TAP Microwave Link Budget Program
OBJECTIVE: To understand the basic operation of the TAP Microwave Link Budget program, including the creation of microwave facilities and the calculation of fade margin and reliability.
In this tutorial, you will select two microwave facilities and calculate a link budget analysis for the path between these two sites. All path calculations assume a clear line of sight path, including adequate clearance for the pertinent Fresnel zone(s) at the frequency under study. Any obstructions or diffraction effects are not considered by this program.
Partially obstructed paths can be examined by including the grazing or diffraction loss from the obstruction in one of the user-specified loss values in the Path Specifications tab folder of the link budget program. (SoftWright’s Bullington propagation program computes path obstruction loss based on Bullington, 1977). Note that unobstructed paths are always to be preferred. The outage times and percent reliability computed with this program for obstructed paths may not be dependable.
TAP will calculate a thermal fade margin based on the temperature, humidity, terrain and other factors described in this tutorial. TAP will also compute a composite fade margin based on thermal fade margin and other fade margins such as dispersive, adjacent channel interference, external interference, etc. These additional fade margins are particularly relevant in digital systems operating at 90 megabits per second and above. A value of zero (0) for each of these fade margins indicates that these fields will not be included in the fade margin calculations, and only the thermal fade margin will be computed. For more information about these composite fade margin calculations, see your TAP technical reference manual.
TAP will calculate a diversity improvement factor representing the improvement in the overall reliability of a microwave system that has incorporated space or frequency diversity in its design. Use space or diversity frequency as countermeasure against multipath fading due to cancellation in microwave systems. Incorporate space diversity by specifying a diversity receiver antenna center of radiation that differs from the antenna center of radiation of the primary receive antenna. Incorporate diversity frequency in your study by specifying a nonzero diversity frequency value for your transmitter facility record. The effect of space and frequency diversity will be demonstrated in this tutorial.
From the main menu of TAP, use your left mouse button to click on the Path menu, then on the Microwave Link Budget option to display the TAP Microwave Path Budget Study screen pictured below.
Your screen may or may not display previously entered site descriptions in the Sites tab depending on whether you have run any of the Path programs prior to this time. You will use TAP’s fixed facility lookup program to access two previously defined microwave sites.
As you can see, the default coordinates for the transmitter and receiver sites are identical. We will use the Fixed Facility Lookup button in BOTH the transmitter and receiver frames in order to access the facility lookup screen where you can view, edit and create new fixed facility data base records for use with this and other TAP programs (see the Fixed Facility Data Base tutorial for more information). When you close the lookup screen, site parameters stored in the current data base record will appear here in the Sites tab. Recording site parameters in the facility data base allows you to access site information from a centralized data base thereby reducing your data entry time when you use the same site in another TAP program.
We will use the Fixed Facility Lookup buttons to select transmitter and receiver microwave facilities from existing tutorial records in the fixed facility data base. Press the Fixed Facility Lookup button in the TRANSMITTER frame first.
Your Fixed Facility Lookup program may display the Blue Mountain facility as ours does below. This depends upon the extent to which you have added and deleted fixed facility records prior to running this tutorial. We will navigate to the transmitter record used in this tutorial.
Press the Find button at the bottom of the lookup screen.
Use your left mouse to pull down a list of facility descriptions in the Record Selection List. Select the Red Mountain Microwave Transmitter record with your left mouse as we have done. Then press the Select button to reposition the lookup screen to this record.
After confirming that the Red Mountain site is displayed in the facility lookup program, press Close to close the Find program. Your lookup should appear like ours below. Note that Red Mountain has a 13 GHz transmitter operating with a 68.78 dBm effective radiated power. Note also that this facility and the receiver facility that we will select in just a moment have predefined gains, losses and other fields set up for purposes of this tutorial.
Press the Close button in the lookup to transfer the Red Mountain site back to path budget screen pictured below. Now we will select a receiver facility.
