Channel Visualization

Communications Toolbox™ software provides a plotting function that helps you visualize the characteristics of a fading channel using a GUI. See Fading Channels for a description of fading channels, objects, and blocks.

Select the desired visualization setting to plot the Impulse Response, Frequency Response, or Doppler Spectrum of the channel.

Impulse Response

The Impulse Response plot displays the path gains, the channel filter coefficients, and the interpolated path gains of the channel. The path gains shown in magenta occur at time instances that correspond to the specified path delays. These might not be aligned with the input sampling time. The channel filter coefficients shown in yellow are used to model the channel. They are interpolated from the actual path gains and are aligned with the input sampling time. When the path gains align with the sampling time, they overlap the filter coefficients. Sinc interpolation is used to generate the blue points that appear between the channel filter coefficients. These points are used solely for display purposes and not used in subsequent channel filtering. For a flat fading channel (one path), the sinc interpolation points are not displayed. For all impulse response plots, the frame and sample numbers appear in the upper left corner of the display. The Impulse Response plot shares the same toolbar and menus as the System object it was based on, dsp.ArrayPlot.

The figure shows the impulse response of a channel in which the path gains align with the sample time. The path gains and channel filter coefficients overlap.

The next plot shows when the specified path gains are not aligned with the sample rate. Observe that the path gains and the channel filter coefficients do not overlap and that the filter coefficients are equally distributed.

The impulse response for a frequency-flat channel is shown next. Because the channel is represented by a single coefficient, no interpolation is done, and the interpolated path gains do not appear.

Note

The displayed and specified path gain locations can differ by as much as 5% of the input sample time.
For MIMO, 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.
The visualization display speed is controlled by the combination of the samples to display property and the Playback > Reduce Updates to Improve Performance menu item. Reducing the percentage of samples to display and enabling reduced updates speeds up the rendering of the impulse response.
After the Impulse Response plots are manually closed, the channel model executes at its normal speed.
Code generation is available only when the visualization property is set to Off.

Frequency Response

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 Frequency Response plot shares the same toolbar and menus as the System object it was based on, dsp.SpectrumAnalyzer. The default settings use a rectangular window. The window length is set based on the channel model configuration. Use the View > Spectrum Settings menu to change property values from their default settings.

The frequency response plot for a frequency-selective channel is shown.

Note

The visualization display speed is controlled by the combination of the samples to display property and the Playback > Reduce Plot Rate to Improve Performance menu item. Reducing the percentage of samples to display and enabling reduced updates speeds up the rendering of the frequency response.
After the Frequency Response plots are manually closed, the channel model executes at its normal speed.
Code generation is available only when the visualization property is set to Off.

Doppler Spectrum

The Doppler Spectrum plot displays both the theoretical Doppler spectrum and the empirically determined data points. The theoretical data is displayed as a yellow line for nonstatic channels and as a yellow point for static channels. The empirical data is shown in blue. 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. For nonstatic channels, the number of input samples needed before the next update is displayed in the upper-left corner. The samples needed is a function of the sample rate and the maximum Doppler shift. The Doppler Spectrum plot shares the same toolbar and menus as the System object it was based on, dsp.ArrayPlot.

For static channels, the text Reset fading channel for next update is displayed.

Note

After the Doppler Spectrum plots are manually closed, the channel model executes at its normal speed.
Code generation is available only when the visualization property is Off.