OFDM Pre-Distortion Using Symbol Perturbation for Lower PA Back-Off
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Solution Overview
Problem
Power amplifiers in OFDM-based communication systems are inefficient due to non-linearity, leading to high energy consumption and increased costs, and require large Input Back Off (IBO) to manage Peak-to-Average Power Ratios (PAPR), which affects Error Vector Magnitude (EVM) and spectral mask compliance.
Innovation Solution
A digital pre-distortion method that computes weighting coefficients based on vector metrics and data tables to perturb OFDM symbols, reducing PAPR and optimizing energy consumption while maintaining spectral mask compliance, by performing an Inverse Fast Fourier Transform (IFFT) on perturbed symbols for transmission through a power amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If PA is operated far from its ideal operated point to combat non-linearity, then linearity is improved, but power efficiency deteriorates
Solution Approach 1:
The patent applies digital pre-distortion to the OFDM symbols before transmission through the power amplifier. By pre-modifying the signal characteristics (amplitude and phase) in the digital domain, the signal is prepared in advance to compensate for the PA's nonlinear behavior, allowing the PA to operate at higher efficiency points while maintaining acceptable linearity performance.
Solution Approach 2:
The patent modifies signal parameters (amplitude scaling factors and phase rotation angles) through pre-distortion processing. By changing these parameters digitally before amplification, the system achieves a trade-off between PA operating point efficiency and output signal linearity, enabling operation closer to the PA's optimal efficiency region.
2Reliability
If large Input Back Off (IBO) is used to manage PAPR, then spectral mask compliance is improved, but power efficiency deteriorates
Solution Approach 1:
The patent applies pre-distortion processing to modify OFDM symbols before transmission, preliminarily adjusting their amplitude and phase characteristics. This allows reduction of the required Input Back Off while maintaining spectral mask compliance, thereby improving power amplifier efficiency without violating spectral regulations.
Solution Approach 2:
The patent converts the potentially harmful high PAPR characteristics of OFDM signals into a benefit by using pre-distortion to shape the signal in a controlled manner. The high PAPR property is utilized to enable operation at lower IBO settings while still meeting spectral mask requirements through intelligent signal preprocessing.
3Device complexity
If simple metric-based predistortion is used, then implementation complexity is reduced, but performance improvement is limited
Solution Approach 1:
The patent applies different pre-distortion strategies to different subsets of subcarriers based on their local characteristics and contributions to PAPR. By identifying and treating only the most problematic subcarriers (those with largest positive metrics) rather than all subcarriers uniformly, the system achieves good performance improvement with reduced computational complexity compared to full-spectrum pre-distortion.
Data Source
AI summary
It is provided a method designed, for OFDM-based communication systems, to slightly modify the signal to be emitted before the power amplifier input of the transmitter. The guiding idea is that the minimal IBO value for which the slightly modified signal still fulfills the standard requirements is inferior to the smallest IBO value for which the transmitted signal still meets the standard requirements in terms of EVM and spectral mask without implementing the proposed method.