Equal Phase Combining for Robust OFDM Systems
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Solution Overview
Problem
OFDM systems face deteriorated bit error rate performance under severe frequency selective channels due to multi-path fading and inter symbol interference, which conventional methods struggle to address effectively.
Innovation Solution
The implementation of an equal phase combining technique at the receiver, where sub-carriers are equalized with the complex conjugate of estimated channel coefficients, giving less weight to sub-carriers with deep fades and more weight to those with low fades, to improve bit error rate performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional OFDM equalization is used, then system simplicity is maintained, but bit error rate performance deteriorates under severe frequency selective channels
Solution Approach 1:
The received OFDM signal is segmented into multiple sub-carriers in the frequency domain, and each sub-carrier is equalized independently using one-tap equalization. This segmentation allows the system to handle frequency selective fading on individual sub-carriers while maintaining overall system simplicity.
Solution Approach 2:
The patent changes the equalization parameter by using the complex conjugate of the channel estimate as the equalizer coefficient for each sub-carrier. This parameter change (using H*k* instead of simple inversion) improves bit error rate performance under severe fading conditions while maintaining the simplicity of one-tap equalization.
2Ease of manufacture
If simple one-tap equalization per sub-carrier is used, then system cost and complexity are reduced, but performance under frequency selective channels deteriorates
Solution Approach 1:
The system divides the broadband OFDM signal into multiple narrowband sub-carriers, applying simple one-tap equalization to each. This segmentation enables low-cost implementation while the collective effect of equalizing all sub-carriers maintains good performance under frequency selective channels.
Solution Approach 2:
The patent moves the equalization problem from the time domain to the frequency domain through FFT-based sub-carrier decomposition. This dimensional change allows simple per-subcarrier equalization to effectively handle frequency selective fading that would be complex to address in the time domain.
3Reliability
If equal phase combining is applied to improve bit error rate, then reliability is enhanced, but noise enhancement may occur
Solution Approach 1:
The patent uses the complex conjugate of the channel estimate (H*k*) as the equalizer coefficient rather than the inverse. This parameter choice provides noise robustness because it does not amplify noise as much as inversion would, while still achieving phase alignment for coherent combining of sub-carriers.
Solution Approach 2:
The system uses pilot symbols to estimate the channel response for each sub-carrier, and this channel estimate is fed back to determine the equalizer coefficients. This feedback mechanism allows the equalizer to adapt to channel conditions without excessive noise amplification.
Data Source
AI summary
A technique for an equal phase combining for a robust orthogonal frequency division multiplexing (OFDM) system under high delay spread channel is disclosed. In one embodiment, a method includes receiving and synchronizing transmitted OFDM symbols having cyclic prefixes via a symbol synchronizer module, removing the cyclic prefixes from the OFDM symbols, via a cyclic prefix remover module, to form time domain symbols, converting the time domain symbols to frequency domain symbols via an FFT module, equalizing a received sub-carrier with complex conjugate of estimated channel coefficients in frequency domain at the same sub-carrier frequency via an equalizer, combining the frequency domain symbols spread over multiple sub-carriers during transmission via a combiner, and detecting phase information from the mapped combined PSK constellation symbol via a detector module.