
Innovation Solution
A decision feedback equalizer (DFE) is implemented in OFDM receivers, using a feedback filter with impulse response calculated from the channel response, which removes only distant delays causing inter-OFDM-symbol interference while preserving short delays, and includes a DFT device, forward and reverse processors, and a feedback filter to correct channel and phase errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a linear equalizer with feedforward filter coefficients inverted from channel impulse response is used, then inter-OFDM-symbol interference is reduced, but computational complexity increases and adaptation time becomes insufficient
Solution Approach 1:
The equalizer is segmented into two distinct parts: a feedforward filter that processes current symbols and a feedback filter that processes previous symbols. This segmentation allows the feedback filter to use a simplified coefficient calculation method (direct inversion of channel impulse response) while the feedforward filter handles the complex adaptive coefficient calculation, thereby reducing overall computational complexity while maintaining interference removal effectiveness
Solution Approach 2:
The feedback filter coefficients are calculated in advance from the channel impulse response obtained during channel estimation, before the actual data transmission begins. This preliminary calculation eliminates the need for real-time coefficient computation during data reception, reducing computational complexity and ensuring coefficients are ready before adaptation time expires
2Reliability
If distant delays are removed to eliminate inter-OFDM-symbol interference, then signal quality improves, but short delays that provide beneficial energy capture are also affected
Solution Approach 1:
The equalizer applies different processing characteristics to different parts of the channel impulse response. The feedforward filter handles short delays with adaptive coefficient adjustment to preserve beneficial energy capture, while the feedback filter specifically targets and removes distant delays causing inter-OFDM-symbol interference. This localized quality differentiation ensures that useful short delays are preserved while harmful distant delays are eliminated
Data Source
AI summary
In an OFDM radio receiver for a wireless packet network, an apparatus to equalize a presently received OFDM signal containing subcarriers using an equalizing signal formed from the past symbols of subcarriers. The apparatus includes a subtractor to subtract the equalizing signal from the present signal, a DFT device to convert to subcarriers, a forward processor to correct the subcarriers for the channel and phase errors, a decision device to make decisions for the subcarriers, a reverse processor to corrupt the decision-based subcarriers' phase in the same manner as the forward processor corrects the subcarrier's phase, an inverse DFT device to form a time signal from the reverse processed subcarriers, and a feedback filter to generate the equalizing signal. The feedback filter's impulse response is directly calculated from an estimate of the channel response.