Communications Blockset™

I/Q Imbalance - Create complex baseband model of signal impairments caused by imbalances between in-phase and quadrature receiver components

Library

RF Impairments

Description

The I/Q Imbalance block creates a complex baseband model of the signal impairments caused by imbalances between in-phase and quadrature receiver components. Typically, these are caused by differences in the physical channels for the two components of the signal.

The I/Q Imbalance block applies amplitude and phase imbalances to the in-phase and quadrature components of the input signal, and then combines the results into a complex signal. The block

1. Separates the signal into its in-phase and quadrature components.

2. Applies amplitude and phase imbalances, specified by the I/Q amplitude imbalance (dB) and I/Q phase imbalance (deg) parameters, respectively, to both components.

3. Combines the in-phase and quadrature components into a complex signal.

4. Applies an in-phase dc offset, specified by the I dc offset parameter, and a quadrature offset, specified by the Q dc offset parameter, to the signal.

The block performs these operations in the subsystem shown in the following diagram, which you can view by right-clicking the block and selecting Look under mask:

Let

I_a = I/Q amplitude imbalance

I_p = I/Q phase imbalance

I_DC = inphase DC offset

Q_DC = quadrature DC offset

Also let x = x_r + x_i be the complex input to the block, with x_r and x_i being the real and imaginary parts, respectively, of x. Let y be the complex output of the block.

Then, for an I/Q amplitude imbalance, I_a

y _{AmplitudeImbalance} =
y _{rAmplitudeImbalance} + j*y_{iAmplitudeImbalance}

For an I/Q phase imbalance I_p

y_{PhaseImbalance} =
y_{rPhaseImbalance}+ j * y_{iPhaseImbalance}

For DC offsets I_DC and Q_DC

y_{PhaseImbalance} = y_{r PhaseImbalance} + j*y_{i PhaseImbalance}

y = y_{r PhaseImbalance} + I_DC +j *y_{iPhaseImbalance} + Q_DC

The value of the I/Q amplitude imbalance (dB) parameter is divided between the in-phase and quadrature components:

• If you enter a positive value X for the I/Q amplitude imbalance (dB), the block applies a gain of +X/2 dB to the in-phase component and a gain of -X/2 dB to the quadrature component.

• If you enter a negative value X for the I/Q amplitude imbalance (dB), the block applies a gain of -X/2 dB to the in-phase component and a gain of +X/2 dB to the quadrature component.

The effects of changing the block's parameters are illustrated by the following scatter plots of a signal modulated by 16-ary quadrature amplitude modulation (QAM) with an average power of 0.01 watts. The usual 16-ary QAM constellation without distortion is shown in the first scatter plot:

The following figure shows a scatter plot of an output signal, modulated by 16-ary QAM, from the I/Q block with I/Q amplitude imbalance (dB) set to 8 and all other parameters set to 0:

Observe that the scatter plot is stretched horizontally and compressed vertically compared to the undistorted constellation.

If you set IQ phase imbalance (deg) to 30 and all other parameters to 0, the scatter plot is skewed clockwise by 30 degrees, as shown below:

Setting the I dc offset to 0.02 and the Q dc offset to 0.04 shifts the constellation 0.02 to the right and 0.04 up, as shown below:

See Scatter Plot Examples for a description of the model that generates this plot.

Dialog Box

I/Q amplitude imbalance (dB)

Scalar specifying the I/Q amplitude imbalance in decibels.

I/Q phase imbalance (deg)

Scalar specifying the I/Q phase imbalance in degrees.

I dc offset

Scalar specifying the in-phase dc offset.

Q dc offset

Scalar specifying the amplitude dc offset.

See Also

Memoryless Nonlinearity

Interlacer

Kasami Sequence Generator