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Channel Visualization

These channel modeling System objects and blocks in Communications Toolbox™ include an option to visualize the characteristics of a fading channel.

You can use the channel visualization option to view the impulse response and frequency response individually or side-by-side in one plot window. You can also view the Doppler spectrum. For more information about fading channels and the features available to model them, see Fading Channels.

Note

  • In the plot window, the displayed and specified path gain locations can differ by as much as 5% of the input sample time.

  • For MIMO channels, when the antenna selection property is set to any value other than Off and the specified transmit-receive pair is not selected for the current frame transmission, nothing will be displayed. Antenna selection is not applicable for SISO channels.

  • After you close channel visualization plots, the channel model executes at its normal speed.

  • Code generation is available only when the visualization property is set to Off.

Impulse Response Plot

The impulse response plot displays the path gains and the channel filter coefficients. The path gains occur at time instances that correspond to the specified path delays and might not align with the input sampling time. The implementations interpolate the channel filter coefficients from the actual path gains, use the coefficients to model the channel, and align the coefficients with the input sampling time. When the path gains align with the sampling time, they overlap the channel filter coefficients.

This plot shows the impulse response for a Rayleigh fading channel configured with path gains that align with the sample timing. The path gains overlap with the channel filter coefficients, and the coefficients are equally distributed.

This plot shows the impulse response for a Rayleigh fading channel configured with path gains that do not align with the sample timing. The path gains do not overlap with the channel filter coefficients, but the coefficients are still equally distributed. The implementation oversamples the interpolation of the coefficients to faithfully reproduce the desired path gains.

The next plot shows the impulse response for a frequency-flat channel. A frequency-flat channel is represented by a single coefficient; no interpolation is required.

Frequency Response Plot

The frequency response plot displays the channel spectrum by taking a discrete Fourier transform of the channel filter coefficients. For the MIMO case, this transform is performed for the specified transmit-receive antenna pair. The default settings use a rectangular window. The window length is set according to the channel model configuration. The y-axis limits of the plot are computed based on the normalized and average path gain values.

This plot shows the frequency response plot for a frequency-selective channel.

Doppler Spectrum Plot

The Doppler spectrum plot displays both the theoretical Doppler spectrum and the empirically determined data points. When the internal buffer is completely filled with filtered Gaussian samples, the empirical plot is updated. The empirical plot is the running mean of the spectrum calculated from each full buffer. The samples needed before the next update is displayed is a function of the sample rate and the maximum Doppler shift.

This plot shows a channel with Jakes Doppler spectrum.

For static channels, the Doppler spectrum reduces to a single point. The bottom toolbar of the plot window displays the message Reset fading channel for next update.

See Also

Blocks

Objects