OFDM Diversity Demodulation Using AGC-Weighted Correlation Combining
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Solution Overview
Problem
In OFDM receivers with diversity configurations, clock recovery and fast Fourier transform window position recovery are compromised when signal levels drop, leading to demodulation performance issues due to the reliance on a single module and the challenges of combining correlation waveforms with phase differences across branches.
Innovation Solution
A demodulator with multiple modules that combines demodulated data using AGC-adjusted complex baseband OFDM signals, employing correlation detection and combining mechanisms that calculate and apply coefficients based on AGC control signals to suppress noise and ensure accurate clock recovery and FFT window position recovery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If correlation waveforms are combined by multiplying by peak values or selecting signals with large peak values, then the combining process becomes simple, but noise in correlation waveforms with low received signal power is amplified, degrading clock recovery and FFT window position recovery performance
Solution Approach 1:
The patent changes the combining parameter from peak value multiplication to AGC gain-based weighting. Instead of using peak values of correlation waveforms, the system uses the inverse of AGC control signal levels as weighting coefficients. This parameter change ensures that signals with lower received power (higher AGC gain) are appropriately weighted to prevent noise amplification, while maintaining a relatively simple combining process.
2Reliability
If OFDM signals from multiple branches are combined before correlation detection, then diversity gain is improved, but phase difference detection and correction circuits are required, increasing device complexity
Solution Approach 1:
The patent segments the diversity combining process into two independent stages: (1) separate correlation detection in each branch using locally generated correlation waveforms, and (2) weighted combining of correlation results. This segmentation avoids the need for phase alignment circuits by performing correlation detection independently in each branch before combining, thus achieving diversity gain without increasing device complexity with phase detection and correction circuits.
3Device complexity
If a single master module performs clock recovery and FFT window position recovery, then device complexity is reduced, but demodulation performance suffers when signal level drops in the master module
Solution Approach 1:
The patent merges the clock recovery and FFT window position recovery functions from a single master module into multiple distributed modules. Each demodulation module performs its own correlation detection using locally generated correlation waveforms, and the results are combined with appropriate weighting. This merging approach ensures that if one module experiences low signal level, other modules can compensate, thereby improving demodulation performance under low signal conditions while maintaining reasonable device complexity.
Data Source
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AI summary
In an OFDM receiver with a diversity configuration having a plurality of demodulation modules, to perform FFT window position recovery and clock recovery, the gain values calculated by the AGC units (16, 26) provided in the demodulation modules for adjusting the level of the received signal are supplied to a correlation combiner (35); the correlation combiner (35) multiplies the correlation signals supplied from the correlation detectors (17, 27) in the demodulation modules by coefficients calculated from the gain values, then adds the products to generate a combined correlation signal and supplies it to an FFT window position recovery unit (32) and a clock error detector (33). FFT window position recovery and clock recovery can thus be performed properly without depending on any one demodulation module.