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

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 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).

Note CFR can only be applied when using the LTE, NR, and Multi Carrier modulation schemes.
  • 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.

Note
  • 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.

Note A CFR filter is applied to each sub block. Sub Block parameters as specified in Multi-Carrier tab are considered for computations.

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.