CFR Methods

NI-RFmx Waveform Creator 3.0 Help

Edition Date: January 2019

Part Number: 377064E-01

»View Product Info
Download Help (Windows Only)
RFWC supports two CFR methods namely Clipping and Peak Windowing.

Clipping by Level

This method allows you to clip the I and Q values of a waveform.
  • If the clipping type is Circle, the overall IQ Vector will be clipped to the percentage specified.
  • If the clipping type is Square, the I and Q values will be individually clipped to the percentage specified.

Clipping by Target PAPR

This method hard clips the signal to achieve the target PAPR. The following figure shows how the PAPR of a signal can be reduced by using clipping by Target PAPR.


Clipping operation can be expressed as:



where,

X[n] is the signal on which CFR is applied.

Xclipping[n] is the clipped signal.

A is the threshold level determined by target PAPR.

The clipping transfer function is represented by the following figure.

Peak Windowing

Peak Windowing is a CFR method that scales the peaks in signal using a weighted window function to get smooth peaks and achieve the target PAPR. This technique involves the following steps:

  1. Detect peaks above the threshold, A, determined by the target PAPR. Samples above the threshold, in the vicinity of the detected peaks are referred to as the peak region.
  2. Scale all the peak regions with a weighted window function such that the resultant peaks are equal to the threshold level, A. The window type and maximum window length are user configurable.

The following figure shows the input signal, the threshold level, and the signal obtained after applying the Peak Windowing method of CFR.

CFR with Filtering

CFR is a non-linear operation that introduces out-of-band distortion in the signal. This distortion can be filtered out after the CFR operation.

CFR with filter can be applied on the waveform by enabling filter. The block diagram below shows one iteration of CFR operation with filtering.

CFR applies filtering per sub Block. The diagram below shows an example of sub block configuration for a signal sampled at S samples per second.

Guidelines for Using CFR

 CFR method  Usage
 Clipping  Faster CFR

Trade-off ACP for EVM
 Peak Windowing  Trade Off between ACP and EVM

NI recommends the Clipping method of CFR, with filter enabled, to obtain waveform with desired PAPR.

For more concept help refer to Crest Factor Reduction topic in NI-RFmx SpecAn Help.

References

[1] Sperlich, Roland, et al. Power amplifier linearization with digital pre-distortion and crest factor reduction. Microwave Symposium Digest, 2004 IEEE MTT-S International. Vol. 2. IEEE, 2004.

[2] Braithwaite, R. Neil. A combined approach to digital predistortion and crest factor reduction for the linearization of an RF power amplifier. IEEE Transactions on Microwave Theory and Techniques 61.1 (2013): 291-302.

[3] Armstrong, Jean. Peak-to-average power reduction for OFDM by repeated clipping and frequency domain filtering. Electronics letters 38.5 (2002): 246-247.

WAS THIS ARTICLE HELPFUL?

Not Helpful