Multi-band Crest Factor Reduction via Carrier Repositioning
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Solution Overview
Problem
Existing signal transmission systems face challenges in reducing the peak-to-average power ratio of multi-carrier signals, particularly with widely-separated carriers, which requires high sampling speeds that are impractical with current hardware, and existing crest factor reduction methods introduce unwanted emissions and clip distortion.
Innovation Solution
A method and apparatus for crest factor reduction in multi-carrier signals involve positioning carriers at a specific frequency separation, combining them, clipping the composite signal, repositioning, and filtering the distortion, allowing for lower sampling rates and reduced unwanted emissions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If hard clipping is used to reduce peak power, then crest factor is reduced, but unwanted emissions and clip distortion increase
Solution Approach 1:
The patent segments the multi-carrier signal processing into distinct stages: positioning carriers at specific frequency separations, combining them, clipping the composite signal, and then separating/repositioning them. This segmentation allows clipping to be applied to the combined signal at controlled frequency separations, reducing unwanted emissions compared to traditional hard clipping approaches
Solution Approach 2:
The patent applies preliminary positioning of carriers at specific frequency separations before clipping occurs. By pre-positioning carriers at frequencies that are integer multiples of a base frequency, the clipping operation can be performed more effectively, reducing spectral regrowth and unwanted emissions outside the intended spectrum
2Area of stationary object
If widely-separated carriers are transmitted simultaneously, then frequency bandwidth is increased, but sampling speed requirements become impractical
Solution Approach 1:
The patent divides the widely-separated carrier signal into multiple sub-carriers that are processed individually at lower sampling rates. Each sub-carrier is positioned at a specific frequency and processed separately, then combined after clipping. This segmentation allows the system to handle wide frequency bandwidth without requiring impractically high sampling speeds for the entire bandwidth simultaneously
Solution Approach 2:
The patent transforms the time-domain processing problem into a frequency-domain solution by positioning carriers at specific frequency separations (integer multiples of base frequency). This dimensional transformation from time-domain clipping to frequency-positioned clipping enables practical sampling rates while maintaining effective crest factor reduction
3Reliability
If power is reduced to prevent amplifier breakdown, then amplifier reliability is improved, but radio equipment efficiency decreases
Solution Approach 1:
The patent applies preliminary crest factor reduction through clipping of the combined multi-carrier signal before amplification. By reducing the peak power levels in advance through digital clipping operations on the positioned and combined carriers, the amplifier operates within its linear region more consistently, improving reliability without significantly reducing average output power and maintaining efficiency
Data Source
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AI summary
Clipping a widely-separated,multi-carrier signal is effectively performed without having to use a high sampling speed. Clipping is performed in a first stage at a combined signal level, but with a predetermined carrier separation of at least twice the channel bandwidth (2CBW), followed by repositioning the carriers at baseband zero frequency. After clipping, carriers are placed at their respective center frequencies with full carrier separation reintroduced in a second stage. Iterative clipping stages smooth out signal reshaping and re-settled amplitudes for combined carriers.