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Apply memoryless nonlinearity to complex baseband signal

RF Impairments

The Memoryless Nonlinearity block applies a memoryless nonlinearity to a complex, baseband signal. You can use the block to model radio frequency (RF) impairments to a signal at the receiver.

This block accepts a column vector input signal.

All values of power assume a nominal impedance of 1 ohm.

The Memoryless Nonlinearity block provides five different
methods for modeling the nonlinearity, which you
specify by the **Method**
parameter. The options for the
**Method** parameter are

`Cubic polynomial`

`Hyperbolic tangent`

`Saleh model`

`Ghorbani model`

`Rapp model`

The block implements these five methods using subsystems underneath the block mask. For each of the first four methods, the nonlinearity subsystem has the same basic structure, as shown in the following figure.

**Nonlinearity Subsystem**

For the first four methods, each subsystem applies a nonlinearity to the input signal as follows:

Multiply the signal by a gain factor.

Split the complex signal into its magnitude and angle components.

Apply an AM/AM conversion to the magnitude of the signal, according to the selected

**Method**, to produce the magnitude of the output signal.Apply an AM/PM conversion to the phase of the signal, according to the selected

**Method**, and adds the result to the angle of the signal to produce the angle of the output signal.Combine the new magnitude and angle components into a complex signal and multiply the result by a gain factor, which is controlled by the

**Linear gain**parameter.

Each subsystem implements the AM/AM and AM/PM conversions differently, according to the Method you specify. The Rapp model does not apply a phase change to the input signal. The nonlinearity subsystem for Rapp model has following structure:

**Nonlinearity Subsystem for Rapp Model**

The Rapp Subsystem applies nonlinearity as follows:

Multiply the signal by a gain factor.

Split the complex signal into its magnitude and angle components.

Apply an AM/AM conversion to the magnitude of the signal, according to the selected

**Method**, to produce the magnitude of the output signal.Combine the new magnitude and angle components into a complex signal and multiply the result by a gain factor, which is controlled by the

**Linear gain**parameter.

If you want to see exactly how the Memoryless Nonlinearity block implements the conversions for a specific method, you can view the AM/AM and AM/PM subsystems that implement these conversions as follows:

Right-click on the Memoryless Nonlinearity block and select

**Mask**>**Look under mask**. This displays the block's configuration underneath the mask. The block contains five subsystems corresponding to the five nonlinearity methods.Double-click the subsystem for the method you are interested in. This displays the subsystem shown in the preceding figure, Nonlinearity Subsystem.

Double-click on one of the subsystems labeled AM/AM or AM/PM to view how the block implements the conversions.

