Intra system interference is calculated in the Network module. The calculation considers only the displayed layers. All sites and links which are not on an active layer will be ignored. The calculations use antenna and radio codes. An antenna or radio code is the data file name without an extension. These contain the parameters required for an interference calculation and are described in detail in this section. The minimum conditions to calculate intra system interference are listed below:
A radio code is optional; however, only the interfering level will be calculated without this file. A radio code is required to calculate filter improvement and threshold degradation.
Note that there is no provision to calculate the interference from a transmitter into a receiver located at the same site.
On completion of an interference calculation, the user is prompted to save the calculation results. The default file name is the gr5 file name with the extension ifr. The file can be reloaded provided that the current gr5 file was used to create the interference file or the network display is blank.
Interference calculation procedure
Select Interference - Calculate Interference from the Network display menu bar. The "Intra System Interference" dialog box sets the options for the calculation.
Study scope
Interference is calculated between two sets of links. One set of links can act as the interfering transmitters and the other as the victim receivers or both sets can act as interfering transmitters. These sets of links can be a selection, a named group of links, all links or the master data base.
Select the two sets of sites and click on the green arrow to change between a single direction and bidirectional analysis. Note that if the two sets are the same, then the calculation is inherently bidirectional and the specified direction has no effect.
When analysing one set of links against second set of links, it is expected, that the user will define two independent sets without any overlapping links. If there are overlapping links an intra system calculation will be carried out in the overlap area will be carried out and this can result in duplicate interference cases. These duplicates are removed from the results; however, as a cautionary measure, the user is advised to examine the cross reference report to verify that duplicates are not present.
When an interference analysis between the network display and the master data base is carried out, duplicate interference cases can also occur. In this case, the test for duplicate interference cases can be ambiguous and the user is responsible to delete any duplicates.
Digital interference objective
The objective is specified in terms of the allowable receiver threshold degradation. For frequency coordination with other operators, the usual value is 1 dB; however, for intra system interference, the final criteria is determined by the increased outage times resulting from the actual threshold degradation.
Note that the allowable threshold degradation determines the composite interfering level. A calculation margin described below is subtracted from this composite interfering level to established a reporting threshold level. The following example illustrates this procedure.
Sometimes an analysis shows that there are no interference cases and the user would like to examine all interference calculations. This can be accomplished by lowering the reporting threshold using a threshold degradation of 0.01 dB and a calculation margin of 200 dB. In the above example, these values would result in a reporting threshold of -333.5 dBm which will show all cases in any practical system
Default Minimum Interference Level
In the above reporting threshold example, the interfering level required to meet the threshold degradation objective was calculated from the receiver noise floor. In the event that the receiver noise floor is not available, the default minimum interference level will be used as the interfering level required to meet the threshold degradation objective. The reporting threshold level will then be given by the default minimum interference level minus the calculation margin.
Calculation Margin
The calculation margin sets a tolerance on the reporting of interference cases. If the interference level objective for the receiver under test is -104 dBm and the calculation margin is set to 10 dB, then all interference cases greater than -114 dBm (-104 - 10) will be reported.
The threshold degradation objective will be converted to an interference level objective for each receiver in the calculation.
Coordination Distance
Interference is not calculated if the interfering path length is greater than the specified coordination distance.
Maximum Frequency Separation
Interference is not calculated if the difference between the interfering transmitter and victim receiver frequencies is greater than the specified maximum value.
Note that if a radio data file is not available, a cochannel interference analysis can be carried out by setting the maximum frequency separation to some value less than the TX to RX frequency spacing.
Ignore Diversity Antennas
This option ignores all receive frequencies associated with a space diversity receive only antenna. In the initial frequency analysis, this option will reduce the number of cases by 50%. If the main and diversity antenna gains are different, then the final analysis should consider the diversity antennas.
Ignore Adjacent Channels
This option applies to 1 for N systems. Once the threshold degradation of the adjacent channels has been established, use this option to limit the number of interference cases.
Calculate OHLOSS automatically
This option will automatically calculate the over the horizon loss (OHLOSS) on all interfering paths which do not have a direct link to the affected receiver. Click the OHLOSS option button to set the specific options for the OHLOSS calculation.
OHLOSS calculations can be a contentious item when resolving interference case between different organizations It is important to note that if an OHLOSS calculation results in a interfering level below the reporting threshold, the interference case will not appear in any report. If the OHLOSS calculations are carried out in the case detail report screen after the main calculation is complete, the OHLOSS cases will remain in the analysis.
OHLOSS options
Click the OHLOSS options button in the Intra system interference dialog. These options will be used in all OHLOSS calculations in the present analysis. Refer to the help in the diffraction loss section of the PL50L program for complete details on the OHLOSS calculation procedure.
An OHLOSS calculation takes time variability into account. Current practice is to compute a long term and a short term time variability. The long term is normally set to 80%. Threshold degradation objectives refer to this long term objective. In the short term, the allowable threshold degradation can be significantly higher.
If required set the diffraction algorithm, the climatic region and the short and long term time percentages.
Correlation options
When an interference path is the same as the main path, the interference case is defined as correlated. In this case it is expected that fading on the interfering main paths will occur at the same time. In the case of multipath fading, the actual fade depths on the two paths will depend on the type of correlation. If the interfering transmitter antenna heights are the same as the main transmitter heights, then the two paths are completely correlated. In this case it is common practice to ignore the case, particularly if automatic transmit power control is employed. The main path will increase power in response to the fade; however the interfering transmitter may not change.
