HERALD Lab #5

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Multipath Fading

HERALD Functions

Exercise 5.1 : Estimate multipath occurrence

Exercise 5.2 : Estimate multipath outage

Exercise 5.3 : Frequency selective multipath (Herald Professional only)

Exercise 5.4 : Design space diversity

Exercise 5.5 : Use frequency diversity (Herald Professional only)

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HERALD Functions

The following Herald functions are related to Multipath analysis :

· The Evaluate / Multipath command displays the "Multipath Occurrence Model" dialog, where the operator sets the parameters useful to estimate the Multipath Occurrence Factor P₀, according to the selected model (Note: if the "Multipath Outage" dialog is displayed, then click the "Revise Po estimate" button to return to the "Multipath Occurrence Model" dialog)..

· Using the Barnett-Vigants model, the required parameters are the climatic region (to be selected among the five regions proposed by that model) and the path roughness (if the path profile is defined, then the roughness is automatically computed). The model is also based on the path length and operating frequency (both are already defined at this design stage).

· (Herald Professional only) After definition of model parameters, the Multipath Occurrence Statistics diagram is displayed. Note that the statistics diagram is plotted, according to ITU-R Rec. P-530, with time percentages as the abscissa axis and fade depth as the ordinate axis. The low attenuation region (approximately, for fade depth lower than 15 dB) deviates from the 10 dB / decade slope, according to Rec. P-530 (sect. 2.3.2) formulas for "shallow-fading".

· Once the Multipath Occurrence Factor P₀ has been estimated, the Multipath Outage diagram is displayed; on the right of the diagram, the available Flat Fade Margin and the outage prediction (in time percentage and seconds in the worst month) are given.

· In order to define a space diversity receiver configuration, the Define / Active Hop Config command must be executed to display the "Hop Configuration" dialog, where the option for a diversity antenna can be set at one or both receiver sites. Space diversity is completed by selecting the antenna code(s) and the antenna spacing.

· Both the Multipath Occurrence Statistics and the Multipath Outage diagrams take account of space diversity, when configured.

· In the Hop Report (Configuration section) the "Environmental Parameters and Propagation Models" lines indicate the assumptions on Multipath analysis, while at the end of the Performance section the predicted outage (single Rx and diversity Rx, if configured) is reported.

Exercise 5.1: Estimate multipath occurrence

Run Herald and open the "Multipath1_Sample.hpf" project.

· Select the DELTA-ECHO hop (11 GHz, 48 km) and display the Hop Report. Check that no reference to multipath analysis is reported in the Configuration or in the Performance sections of the Hop Report.

· Execute the Evaluate / Multipath command.

· In the "Multipath Occurrence Model" dialog, you have to set the Barnett-Vigants model parameters. The profile roughness is already computed (10 m) from the terrain profile previously defined. Test how the selection of the Climatic Region affects the Occurrence Factor P₀, shown in the same dialog. Finally, select the "Continental temperate" region. Click the OK button.

· The Multipath Occurrence Statistics diagram is displayed. Note the deep fading region (10 dB / decade slope) and the shallow-fading region (low attenuation). Click the Exit button (right bottom corner of the screen).

· After initialization of multipath analysis, save the project as "Multipath2.hpf" (if your HERALD release does not support the "Save" function, don't worry; you'll find the "Multipath2_Sample.hpf" file in your Project examples).

· To test how the profile roughness affect the Multipath Occurrence Factor, delete the path profile (Define / Profile command, then delete all points). In the "Multipath Occurrence Model" dialog, you can now modify the Profile Roughness parameter (the acceptable range in the Barnett model is 6 to 42 m).

Exercise 5.2: Estimate multipath outage

Run Herald and open the " Multipath2.hpf" (or the "Multipath2_Sample.hpf") project, as revised in the previous exercise; select the DELTA-ECHO hop.

· Execute the Evaluate / Multipath command.

· The "Multipath Outage" dialog is displayed; do not check any checkbox nor set any plotting comparison.

· The Multipath Outage diagram is displayed; on the right you find a summary of the hop parameters, propagation model, and outage predictions. Note that only the "narrowband" or "non-selective" multipath model is applied in this example.

· Display the Hop Report and find results of multipath analysis at the end of the Performance section.

Exercise 5.3: Frequency selective multipath (Herald Professional only)

Go on with the project carried on in the previous exercise.

· Go to the "Hop Configuration" dialog to select the equipment with code "D/11000a" (signature data are defined). Then repeat the Outage estimation process, using the "frequency selective" multipath model.

· When you execute the Evaluate / Multipath command, in the Multipath Outage dialog, set the Plotting comparison checkbox "Narrow vs. Wide-band (Selective fading)".

· Comment on the results in the Multipath Outage diagram.

Tip : the outage prediction produced by the Narrowband model is a decreasing function of the Fade Margin. On the other hand, the prediction produced by the Wideband (frequency selective) model has a flat asymptote. This is due to the frequency selective outage component (the outage probability due to signal distortion).

Exercise 5.4: Design space diversity

Run Herald and open the "Multipath2_Sample.hpf" project, select the DELTA-ECHO hop. The predicted outage with single receiver and the Narrowband multipath model is very high (about 10 minutes/month) and probably not acceptable

· Display the "Hop Configuration" dialog (Define / Active Hop Config command) and configure the diversity antenna at both radio sites. Select the same antenna code, used as the main antenna, set the diversity spacing at 180 wavelengths (about 5m), as a tentative value.

· Execute the Evaluate / Multipath command and test the new outage predictions. More complete results are given in the Hop Report (comparison of outage predictions in single and diversity reception, diversity improvement.

· Execute the Evaluate / Multipath command and click the “Revise P₀ estimate” butto to return to the “Multipath occurrence Model” dialog; click OK to display the Multipath Occurrence Statistics diagram; note that the diversity curve slope (in deep fading region) is 5 dB / decade.

· Repeat the analysis with different antenna spacing and test how the diversity improvement varies.

Tip : set a different spacing at the two sites, then display the Hop Report, so you can immediately compare the diversity improvements (this is correct as far as the fade margin is the same at the two Rx sites).

Exercise 5.5: Use frequency diversity (Herald Professional only)

Run Herald and open the "Multipath2_Sample.hpf" project, select the DELTA-ECHO hop.

· Display the "Hop Configuration" dialog (Define / Active Hop Config command) and click the "Frequency Diversity / Define" button. Select the 1+1 frequency diversity configuration, with 80 MHz channel spacing (this is the co-channel RF spacing in the ITU 11 GHz channel arrangement).

· Close the open dialogs and execute the Evaluate / Multipath command to test the new outage predictions. Set the Plotting Comparison checkbox "With and W/o Frequency diversity"

· Repeat the exercise with a different channel spacing.

· Repeat the exercise with the Wideband (frequency selective) multipath model, as introduced in Exercise 5.3.

End of HERALD Lab #5

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