Contents

This model shows a satellite link, using the blocks from the Communications System Toolbox™ RF Impairments Library (red blocks) to simulate the following impairments:

  • Free space path loss

  • Receiver thermal noise

  • Memoryless nonlinearity

  • Phase noise

  • In-phase and quadrature imbalances

  • Phase/frequency offsets

By modeling the gains and losses on the link, this model implements link budget calculations that determine whether a downlink can be closed with a given bit error rate (BER). The gain and loss blocks, including the Free Space Path Loss block and the Receiver Thermal Noise block, determine the data rate that can be supported on the link in an additive white Gaussian noise channel.

Structure of the Example

The example highlights both the satellite link model and its signal scopes. The model consists of a Satellite Downlink Transmitter, Downlink Path, and Ground Station Downlink Receiver.

The blocks that correspond to each of these sections are

Satellite Downlink Transmitter

  • Random Integer Generator - Creates a random data stream.

  • Rectangular QAM Modulator Baseband - Maps the data stream to 16-QAM constellation.

  • Raised Cosine Transmit Filter - Upsamples and shapes the modulated signal using the square root raised cosine pulse shape.

  • Memoryless Nonlinearity (High Power Amplifier) - Model of a traveling wave tube amplifier (TWTA) using the Saleh model.

  • Gain (Tx. Dish Antenna Gain) - Gain of the transmitter parabolic dish antenna on the satellite.

Downlink Path

  • Free Space Path Loss (Downlink Path) - Attenuates the signal by the free space path loss.

  • Phase/Frequency Offset (Doppler and Phase Error) - Rotates the signal to model phase and Doppler error on the link.

Ground Station Downlink Receiver

  • Receiver Thermal Noise (Satellite Receiver System Temp) - Adds white Gaussian noise that represents the effective system temperature of the receiver.

  • Gain (Rx. Dish Antenna Gain) - Gain of the receiver parabolic dish antenna at the ground station.

  • Phase Noise - Introduces random phase perturbations that result from 1/f or phase flicker noise.

  • I/Q Imbalance - Introduces DC offset, amplitude imbalance, or phase imbalance to the signal.

  • DC Blocking - Estimates and removes the DC offset from the signal. Compensates for the DC offset in the I/Q Imbalance block.

  • Magnitude AGC I and Q AGC (Select AGC) - Automatic gain control Compensates the gain of both in-phase and quadrature components of the signal, either jointly or independently.

  • I/Q Imbalance Compensator - Estimates and removes I/Q imbalance from the signal by a blind adaptive algorithm.

  • Phase/Frequency Offset (Doppler and Phase Compensation) - Rotates the signal to represent correction of phase and Doppler error on the link. This block is a static block that simply corrects using the same values as the Phase/Frequency Offset block.

  • Raised Cosine Receive Filter - Applies a matched filter to the modulated signal using the square root raised cosine pulse shape.

  • Rectangular QAM Demodulator Baseband - Demaps the data stream from the 16-QAM constellation space.

Exploring the Example

Double-click the block labeled Model Parameters to view the parameter settings for the model. All these parameters are tunable. To make changes to the parameters and apply them as the model is running, update the model via ctrl+d. The parameters are

Satellite altitude (km) - Distance between the satellite and the ground station. Changing this parameter updates the Free Space Path Loss block. The default setting is 35600.

Frequency (MHz) - Carrier frequency of the link. Changing this parameter updates the Free Space Path Loss block. The default setting is 8000.

Transmit and receive antenna diameters (m) - The first element in the vector represents the transmit antenna diameter and is used to calculate the gain in the Tx Dish Antenna Gain block. The second element represents the receive antenna diameter and is used to calculate the gain in the Rx Dish Antenna Gain block. The default setting is [.4 .4].

Noise temperature (K) - Allows you to select from three effective receiver system noise temperatures. The selected noise temperature changes the Noise Temperature of the Receiver Thermal Noise block. The default setting is 0 K. The choices are

  • 0 (no noise) - Use this setting to view the other RF impairments without the perturbing effects of noise.

  • 20 (very low noise level) - Use this setting to view how easily a low level of noise can, when combined with other RF impairments, degrade the performance of the link.

  • 290 (typical noise level) - Use this setting to view how a typical quiet satellite receiver operates.

HPA backoff level - Allows you to select from three backoff levels. This parameter is used to determine how close the satellite high power amplifier is driven to saturation. The selected backoff is used to set the input and output gain of the Memoryless Nonlinearity block. The default setting is 30 dB (negligible nonlinearity). The choices are

  • 30 dB (negligible nonlinearity) - Sets the average input power to 30 decibels below the input power that causes amplifier saturation (that is, the point at which the gain curve becomes flat). This causes negligible AM-to-AM and AM-to-PM conversion. AM-to-AM conversion is an indication of how the amplitude nonlinearity varies with the signal magnitude. AM-to-PM conversion is a measure of how the phase nonlinearity varies with signal magnitude.

