This chapter specifies the system used by Xirio to calculate interference. It will describe the method used to estimate the contribution of one or several interference sources as well as the terminology and notation.
Parameters that allow to define interference will be:
•Propagation method. Method that will be used to estimate the signal level with which the interfering transmitters affect the interfered receiver.
•Complementary method. It is possible to use two calculation methods together, one main and the other complementary. This option is useful for attempting to complete the worst case in any situation, since Xirio will keep the interference value predicted by the more pessimistic method.
•Combination method. It will be used in multiple interference calculations (from more than one interference source) to combine individual interferences and get the one produced by all of them. The methods used are: simplified multiplication for audio broadcasting in frecuency modulation (FM) and quadratic sum for all other services.
•Inter-services factor. It defines how the receivers are affected by interference depending on the services of transmitters and receivers.
Interference from one source
The interference from one source is calculated for the main and complementary methods with the following results being generated:
•Field received (Er): This is the field generated by the interfering transmitter at the receiver location.
•Interfering field (Ei): This is the received field, filtered according to the receiver's frequency discrimination, in other words, applying the rejection factor.
•Interfering power (Pi): This is the actual power received by the receiver. The receiver antenna will discriminate the interference signal considering the orientation and the received signal origin.
The multiple interference calculation is done by combining calculations of the individual interference contributed by each interfering transmitter, using the sum of squares as the combination method.
If calculation using the complementary method is selected, all of the interference calculated complementary method combined on the other. Later, the total interference result will be the worst case, that is, the combination that gives the greatest interference as a result.
The multiple interference calculations are: Interfering field (Ei), Interfering power (Pi), and Carrier to interfering power ratio (C/I).
Influence of channels
The way to treat interference in reference to the channels is: their interference contribution is the combination of the interference caused for each channel (or frequencies specified by the user).
The interference contribution of different channels will be calculated separately. Each one will work at their frequency, specified by the user, like it was a new transmitter working at each carrier.
Influence of polarization
In the interference calculations is important to consider the effect of interfering signals polarization. Xirio has two parameters of Discrimination at 45° and 90° (antenna pattern) that establish the attenuation that signals received in other polarizations will suffer.
•XPD or discrimination 45º: Attenuation suffered for receiving the signal with a polarization that differs 45° from polarization emitted by the transmitter (circular or mixed polarization).
•XPD or discrimination 90º: Attenuation suffered for receiving the signal with a polarization that differs 90° from polarization emitted by the transmitter (cross-polar).
Once the discrimination factor corresponding to the interfering signals has been applied, the rejection factor that defines how interfering signals produced by transmitters affect the desired signal of the same or another service depending on its frequency separation.
In fixed services, where the input angle of the received signal is critical, Xirio uses two radiation patterns (copolar and cross-polar) to perform the interference calculations. It is considered to co-polar all types of signals, desired and interfering, that get to the receiver with same polarization and will be received with its copolar diagram, while the crosspolar signals (opposite polarization) will be received with the crosspolar diagram.
Xirio always analyzes the influence of copolar and cross-polar signals, resulting the value of total interference.
Influence of the receiving antenna
In the interference calculation is very important to introduce correctly the receivers radiating system since the interference will be calculated taking into account the receivers antennas orientation and the source of different interferences.
Xirio considers that receivers on each calculation point are pointing with maximum gain to their corresponding transmitters, while interfering transmitters will be entering the radiant system from azimuts with less gain.
The operation of SFN (Single Frequency Networks) requires special treatment since the different stations are broadcasting the same program in unison and signals presence from several sources at a point could be considered to be constructive or destructive interference depending on delays in different signals with respect to the dominant one. For further information, clik here.