Power Amplifier Crest Factor Reduction With Edge Smoothing
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Solution Overview
Problem
Power amplifier circuits face distortion issues with high Peak to Average Power Ratio (PAPR) signals, leading to inefficiency and undesirable signal characteristics such as out-of-band emissions, which conventional methods like dynamic gain reduction and hard clipping fail to adequately address without compromising efficiency or introducing signal peak regrowth.
Innovation Solution
A crest factor reduction system that clips high-PAPR signals to prevent peak level exceedance, employs edge smoothing using a moving average filter to reduce out-of-band emissions, and includes maximum operation to prevent signal regrowth, with optional iteration loops for enhanced smoothing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hard clipping is used to keep signal amplitude below threshold limit, then distortion is reduced, but out-of-band emission increases due to hard edges in the signal
Solution Approach 1:
The patent applies preliminary edge smoothing action before power amplification by detecting signal transitions exceeding a threshold and inserting delay elements (zero-value samples) at these transition points. This preprocessing removes hard edges that would cause out-of-band emissions while maintaining the signal's peak amplitude constraints, thus resolving the contradiction between distortion reduction and emission control.
Solution Approach 2:
The patent introduces an intermediary edge smoothing subsystem between the clipping stage and the power amplifier. This subsystem acts as a mediator that softens the abrupt transitions created by hard clipping without allowing signal peaks to exceed the threshold, thereby eliminating the harmful out-of-band emissions while preserving the benefits of clipping for distortion reduction.
2Object-generated harmful factors
If filtering is applied to smooth hard edges from hard clipping, then out-of-band emission is reduced, but signal peak regrowth occurs in excess of threshold limit
Solution Approach 1:
The patent performs preliminary edge smoothing by detecting transitions and inserting delay elements before the signal is amplified. This approach smooths edges to reduce emissions while maintaining peak constraints through a different mechanism (delay insertion rather than conventional filtering), thus avoiding the peak regrowth problem associated with traditional filtering methods.
Solution Approach 2:
The patent substitutes the mechanical filtering approach with a digital signal processing approach using delay element insertion. Instead of using filters that physically smooth the signal (which cause peak regrowth), the patent uses a digital method of inserting zero-value samples at transition points, achieving edge smoothing without the harmful side effects of conventional filtering.
3Reliability
If dynamic gain reduction is applied at power amplifier input, then signal power is kept within linear operating range, but power amplifier efficiency is appreciably reduced
Solution Approach 1:
The patent applies preliminary crest factor reduction through edge smoothing and delay insertion before the signal enters the power amplifier. By preprocessing the signal to remove peaks and smooth edges, the power amplifier can operate at higher gain levels within its linear region, maintaining both signal linearity and efficiency without requiring dynamic gain reduction.
Solution Approach 2:
The patent segments the signal processing into distinct stages: edge detection, delay element insertion at transition points, and subsequent power amplification. This segmentation allows the power amplifier to receive a pre-conditioned signal with reduced peak factors, enabling efficient operation while maintaining linearity, thus resolving the contradiction between these two performance metrics.
Data Source
AI summary
Techniques are described for crest factor reduction in power amplifier circuits. For example, crest factor reduction can keep the peak signal level of a signal for transmission to below a peak threshold level associated with a power amplifier in the transmission path. The signal is received by the crest factor reduction system and clipped in accordance with the peak threshold level. Edge smoothing is then applied to the clipped signal to reduce out-of-band emissions. The edge smoothing is implemented by a moving average filter, such as a time-domain box filter. In some embodiments, a maximum operation or minimum operation is used to prevent signal peak regrowth after the filtering. Some embodiments also include various iteration loops to further improve crest factor reduction.