  • 7 dB (moderate nonlinearity) - Sets the average input power to 7 decibels below the input power that causes amplifier saturation. This causes moderate AM-to-AM and AM-to-PM conversion.

  • 1 dB (severe nonlinearity) - Sets the average input power to 1 decibel below the input power that causes amplifier saturation. This causes severe AM-to-AM and AM-to-PM conversion.

Phase correction - Allows you to select from three phase offset values to correct for the average AM-to-PM conversion in the High Power Amplifier. The selection updates the Phase/Frequency Offset (Doppler and Phase Compensation) block. The default setting is None. The choices are

  • None - No correction. Use to view uncorrected AM-to-PM conversion.

  • Correct for moderate HPA AM-to-PM - Corrects for average AM-to-PM distortion when the HPA backoff is set to 7 dB.

  • Correct for severe HPA AM-to-PM - Corrects for average AM-to-PM distortion when the HPA backoff is set to 1 dB.

Doppler error - Allows you to select from three values of Doppler on the link and the corresponding correction, if any. The selection updates the Phase/Frequency Offset (Doppler and Phase Error) and Phase/Frequency Offset (Doppler and Phase Compensation) blocks. The default setting is None. The choices are

  • None - No Doppler on the link and no correction.

  • Doppler (0.7 Hz - uncorrected) - Adds 0.7 Hz Doppler with no correction at the receiver.

  • Doppler (3 Hz - corrected) - Adds 3 Hz Doppler with the corresponding correction at the receiver, -3 Hz.

Phase noise - Allows you to select from three values of phase noise at the receiver. The selection updates the Phase Noise block. The default setting is Negligible (-100 dBc/Hz @ 100 Hz). The choices are

  • Negligible (-100 dBc/Hz @ 100 Hz) - Almost no phase noise.

  • Low (-55 dBc/Hz @ 100 Hz) - Enough phase noise to be visible in both the spectral and I/Q domains, and cause additional errors when combined with thermal noise or other RF impairments.

  • High (-48 dBc/Hz @ 100 Hz) - Enough phase noise to cause errors without the addition of thermal noise or other RF impairments.

I/Q imbalance - Allows you to select from five types of in-phase and quadrature imbalances at the receiver. The selection updates the I/Q Imbalance block. The default setting is None. The choices are

  • None - No imbalances.

  • Amplitude imbalance (3 dB) - Applies a 1.5 dB gain to the in-phase signal and a -1.5 dB gain to the quadrature signal.

  • Phase imbalance (20 deg) - Rotates the in-phase signal by 10 degrees and the quadrature signal by -10 degrees.

  • In-phase DC offset (2e-6) - Adds a DC offset of 2e-6 to the in-phase signal amplitude. This offset changes the received signal scatter plot, but does not cause errors on the link unless combined with thermal noise or other RF impairments.

  • Quadrature DC offset (1e-5) - Adds a DC offset of 1e-5 to the quadrature signal amplitude. This offset causes errors on the link even when not combined with thermal noise or another RF impairment. This offset also causes a DC spike in the received signal spectrum.

I/Q imbalance compensator - Allows you to adjust the adaptation step size to control the convergence speed. The estimated compensator coefficient can be obtained by an optional output port.

DC offset compensation - Allows you to enable or disable the DC Blocking block. The default setting is Disabled.

AGC type - Allows you to select the automatic gain control for the link. The selection updates the Select AGC block, which is labeled Magnitude AGC or I and Q AGC, depending on whether you select Magnitude only or Independent I and Q, respectively. The default setting is Magnitude only.

  • Magnitude only - Compensates the gain of both in-phase and quadrature components of the signal by estimating only the magnitude of the signal.

  • Independent I and Q - Compensates the gain of the in-phase signal using an estimate of the in-phase signal magnitude and the quadrature component using an estimate of the quadrature signal magnitude.

Results and Displays

When you run this model, the following displays are active:

Bit error rate (BER) display - In the lower right corner of the model is a display of the BER of the model. The BER computation is reset every 5000 symbols to allow you to view the impact of the parameter changes as the model is running (ctrl+d is required to apply the changes).

Power Spectrum - Double-clicking this Open Scopes block enables you to view the spectrum of the modulated/filtered signal (blue) and the received signal before demodulation (red).

Comparing the two spectra allows you to view the effect of the following RF impairments:

  • Spectral regrowth due to HPA nonlinearities caused by the Memoryless Nonlinearity block

  • Thermal noise caused by the Receiver Thermal Noise block

  • Phase flicker (that is, 1/f noise) caused by the Phase Noise block

End to End Constellation - Double-clicking this Open Scopes block enables you to view the scatter plots of the signal after QAM modulation (red) and before QAM demodulation (yellow). Comparing these scatter plots allows you to view the impact of all the RF impairments on the received signal and the effectiveness of the compensations.

Constellation Before and After HPA - Double-clicking this Open Scopes block enables you to view the constellation before and after the HPA. Comparing these plots allows you to view the effect that the nonlinear HPA behavior has on the signal.

