Time Domain Transmitter Signal Shaping for OFDM Power Amplifiers
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Solution Overview
Problem
Conventional spectral shaping techniques for modulated signals, such as OFDM, face challenges in independently adjusting out-of-band emissions and in-band error vector magnitude (EVM) while limiting complexity and iterations, leading to suboptimal power amplifier output due to distortion and spectral mask violations.
Innovation Solution
The proposed solution involves a spectral shaping method that uses a combination of nonlinear mapping, polar clipping, and noise shaping filters to redistribute spectral errors, allowing independent adjustments of out-of-band emissions and in-band EVM, with digital pre-distortion to compensate for power amplifier nonlinearity, thereby meeting spectral mask and EVM requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If power amplifier power is increased, then output power is improved, but signal errors and distortions grow across the frequency spectrum causing EVM degradation and out-of-band emissions
Solution Approach 1:
The patent applies spectral shaping and digital pre-distortion techniques before the signal enters the power amplifier. By pre-shaping the spectrum to predictably reduce out-of-band emissions and pre-distorting the signal to compensate for amplifier nonlinearity, the system prepares the signal in advance to withstand high-power amplification without generating excessive errors or violations, thus enabling higher output power while maintaining signal quality.
2Shape
If conventional frequency domain spectral shaping techniques are used, then frequency spectrum shaping is achieved, but symbol boundary effects occur that degrade performance and numerous iterations are required
Solution Approach 1:
The patent replaces conventional iterative frequency domain spectral shaping techniques with a time domain approach using pulse shaping filters. This substitution eliminates the need for repeated frequency domain transformations and iterations, avoiding symbol boundary effects entirely. The time domain method directly shapes the spectrum through filter design without requiring multiple passes through the signal processing chain, thereby improving computational efficiency and signal quality.
3Shape
If polar clipping and filtering techniques are used, then spectral shaping is achieved, but independent adjustment of out-of-band emissions and in-band EVM is not possible
Solution Approach 1:
The patent segments the spectral shaping function into distinct controllable components: pulse shaping filters that control out-of-band emissions and digital pre-distortion that manages in-band EVM. This segmentation allows independent optimization and adjustment of each parameter. The pulse shaping filter parameters can be tuned specifically for spectral containment while pre-distortion coefficients are optimized separately for linearization, enabling independent control of out-of-band emissions and in-band EVM performance.
4Device complexity
If spectral shaping is not applied, then signal processing complexity is reduced, but uncontrolled errors in the frequency spectrum limit achievable output power and cause mask violations
Solution Approach 1:
The patent modifies key signal parameters through pulse shaping filter design and digital pre-distortion coefficient optimization. By changing the temporal characteristics of the signal through these parameters, the spectral properties are improved without requiring complex iterative processing. The filter order, tap weights, and pre-distortion coefficients are optimized to achieve the required spectral mask compliance and EVM performance, enabling higher output power with manageable processing complexity.
Data Source
AI summary
High peak-to-average ratio of OFDM signals requires large back-off from an RF power amplifier's saturation power. A spectral shaper device therefore increases the output power and efficiency of the power amplifier. The shaper device performs linearization through digital predistortion, based on an out-of-band regrowth limit, as well as the EVM requirement for a particular data rate. The shaper can distribute the error energy, precisely, over frequencies such that each of the inband and out-of-band requirements is independently and individually met. The shaper distributes error energy to frequency regions in the spectrum to the maximally allowed by the standards and regulations, while not increasing the total error. The error energy is kept to the minimum where it is crucial in meeting EVM requirements. In this way, the shaper maximizes the allowable output power of the nonlinear power amplifier.