Press the Fixed Facility Lookup button in the RECEIVER frame. As mentioned above, you may or may not initially view the same facility record (Blue Mountain) as we do below. If necessary, press the Find button to select Blue Mountain Microwave Receiver from the Record Selection List in the Find program.
Once the Blue Mountain facility is the current record in the facility lookup, press Close to transfer your receiver data to the path budget screen. As you can see below, TAP automatically calculates the azimuth and path length of the link that we will study.
Press Continue in the path budget study screen to access the Microwave Link Budget tab folders pictured below. If you have not previously run the link budget for this path, the following warning will be displayed before the link budget tab folders are displayed. Press OK - we will demonstrate the New button below.
TAP groups your link budget data fields into three major categories: TX Specifications, RX Specifications and Path Specifications. The transmitter and receiver specs folders display summary information about the facilities that form the endpoints of the link. These data fields are "read-only" - they may not be changed in this program. Use your Fixed Facility Lookup program or Fixed Facility Data Base Editor to make modifications to your facility records. The path specifications data fields, however, are not read-only. You will use this link budget program to edit your link budget data base in order to store information specifically related to the path between your facilities.
The ERP displayed in the Facility frame of the transmitter specs was calculated by TAP as a function of the transmitter power output (TPO), antenna gain and losses at the transmitter. Press the Print Transmitter Specifications Report button to view the various facility specs used in this calculation. Use the vertical scroll bar at the right of the Notepad window to view your transmitter information. You will see, for example, that line and circulator losses were assigned to the Red Mountain transmitter.
Press the Close button at the bottom of the text editor to return to the link budget program. When the text editor has disappeared, note the TX Diversity Frequency (MHz) field displayed in the TX Specifications folder. We have introduced frequency diversity into this microwave system by adding a diversity frequency of 13.2 GHz to the transmitter facility record. We will see the effect of frequency diversity on the overall reliability of the system later in this tutorial.
Now use your left mouse to select the RX Specifications tab. Note the Space Diversity field displayed in the receiver Facility frame. Because an antenna center of radiation for the diversity receiver (160 ft) differed from the center of radiation for the primary system (190 ft), space diversity (30 ft) was introduced into the microwave system. We will see the effect of space diversity on the overall reliability of the system later in this tutorial.
Press the Print Receiver Specifications Report button to view specifications for the Blue Mountain receiver like those displayed below.
TAP will calculate a required power for both the primary and diversity receiver systems as a function of the receiver thresholds and receiver gains and losses for the facility. Use the vertical scroll bar at the right of the Notepad window to view your receiver information. You will see, for example, that a 3 dB power amp gain was assigned to both the primary and diversity receivers. In addition, both systems have various connector, line, ether and splitter losses built into the Total Loss values displayed in the text editor and pictured below. When you scroll down to the bottom of the file, the required powers for both receiver systems appear.
Press the Close button at the bottom of the text editor to return to the link budget program. Now use your left mouse to select the Path Specifications tab. Many of the loss and reliability fields displayed in the path specifications tab folders are stored in your link budget data base. You may, for example, wish to contrast the overall reliability calculations of both an optimistic and pessimistic path configuration by creating and saving two link budget path records for the same link. When you enter the link budget program for the first time, your only Record Controls options are New and Cancel. We will use the New button to create and edit a new link budget record.
Press the New button to add a record to the link budget data base. The background color of the Description field becomes white indicating that it is editable.
Enter the description "TAP Microwave Tutorial" in the Path Record Description frame.
Press the Losses tab to view the Free Space Loss calculated by TAP and six miscellaneous loss fields. The Free Space Loss is a read only value computed as a function of frequency and path length. The Other Losses fields allow you to assign customized losses to this particular path.
Enter Ether Losses of 0.5 dB as we have done below. These and other losses added to your link budget record will be incorporated in the fade margin and reliability/outage calculations performed by TAP.