Experimenting with the Example

This section describes some ways that you can change the model parameters in order to experiment with the effects of the blocks from the RF Impairments library and other blocks in the model. You can double-click the block labeled "Model Parameters" in the model and try some of the following scenarios:

Link gains and losses - Change Noise temperature to 290 (typical noise level) or to 20 (very low noise level). Change the value of the Satellite altitude (km) or Satellite frequency (MHz) parameters to change the free space path loss. In addition, increase or decrease the Transmit and receive antenna diameters (m) parameter to increase or decrease the received signal power. You can view the changes in the received constellation in the received signal scatter plot scope and the changes in received power in the spectrum analyzer. In cases where the change in signal power is large (greater than 10 dB), the AGC filter causes the received power (after the AGC) to oscillate before settling to the final value.

Raised cosine pulse shaping - Make sure Noise temperature is set to 0 (no noise). Turn on the Constellation Before and After HPA scopes. Observe that the square-root raised cosine filtering results in intersymbol interference (ISI). This results in the points' being scattered loosely around ideal constellation points, which you can see in the After HPA scatter plot. The square-root raised cosine filter in the receiver, in conjunction with the transmit filter, controls the ISI, which you can see in the received signal scatter plot.

HPA AM-to-AM conversion and AM-to-PM conversion - Change the HPA backoff level parameter to 7 dB (moderate nonlinearity) and observe the AM-to-AM and AM-to-PM conversions by comparing the Transmit RRC filtered signal scatter plot with the RRC signal after HPA scatter plot. Note how the AM-to-AM conversion varies according to the different signal amplitudes. You can also view the effect of this conversion on the received signal in the received signal scatter plot. In addition, you can observe the spectral regrowth in the received signal spectrum analyzer. You can view the AM-to-PM conversion compensation in the receiver by setting the Phase correction parameter to Correct for moderate HPA AM-to-PM. You can also view the phase change in the received signal in the received signal scatter plot scope.

Change the HPA backoff level parameter to 1 dB (severe nonlinearity) and observe from the scopes that the AM-to-AM and AM-to-PM conversion and spectral regrowth have increased. You can view the AM-to-PM conversion to compensate in the receiver by setting the Phase correction to Correct for severe HPA AM-to-PM. You can view the phase change in the received signal scatter plot scope.

Phase noise plus AM-to-AM conversion - Set the Phase Noise parameter to High and observe the increased variance in the tangential direction in the received signal scatter plot. Also note that this level of phase noise is sufficient to cause errors in an otherwise error-free channel. Set the Phase Noise to Low and observe that the variance in the tangential direction has decreased somewhat. Also note that this level of phase noise is not sufficient to cause errors. Now, set the HPA backoff level parameter to 7dB (moderate nonlinearity) and the Phase correction to Correct for moderate HPA AM-to-PM conversion. Note that even though the corrected, moderate HPA nonlinearity and the moderate phase noise do not cause bit errors when applied individually, they do cause bit errors when applied together.

DC offset and DC offset compensation - Set the I/Q Imbalance parameter to In-phase DC offset (2e-6) and view the shift of the constellation in the received signal scatter plot. Set DC offset compensation to Enabled and view the received signal scatter plot to view how the DC offset block estimates the DC offset value and removes it from the signal. Set DC offset compensation to Disabled and change I/Q imbalance to Quadrature DC offset (1e-5). View the changes in the received signal scatter plot for a large DC offset and the DC spike in the received signal spectrum. Set DC offset compensation to Enabled and view the received signal scatter plot and spectrum analyzer to see how the DC component is removed.

Amplitude imbalance and AGC type - Set the I/Q Imbalance parameter to Amplitude imbalance (3 dB) to view the effect of unbalanced I and Q gains in the received signal scatter plot. Set the AGC type parameter to Independent I and Q to show how the independent I and Q AGC compensate for the amplitude imbalance.

Doppler and Doppler compensation - Set Doppler error to 0.7 Hz (uncorrected) to show the effect of uncorrected Doppler on the received signal scatter plot. Set the Doppler error to 3 Hz corrected to show the effect of correcting the Doppler on a link. Without changing the Doppler error setting, repeat the following scenarios to view the effects that occur when DC offset and amplitude imbalances occur in circuits that do not have a constant phase reference:

  • DC offset and DC offset compensation

  • Amplitude imbalance and AGC type

Selected Bibliography

[1] Saleh, Adel A.M., "Frequency-Independent and Frequency-Dependent Nonlinear Models of TWT Amplifiers," IEEE® Transactions on Communications, Vol. COM-29, No. 11, November 1981.

[2] Kasdin, N.J., "Discrete Simulation of Colored Noise and Stochastic Processes and 1/(f^alpha); Power Law Noise Generation," The Proceedings of the IEEE, Vol. 83, No. 5, May, 1995.

[3] Kasdin, N. Jeremy, and Todd Walter, "Discrete Simulation of Power Law Noise," 1992 IEEE Frequency Control Symposium.

[4] Sklar, Bernard, Digital Communications: Fundamentals and Applications, Englewood Cliffs, N.J., Prentice Hall, 1988.

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