Applying Crest Factor Reduction
- Updated2025-04-07
- 6 minute(s) read
Crest factor reduction (CFR) is a technique to reduce the peak to average power ratio (PAPR) of a waveform to a desired value. The CFR techniques can be used in conjunction with digital predistortion to improve the overall linearization of the predistorter and power amplifier system.
CFR Methods
RFWC supports two CFR methods namely Clipping and Peak Windowing.
Clipping by Level
- 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:
- 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
- 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.
- 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 with filter enabled to obtain waveform with desired PAPR.
NI also recommends to set the oversampling factor to 4x or more of sampling rate whenever CFR is enabled.
For more concept help refer to Crest Factor Reduction topic in 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.
CFR Configurations
The following controls are available in CFR tab.
CFR Apply: Specifies whether to apply crest factor reduction (CFR).
- CFR is applied per carrier set.
CFR Method: Specifies the method used to perform CFR when you select the CFR Apply checkbox. RFmx Waveform Creator supports two CFR methods, Clipping and Peak Windowing. The default value is Clipping.
Clipping
When the Clip Using control is set to Level, the I and Q values of a waveform are clipped.

With clipping enabled, the I and Q values will be clipped depending on the settings specified.
- 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.
There is an option to apply the clipping parameters before filtering has been applied, or after.
- Applying clip using level behavior is the same as applying the clipping function under IQ Impairments tab.
When the Clip Using control is set to Target PAPR, the signal is hard clipped such that the target PAPR is achieved.

Target PAPR: Specifies the target peak-to-average power ratio when you select the CFR Apply checkbox. This value is expressed in dB. The default value is 8.00 dB. Valid values are 1 dB to 100 dB.
Max Iterations: Specifies the maximum number of iterations allowed to converge waveform PAPR to target PAPR, when you select the CFR Apply checkbox. The default value is 10. Valid values are 1 to 1000.
Filter Enabled: Specifies whether to enable the filtering operation when you select the CFR Apply checkbox. The default value is Disabled.
When using the Multi Carrier modulation scheme, a CFR filter is applied to each input waveform (carrier). Frequency Offset and Bandwidth can also be specified for each carrier.
Peak
Windowing
Peak windowing scales the peaks in the signal using weighted window function to get smooth peaks and achieve the target PAPR.

Window Type: Specifies the window type to be used when you select the CFR Apply checkbox and set CFR Method control to Peak Windowing. The default value is Kaiser-Bessel Window. Select from the following options:
- Flat Top Window - Uses the flat top window function to scale peaks.
- Hanning Window - Uses the Hanning window function to scale peaks.
- Hamming Window - Uses the Hamming window function to scale peaks.
- Gaussian Window - Uses the Gaussian window function to scale peaks.
- Blackman Window - Uses the Blackman window function to scale peaks.
- Blackman Harris Window - Uses the Blackman-Harris window function to scale peaks.
- Kaiser Bessel Window - Uses the Kaiser Bessel window function to scale peaks.
Window Length: Specifies the maximum window length to be used when you select the CFR Apply checkbox and set CFR Method control to Peak Windowing. The default value is 10. Valid values are 1 to 1024.