Press the Microwave tab to view the Attenuation and Reliability frames. This tab folder contains path losses and data specifically related to microwave design. Note the "Loss Mode:" labels that appear to the right of several fields. In these cases, you have the option to direct TAP to calculate these values or directly enter data. If TAP estimates a data field, the mode is designated as "Calc". If you directly enter data, the label "Spec" will appear.
Press the Calculate Loss button to the right of the Absorption Loss (dB) field. TAP will automatically insert a value representing water and vapor loss calculated as a function of frequency and path length.
Now press the Calculate Loss button for Rain Attenuation. The Rain Attenuation Lookup screen appears. You will see that losses were calculated for three different rain attenuation methods based upon default values for rain distance and rain rate supplied by TAP. We will modify these parameters below.
Rain Distance refers to the portion of the path affected by rain attenuation. Make the following modifications so that your rain attenuation lookup looks like ours pictured below:
Use your left mouse to change the Rain Distance units to miles (mi).
Enter a Rain Distance of 11.2 miles, approximately 50% of the path.
Assign a Rain Rate of 10.0 mm/hr. Tab or click off the Rain Rate units when you have assigned mm/hr. Your screen should look like ours.
In our case, all three models have generated estimated path losses due to rain attenuation over 50% of the path. In some cases, an error flag will appear for one or more of the models if, for example, your frequency is beyond the acceptable range for the model. For purposes of this tutorial, use your left mouse to select the 6.68 dB loss predicted by the Medhurst method. Press Close to return to the link budget program.
When the rain attenuation program disappears, you will notice that 6.68 dB has been assigned to the Rain Attenuation field. "Med" was assigned to the Loss Mode as a reminder that the Medhurst rain attenuation method was utilized.
You will notice several View Details buttons displayed in the Microwave tab folder. Use the view feature to remind you of the parameters used to generate the calculated values displayed in this screen. For example, press the View Details button to the right of the Calculate Loss button for Rain Attenuation to verify the Rain Distance and Rain Rate parameters that resulted in a rain attenuation of 6.68 dB. When you are ready, press the Close button on the Calculation Details screen pictured below to return to the microwave program.
Alignment loss refers to attenuation resulting from the improper orientation of the transmit and/or receive antennas. Enter a loss of 0.5 dB in the Alignment Loss (dB) field.
Now we will specify the terrain and climate factors used to predict system reliability. The Terrain(a) value refers to the terrain and humidity factor used in the reliability calculation discussed in Lenkurt (1970). Lenkurt suggests the following values:
4 for very smooth terrain, including over water
1 for average terrain, with some roughness
0.25 for mountainous, very rough, or very dry areas
In the alternative, TAP will calculate the Terrain factor as a function of the terrain roughness and local area humidity using Roelofs (1986). Position your cursor in the Terrain (a) field and press the Calculate button. The Reliability Factors Lookup program will appear with some default values inserted by TAP. Roughness (ft) refers to terrain roughness defined as the standard deviation of the path elevations taken at one mile intervals, not including the end points. Note that we could direct TAP to Calculate roughness using your TAP terrain data bases. We will not do that in this tutorial. When you experiment later with this feature, be sure that your topographic data paths are properly set up BEFORE you direct TAP to calculate roughness (see the TAP on line help topic for topographic data configuration).
Make sure that your Humidity factor is set to Average and enter a Roughness of 50.0 if another value appears there. The calculated Terrain (a) factor should display 1.000 as it does below. Press Close to return to the link budget program that will continue to display a terrain factor of 1.000.
Now we will address the climate factor. Lenkurt (1970) suggests the following values:
0.5 for gulf coast or similar hot, humid areas
0.25 for normal interior temperate or northern areas
0.125 for mountainous or very dry areas
In TAP, the default value for a new link budget record is 0.25 (normal interior temperate). This time, however, we will calculate a climate factor from annual temperature information that differs from the default. Press the Calculate button to the right of the Climate (b) field. Once again, the Reliability Factors Lookup program will appear with some default values. Make sure that the Average Annual Temperature is set to 55.0 and tab or click off the temperature field. The calculated Climate Factor should read 0.275 as ours does below.