The following illustration shows the AM/PM
behavior for the ```
Cubic
polynomial
```

and
`Hyperbolic tangent`

methods:

The AM/PM conversion scales linearly with
input power value between the lower and upper
limits of the input power level (specified by
**Lower input power limit for AM/PM
conversion (dBm)** and **Upper
input power limit for AM/PM conversion
(dBm)**). Beyond these values, AM/PM
conversion is constant at the values corresponding
to the lower and upper input power limits, which
are zero and $$(\text{AM/PMconversion})\cdot (\text{upperinputpowerlimit}-\text{lowerinputpowerlimit})$$, respectively.

The following figure shows, for the Saleh method, plots of

Output voltage against input voltage for the AM/AM conversion

Output phase against input voltage for the AM/PM conversion

You can see the effect of the Memoryless Nonlinearity block on a signal modulated by 16-ary quadrature amplitude modulation (QAM) in a scatter plot. The constellation for 16-ary QAM without the effect of the Memoryless Nonlinearity block is shown in the following figure:

You can generate a scatter plot of the same
signal after it passes through the Memoryless
Nonlinearity block, with the
**Method** parameter set to
`Saleh Mode`

l, as shown
in the following figure.

This plot is generated by the model described in Illustrate RF Impairments That Distort a Signal with the following parameter settings for the Rectangular QAM Modulator Baseband block:

**Normalization method**set to`Average Power`

**Average power (watts)**set to`1e-2`

The following sections discuss parameters specific to the Saleh, Ghorbani, and Rapp models.

The **Input scaling (dB)**
parameter scales the input signal before the
nonlinearity is applied. The block multiplies the
input signal by the parameter value, converted
from decibels to linear units. If you set the
parameter to be the inverse of the input signal
amplitude, the scaled signal has amplitude
normalized to 1.

The AM/AM parameters, alpha and beta, are used to compute the amplitude gain for an input signal using the following function:

$${F}_{AM/AM}(u)=\frac{\text{alpha}*u}{1+\text{beta}*{u}^{2}}$$

where *u* is the magnitude
of the scaled signal.

The AM/PM parameters, alpha and beta, are used to compute the phase change for an input signal using the following function:

$${F}_{AM/PM}(u)=\frac{\text{alpha}*{u}^{2}}{1+\text{beta}*{u}^{2}}$$

where *u* is the magnitude
of the scaled signal. Note that the AM/AM and
AM/PM parameters, although similarly named alpha
and beta, are distinct.

The **Output scaling (dB)**
parameter scales the output signal similarly.

The **Input scaling (dB)**
parameter scales the input signal before the
nonlinearity is applied. The block multiplies the
input signal by the parameter value, converted
from decibels to linear units. If you set the
parameter to be the inverse of the input signal
amplitude, the scaled signal has amplitude
normalized to 1.

The AM/AM parameters,
[x_{1}
x_{2}
x_{3}
x_{4}], are used to compute
the amplitude gain for an input signal using the
following function:

$${F}_{AM/AM}(u)=\frac{{x}_{1}{u}^{{x}_{2}}}{1+{x}_{3}{u}^{{x}_{2}}}+{x}_{4}u$$

where *u* is the magnitude
of the scaled signal.

The AM/PM parameters,
[y_{1}
y_{2}
y_{3}
y_{4}], are used to compute
the phase change for an input signal using the
following function:

$${F}_{AM/PM}(u)=\frac{{y}_{1}{u}^{{y}_{2}}}{1+{y}_{3}{u}^{{y}_{2}}}+{y}_{4}u$$

where *u* is the magnitude
of the scaled signal.

The **Output scaling (dB)
**parameter scales the output signal
similarly.

The **Linear gain (dB)**
parameter scales the input signal before the
nonlinearity is applied. The block multiplies the
input signal by the parameter value, converted
from decibels to linear units. If you set the
parameter to be the inverse of the input signal
amplitude, the scaled signal has amplitude
normalized to 1.

The **Smoothness factor** and
**Output saturation level**
parameters are used to compute the amplitude gain
for the input signal:

$${F}_{AM/AM}(u)=\frac{u}{{\left(1+{\left(\frac{u}{{O}_{sat}}\right)}^{2S}\right)}^{1/2S}}$$

where *u* is the magnitude
of the scaled signal, *S* is
the **Smoothness factor**, and
*O*_{sat}
is the **Output saturation
level**.

The Rapp model does not apply a phase change to the input signal.

The **Output saturation
level** parameter limits the output
signal level.

**Method**The nonlinearity method.

The following describes specific parameters for each method.

**Linear gain (db)**Scalar specifying the linear gain for the output function.

**IIP3 (dBm)**Scalar specifying the third order intercept.

**AM/PM conversion (degrees per dB)**Scaler specifying the AM/PM conversion in degrees per decibel.

**Lower input power limit (dBm)**Scalar specifying the minimum input power for which AM/PM conversion scales linearly with input power value. Below this value, the phase shift resulting from AM/PM conversion is zero.

**Upper input power limit (dBm)**Scalar specifying the maximum input power for which AM/PM conversion scales linearly with input power value. Above this value, the phase shift resulting from AM/PM conversion is constant. The value of this maximum shift is given by:

$$(\text{AM/PMconversion})\cdot (\text{upperinputpowerlimit}-\text{lowerinputpowerlimit})$$

**Linear gain (db)**Scalar specifying the linear gain for the output function.

**IIP3 (dBm)**Scalar specifying the third order intercept.

**AM/PM conversion (degrees per dB)**Scalar specifying the AM/PM conversion in degrees per decibel.

**Lower input power limit (dBm)**Scalar specifying the minimum input power for which AM/PM conversion scales linearly with input power value. Below this value, the phase shift resulting from AM/PM conversion is zero.

**Upper input power limit (dBm)**Scalar specifying the maximum input power for which AM/PM conversion scales linearly with input power value. Above this value, the phase shift resulting from AM/PM conversion is constant. The value of this maximum shift is given by:

$$(\text{AM/PMconversion})\cdot (\text{upperinputpowerlimit}-\text{lowerinputpowerlimit})$$

**Input scaling (dB)**Number that scales the input signal level.

**AM/AM parameters [alpha beta]**Vector specifying the AM/AM parameters.

**AM/PM parameters [alpha beta]**Vector specifying the AM/PM parameters.

**Output scaling (dB)**Number that scales the output signal level.

**Input scaling (dB)**Number that scales the input signal level.

**AM/AM parameters [x1 x2 x3 x4]**Vector specifying the AM/AM parameters.

**AM/PM parameters [y1 y2 y3 y4]**Vector specifying the AM/PM parameters.

**Output scaling (dB)**Number that scales the output signal level.

**Linear gain (db)**Scalar specifying the linear gain for the output function.

**Smoothness factor**Scalar specifying the smoothness factor

**Output saturation level**Scalar specifying the output saturation level.

[1] Saleh, A.A.M., "Frequency-independent and frequency-dependent nonlinear models of TWT amplifiers," IEEE Trans. Communications, vol. COM-29, pp.1715-1720, November 1981.

[2] A. Ghorbani, and M. Sheikhan, "The effect of Solid State Power Amplifiers (SSPAs) Nonlinearities on MPSK and M-QAM Signal Transmission", Sixth Int'l Conference on Digital Processing of Signals in Comm., 1991, pp. 193-197.

[3] C. Rapp, "Effects of HPA-Nonlinearity on a 4-DPSK/OFDM-Signal for a Digital Sound Broadcasting System", in Proceedings of the Second European Conference on Satellite Communications, Liege, Belgium, Oct. 22-24, 1991, pp. 179-184.