Applying IQ Impairments

RFmx Waveform Creator allows the user to distort waveform types by adding impairments to them. If you wish to do this, select the IQ Impairments tab. This prompts the screen as shown in the following image.

There are three types of impairments that can be applied to a waveform:

  • IQ errors
  • Noise
  • Clipping

Adding IQ Errors

This section of the IQ Impairments tab allows you to add errors to a waveform. Select the Apply IQ Errors checkbox to enable the parameters.

There are four types of IQ error that you can add. These are as follows:

Skew

An ideal base band modulation scheme relies on the in phase (I) and quadrature (Q) components to be orthogonal. In practice this may not be the case. Skew is offset angle φ that forces the I and Q components of signal to become non–orthogonal. Refer to the following figure.

The in-phase component (I) axis remains unchanged while the quadrature component (Q) is rotated by φ degrees. Mathematically, this is described as follows:

where I' and Q' are the modified I and Q values with a skew of φ degrees applied. It is assumed that the amplitude of the axes remains unchanged.

Carrier Leak

Carrier leak after IQ modulation occurs if there is a DC component on either the I or Q channel. RFmx Waveform Creator allows you to add a DC value to either channel. The amount of DC added is defined as a percentage of the rms vector. The figure below depicts this, where r is the rms value voltage of the signal, ki and kq are the percentages of DC offsets to be added to the I and Q channels respectively.

Mathematically the amount of DC added to either the I or Q channel is given by

where

and N is the number of IQ sample pairs. Note that it is possible to have negative DC offsets.

IQ Gain Imbalance

IQ gain imbalance results when the I and Q channels have a different gains applied to them. The result of this distortion can be seen below. This figure shows a vector plot of an MSK signal that has had a gain imbalance of -5 dB applied. An ideal plot would have been a circle. In RFmx Waveform Creator, the gain imbalance is set with reference to the I channel. This implies that a positive gain imbalance gives an I gain that is greater than the Q gain.

Adding Noise

This section of the IQ Impairments tab provides the facility to inject additive white Gaussian noise (AWGN) into the waveform. To enable SNR, set the State to On.

You have to set the SNR parameter in dB.

IQ Clipping

This section of the IQ Impairments tab allows you to clip the I and Q of a waveform. Select the Apply Clipping checkbox to enable the parameters.

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  
  • Clipping is not enabled for all modulations. For example, clipping is not enabled for the Generic modulation.
  • For LTE, NR, and multi carrier plugin, clipping is available in CFR tab. In order to apply clipping, set CFR method to Clipping and Clip Using to Level.