Analyzing and resolving cosite interference to improve system performance and optimize network costs

Naveed Mufti · August 27

Analyzing and Resolving Cosite Interference to Improve System Performance and Optimize Network Costs

In real life scenarios, there is a high probability of multiple transmitting and receiving antennas mounted on the same device, system or platform, including moving (like aircraft, ship or vehicles) and stationary (for example, towers or masts) ones.

In these cases, the antennas will mostly be designed for and connected with transmitters and receivers for different applications and frequencies. Yet, due to their proximity, there is a probability of interference (typically called cosite interference), mainly due to:

Coupling between the antennas
Non-linear electronic effects

Altair Feko not only enables antenna design, placement and coupling but also allows the user to analyze and mitigate interference in such scenarios, with the co-site/collocation interference tool inside WRAP (a Feko Component). This technical paper explains how multiple antennas placed on a Learjet, simulated and optimized in Altair Feko, can be analyzed for mutual interference in Altair WRAP.

Altair WRAP is a comprehensive tool for electromagnetic propagation, antenna collocation, and spectrum management. It’s part of Altair’s software and service offering, designed to simplify technical and administrative tasks related to radio and radar coverage planning, interference analysis, frequency administration, and network monitoring. WRAP has been used in different industries for over two decades, including, but not limited to, spectrum regulatory agencies (defense and civilian), telecommunication service providers, armed forces and defense agencies, aviation, public utilities, transportation etc.

The Collocation Interference tool inside WRAP is used for multiple users, multiple service sites where several independent radio networks meet under severe, and high-level interference conditions. Output data is useful to provide guidance on changes in frequency assignments and changes in site configuration of antennas.

In addition to co- and adjacent- channel interference calculations, the collocation interference tool inside WRAP can consider Intermodulation, Harmonics, Receiver Blocking, IF breakthrough and Image frequencies, using relevant equipment properties and coupling between the relevant antennas as inputs.

The coupling depends on the values inside the Coupling loss matrix, which defines the system losses between stations/terminals. WRAP can calculate these values automatically, based on how the Tx/Rx are defined inside the WRAP project (indicated by * next to the value), as it happens in this example initially.

The entries can also be manually defined/modified inside WRAP. Alternatively, the loss matrix values can be imported (from a touchstone file), as illustrated later in the example.

Figure 3 shows typical workflows inside Feko and WRAP, for a scenario requiring investigation of co-platform/co-site interference:

This paper briefly guides the reader about the process of co-site interference calculations for a project created in Altair Feko.

A Learjet aircraft with 3 antennas has been analyzed, with UHF Blade antenna on fuselage bottom and VHF Blade and GPS Receiver antennas on top of the fuselage (see Figure 4). The antenna design and placement have been optimized for specific frequencies inside Feko.

For detailed information, please check the Altair community blog [1], and the webinar recording [2]. Another webinar also provides further insight [3]. From within Feko, the S-parameters are configured and imported as a touchstone file, as shown in Figure 5 (the procedure is mentioned in Feko HTML helpfile [4]).

The workflow inside WRAP has been outlined in Figure 3, while also discussed in the article [1] and webinar [2]. The steps to follow inside WRAP are explained as detailed workflow below:

Step 1:

Create Equipment inside WRAP (Antenna, Transmitter, Receiver):

Step 2:

Create Stations as A Group, with relative coordinates:

Step 3:

Import touchstone file inside WRAP:

Step 4:

Map the ports (from Altair Feko S parameter file) to the stations already defined inside WRAP:

Note: After this import and mapping, the user has the coupling loss matrix in WRAP project with values from the Feko project:

Now, WRAP is ready to perform collocation interference calculations.

Step 5:

Collocation Interference Analysis:

Sub-Step 1: Collocation Interference Settings:

Sub-Step 2: Calculate Interference:

Sub-Step 3: Interpret the Results:

UHF receiver suffers from at least one type of interference (co/adjacent-channel) caused by VHF Transmitter while GPS antenna suffers from co/adjacent channel interference caused by both VHF and UHF Transmitters.

The most important value to consider in these calculations is the Margin. It indicates by how much the receiver sensitivity (ability to detect a weak wanted signal) has been degraded. The interfering signal would have to be reduced by at least this much to have no effect on system performance.

Sub-Step 4: Possible Remedies

For harmonics, adding a relevant (bandpass) filter to the interfering transmitter producing harmonics can reduce all such interference involving harmonics. For the co-channel interference, the operating frequency needs to be changed. This can be done manually or using functions inside WRAP.

In this case, adding filters to VHF and UHF Transmitters leads to substantial interference reduction, while adding filters to UHF and GPS Receivers eliminates any type of interference arising from collocation scenarios. The calculations after these changes show no interference.

Interference Matrix diagram gives a quick overview of the worst interference cases (in this case, it has been well mitigated).

As can be concluded, Altair Feko and WRAP provide a comprehensive solution for co-site interference analysis, which applies to co-platform, co-site, co-premises and co-located scenarios. Feko provides the tool for antenna and port design and optimization, while WRAP quantifies co-site interference, taking non-linear equipment properties into account.

More Resources about Co-Site Interference using Altair Solutions:

Analyzing and resolving cosite interference to improve system performance and optimize network costs

Analyzing and Resolving Cosite Interference to Improve System Performance and Optimize Network Costs

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