HERALD Lab #6

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Rain Attenuation

HERALD Functions

Exercise 6.1 : Predict rain unavailability

Exercise 6.2 : Optimize hop design

Exercise 6.3 : Include atmospheric absorption

Exercise 6.4 : Use Freq. / Pol. Scaling (Herald Professional only)

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HERALD Functions

The following Herald functions are related to Rain attenuation :

· The Evaluate / Rain command displays the "Rain Model" dialog, where the operator sets the parameters required to implement the ITU-R rain attenuation model, based on rain intensity statistics.

· The main model parameter is the rain intensity for 0.01% of the time: it can be set directly by the operator; or the ITU-R rain region can be selected (the rain intensity is automatically set), or the ITU-R rain database can be accessed, on the basis of hop coordinates.

· The hop coordinates are also required if the most recent version of Rec. 530 is applied, in order to specify if the hop operates in the temperate or in the equatorial/tropical zone (latitude lower than 30°, North or South).

· Other parameters are he Rx signal polarization at each receiver site and the wet radome loss (if any).

· The Rain Unavailability diagram shows the time percentage as a function of the rain attenuation, both for horizontal and vertical polarizations; on the right, the predicted unavailability is reported, taking account of the specified polarization at each Rx site and of the wet radome loss.

· The Hop Report, at the Environmental Parameters & Propagation Models lines (Configuration section), indicates the assumptions made about the rain attenuation model. At the end of the Performance section, the rain unavailability predictions are reported.

Exercise 6.1 : Predict rain unavailability

Run Herald and open the "Rain1_Sample.hpf" project, received with the Course material.

· Select the BETA-DELTA hop (38 GHz, 2.5 km) and display the Hop Report

· Execute the Evaluate / Rain command.

Tip : Click the HERALD icon

· In the "Rain Model" dialog, set the model parameters; to set rain intensity, you can :

(a) enter directly the desired value,

(b) select a rain region (ITU-R Rec. 837-2) ,

· The Rain Unavailability diagram is displayed. Note the unavailability prediction for each Rx site, on the right, and the significant difference between the two curves for H-pol. and V-pol.

. · Return to the "Rain Model" dialog, to modify the model parameters, and display again the Rain Unavailability diagram.

. · Check the results reported in the Hop Report (model parameters in the Configuration section and predicted unavailability in the Performance section).

Exercise 6.2 : Optimize hop design

Run Herald and open the "Rain1_Sample.hpf" project; select the ECHO-DELTA (22 GHz, 6 km) hop.

· In the Hop Configuration dialog, note that the antenna are not selected yet. Make a tentative selection, then display the Hop Report and take note of the available Fade Margin.

· Execute the Evaluate / Rain command. In the "Rain Model" dialog, set the "L" rain region (rain intensity 60 mm/h) and the H-pol. at both sites.

· Examine the Unavailability results. Modify antenna selection (return to Hop Configuration), if necessary, to get a rain unavailability objective below 30 min/year.

· Revise the previous design to the objective of minimum cost antennas, with accepted rain unavailability up to 45 min/year.

Exercise 6.3 : Include atmospheric absorption

Go on with the "Rain1_Sample.hpf" project, as updated in the previous exercise. Select the ECHO-DELTA (22 GHz, 6 km) hop.

· Estimate the water vapour specific attenuation at about 22 GHz, for 60% relative vapour density at about 15 °C.

· Display the "Hop Configuration" dialog and click the "Propagation Losses / Define" button. In the "Various Losses" dialog, set the Atmospheric Absorption at the value computed above.

· Close the "Hop Configuration" dialog and check the Fade Margin (reduced by the additional loss).

· Revise the rain unavailability predictions with the reduced fade margin.

Exercise 6.4 : Use Freq./Pol. Scaling (Herald Professional only)

Run Herald and open the "Rain1_Sample.hpf" project; select the BETA-DELTA (38 GHz, 2.5 km) hop.

· Execute the Customize / Propagation Models command. In the "Propagation Models" dialog, set the Rain Attenuation model (upper-left frame) to "Frequency / Polarization Scaling"

· Execute the Evaluate / Rain command. Set required parameters in the "Rain Model" dialog:

- Reference frequency = 23 GHz;

- Reference polarization = H;

- Reference attenuation (0.01%) = 15 dB

· Examine the Unavailability results. Note that the 0.01% attenuation (15 dB at the reference frequency 23 GHz, H-pol.) is about 30 dB at the operating frequency 38 GHz, same pol.

· Return to the Rain Intensity (Rec. 530-15) model (Customize / Propagation Model command), then repeat the Rain Unavailability prediction process. Set Rain Intensity = 55 mm/h and display the Rain Unavailability diagram. Note that the 0.01% attenuation is again 30 dB.

This means that the "Frequency Scaling" model (as applied with the above data at 23 GHz) is equivalent to the "Rain Intensity" model with a 0.01% rain intensity = 55 mm/h.

End of HERALD Lab #6

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