Press Close in the climate factor lookup program to accept 0.275 and display this value in the link budget program.
Now press the Save button at the bottom of the link budget program to save these path parameters to the underlying link budget data base. The background colors of the microwave fields will turn gray to indicate that you are no longer editing a record. The next time you run the link budget program for this path, these path parameters will be available in your link budget data base.
Press the Fade Margin tab to view the received signal, fade margin, reliability and outage times calculated by TAP. Remember that in addition to the path attenuation values we have just specified, the gains, losses and other parameters defined for the transmitter and receiver facilities have been incorporated in the link budget calculations displayed on this screen.
Received Signal Level("RSL") refers to the power in dBm at the receiver input terminal calculated as the sum of Transmitter Power Output plus transmitter and receiver gains (including antenna gains) less transmitter, receiver and path losses. If you have specified vertical separation for the primary and diversity receive facility antennas, an RSL is displayed both with ("Div") and without ("Pri") space diversity.
A Thermal Fade Margin ("TFM") is calculated as the Received Signal Level less the primary receiver Threshold in dBm. If you employ space diversity, TFM is the greater of the fade margins calculated both with and without space diversity.
A Composite Fade Margin ("CFM") is computed as a function of Dispersive Fade Margin ("DFM"), Adjacent Channel Interference Fade Margin ("AIFM"), External Interference Fade Margin ("EIFM") and Thermal Fade Margin ("TFM"). DFM, AIFM and EIFM are all fixed facility data base fields which can be edited either in a Fixed Facility Lookup or in the Fixed Facility Data Base Editor. In our case, we entered a value of 0 for these specs resulting in a composite fade margin that is identical to the thermal fade margin value.
The reliability percentages displayed in the Fade Margin tab are calculated as a function of the Terrain/Humidity factor, the Climate factor, frequency, path length and the Thermal Fade Margin. The outages displayed refer to unreliability of the system expressed in seconds per year.
With space diversity, reliability is calculated using the greater of the two fade margins calculated both with and without space diversity. When space diversity is not employed, the Thermal Fade Margin displayed in the Fade Margin frame is used in the reliability calculation.
The overall reliability of the system generally can be improved by the use of space diversity and/or frequency diversity. A space diversity improvement factor Isd is calculated as a function of frequency, vertical spacing between the primary and diversity receive antennas, path length and fade margin. In this case, fade margin is the smaller of the fade margins calculated with and without space diversity. The Isd factor is used to calculate the overall reliability of the system. In our case, an improvement factor (Isd) of 18.5 is an indication of significant reliability improvements anticipated for a space diversity of 30 ft.
A frequency diversity improvement factor Ifd is calculated as a function of frequency, frequency spacing between the transmitter and diversity frequencies, and the composite fade margin. The Ifd factor is also used to calculate the overall reliability of the system. In our case, an improvement factor (Ifd) of 1.3 is an indication of slight reliability improvements anticipated for a frequency diversity of 13.2 GHz.
When both space and frequency diversity are employed, a combined improvement factor computed as the product of Isd and Ifd is used to calculate the overall reliability of the system. Together the space and frequency diversities combine for an improvement factor of 23.6. The estimated outage time improves to 135 seconds per year compared to nearly 3200 seconds when diversity is not employed.
This concludes the microwave link budget introduction. Consult your TAP technical reference manual for a more detailed discussion of microwave concepts and system equations used by TAP. You will also find there references to the microwave design literature used in the development of the TAP microwave path budget program. When you are ready to exit the link budget program, select the Close button in the Record Controls buttons at the bottom of this program.
Copyright 2000 by SoftWright LLC