A partially correlated situation exists when the interfering transmitter antenna heights are different than the main transmitter antenna height. In this case, an additional loss in the order of 5 to 10 dB is added to the interfering signal
Although the same correlation options are used for both mulipath and rain, the definition of correlation is actually whether the interfering and main paths are in the same rain cell.
Click the Correlation options button in the Intra system interference dialog to set the correlation options
Interference calculation
The calculation starts by building transmitter and receiver tables for the two sets of links. If the two sets are the same, then only one set of transmitter and receiver tables is required.
If the integrated PL50L program is running with one of the required links, the transmit and receive data is taken from the PL50L data; otherwise, the data is read from the Pathloss data file associated with each link. The Pathloss data file in memory can be edited and when the interference is recalculated to see the effects of the changes.
Interfering level objective
The interfering level objective Iobj is calculated for each receiver as follows:
where:
The receiver noise threshold calculation will depend on the specific data available in the radio data file
Threshold to Interference T_I and the 10-6 BER receiver threshold RXthr
Nrx = RXthr -- T_I + 5.868 dBm
3 dB carrier to interference C_I3dB measured at the 10-6 BER receiver threshold RXthr10^6
3 dB carrier to interference C_I3dB measured at the 10-3 BER receiver threshold RXthr10^3
Receiver noise figure NF and 3 dB receiver bandwidth BW3dB
Nrx = 10 * log10(n_f) + 30. + NF dBm
and the 3 dB bandwidth is expressed in MHz
This interfering level objective represents the total power in the victim receiver passband which will degrade the receiver threshold by the specified amount. At this point, the frequencies and the bandwidths of the interfering transmitter and victim receiver are not considered. These will be used later to calculate the filter improvement.
If the receiver noise threshold is not available due to missing data, the default minimum interfering level will be used as the objective.
Note that Iobj - calculation margin is the reporting threshold.
Interference case rejection
As the calculation proceeds, an interference case will be rejected at any point if its interfering level is less than the reporting threshold, Several other conditions to reject an interference case are as follows:
Free Space Loss Interfering Signal
The free space loss interfering signal level is calculated as follows:
Ifs = TX power + TX antenna gain + RX antenna gain - TX loss - RX loss - free space loss.
Antenna Discrimination
The effects of the transmit and receive antenna discriminations are now considered:
Antenna discrimination is characterized by the four polarization combinations HH, HV, VV, and VH. The first letter is the polarization of the antenna under test. The second letter is the polarization of the signal being received or transmitted. For example, the term HV is the response of a horizontally polarized antenna to a vertically polarized signal.
At first glance, the total antenna discrimination would be obtained by adding the appropriate polarization combinations of the interfering transmit antenna and the victim receiver antenna. Unfortunately, this is not the case, as the ratio of horizontal and vertical polarized signals is unknown.
The following polarization combinations are determined and the minimum value is assigned as the total antenna discrimination:
An example of this analysis is given below for a free space loss interfering signal level of -38.59 dBm. The first letter in the polarization combination is the transmit polarization; the second letter is the receive polarization.
Antenna Discrimination HH HV VV VH
Interfering TX 0.00 32.00 0.00 32.00
Victim RX 65.00 69.00 67.00 69.00
Total Discrimination (dB) 65.00 69.00 67.00 69.00
Interfering Signal (dBm) -103.59 -107.59 -105.59 -107.59
If the interfering level for the specified polarization is less than (Iobj - calculation margin), then the interference case is rejected.
If this is a cochannel case (same transmit and receive frequencies) and the same radio codes are used for the transmitter and receiver, the calculation for this interference case is complete.
Filter Improvement
Radio data files are required to calculate the filter improvement. If a radio data file does not exist for the victim receiver, the calculation terminates. The calculation sequence for filter improvement proceed with the following steps.
If the required T to I or interference reduction factor curve is not available, the filter improvement will be calculated by convoluting the spectrum of the interfering transmitter against the victim receiver selectivity. There are two levels of default for both the transmitter and receiver.
If the transmitter radio data file includes a measured transmit spectrum, this will be used; otherwise, the default emission mask is used. In both cases, the data is normalized, so that the area under the curve is unity as shown in Equation (2).
where PI(f) is the power spectral density of the interfering transmitter. This normalization was carried out when the radio data file was created from the ASCII version
The receiver selectivity calculation curve (RX_SELECTIVITY_CALC) is generated when the radio data file is created using the following priorities for the source data:
This curve is optimized for the convolution process by using a variable point density concentrated around the receiver passband.
The receiver selectivity and interfering power spectral density are convoluted together to calculate the filter improvement FI as follows:
where:
Threshold Degradation
The threshold degradation is calculated using the following formula.
where
The composite threshold degradation is calculated in the same manner using the power sum of all interfering signals.
Fade correlation
Once the basic interference calculation has been completed, the rain and multipath fade correlation can be set for each interference case. Select Interference - Fade correlation on the network display menu bar. Step through the cases and sub cases to view the interference cases.
Use the go to case button to set a specific case number. on the cross reference report
The current interference case - sub case is shown on the network display
The following abbreviations are used in the display:
Refer to the Fade correlation options section for details of the concept and typical values for fade correlation
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