I Can’t Hear You! Radio Frequency Interference (RFI) in Sports Arenas

During a recent NFL game, the visiting team complained about picking up the home team’s radio broadcast on their coach-to-coach headsets preventing the coaches from communicating with one another. The home team indicated that there were also issues communicating with the quarterback using their radio system.

RF interference stadiumsRadio frequency interference problems in major sports stadiums are unfortunately very common given the large number of radios present in a relatively small area. A typical sports stadium includes systems transmitting and receiving signals for game day operations, referee and commercial coordination, coach and player communication, a variety of cell phone networks, Wi-Fi services, and a number of other wireless services.

The problem is so severe that stadiums will typically have a dedicated frequency coordinator whose responsibility is to keep track of all of the wireless systems operating in the stadium and to assign specific channels to each user in order to avoid interference. A major sporting event can require the careful coordination of thousands of radio frequencies. As we will see, even the best laid plans of the frequency coordinator can fail if important, not always obvious, mechanisms for interference are not considered.

In a perfect world, all of the various radios in the stadium would happily coexist without interfering with one another. Unfortunately, even the best radios and best frequency plans can still suffer from interference for a wide variety of reasons. To understand why, we have to consider what comes out of the transmitters (the emissions), where the receivers can detect signals (the susceptibility) and how energy is transferred from the transmitting antenna to the receiving antenna (the coupling). We will illustrate this idea through an example involving two UHF radios and one VHF radio. The UHF radios are used for communication between coaches and the quarterback while the VHF radio is used by a security guard in the stands.

VHF radio is used by a security guard in stands

Before we consider the details of the problem, let’s take a moment to consider real world performance of transmitters and receivers. Ideally, a transmitter would only broadcast signals at the desired frequency or channel of operation. In reality, transmitters broadcast the desired signal but also harmonics (i.e., multiples of the desired signal), spurious emissions and broadband noise. Similarly, a receiver is designed to be most sensitive at a specific frequency or channel, but it also has undesired spurious responses at other frequencies. These are extremely important characteristics to consider when investigating an interference problem.

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Back to our problem. Using the ANSYS HFSS, Savant and EMIT products, we can predict interference between all of the radios in the stadium. As an example, we will take a look at interference that can occur between the VHF radio used by the security guard and the UHF radios used by the quarterback and coaches. We first used HFSS to model the antennas on the UHF and VHF radios. We also computed the broadband coupling between all of the antennas, an example of which is shown below.

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Given the antenna coupling information, we can now use EMIT to perform an RF system analysis to determine if interference is occurring in the quarterback’s radio. Using the antenna coupling data from HFSS and radio models from its built-in library, EMIT is used to predict where signals from the transmitter will cause interference with the receiver. The EMIT simulation shows that the 3rd harmonic (3 x 152 MHz) of the security guard’s radio falls directly in the desired channel of the quarterback’s radio (456 MHz) and produces a large source of interference preventing the quarterback from hearing the intended communication from the coaches.

Fortunately, the radios used by the coaches and the quarterback can be tuned to different channels. The EMIT analysis considers all possible channels that the quarterback’s radio can operate over and presents the results to the user. This information can be used to formulate a solid frequency plan in the stadium. As can be seen, simply tuning to the adjacent channels (455.5 MHz or 456.5 MHz) does not solve the interference problem (as indicated by the red and yellow “bubbles” beside the receive frequency channels in the list) because the coupling between the antennas is still quite strong and noise transmitted by the security guard’s radio presents a different type of interference to the quarterback’s radio. One needs to tune to a number of channels away from 456 MHz before an interference free channel (those colored green) can be found.

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In this example, we considered one interference scenario. Sports stadiums are harsh RF environments given the large number of transmitters and receivers and the multipath environment (signals bouncing off the interior surfaces of the stadium). To properly model such a complex and multi-scale problem, one needs full-wave (HFSS), ray tracing (Savant) and RF system (EMIT) simulation tools. ANSYS has the best in class solutions for all of these domains and can be used to identify sources of interference and ultimately mitigate those problems. In this video, more details of the stadium interference and propagation problems are examined.

The ANSYS HFSS, Savant and EMIT products are applicable to a wide range of problems and industries. Our customers are using these products to solve interference and installed antenna performance problems on airborne, sea, space, ground and handheld platforms. With the huge increase of RF devices due to the Internet of Things and other advances in technology, we can only expect radio frequency interference (RFI) problems to increase in the future.

Designing new technologies right the first time requires powerful simulation tools like HFSS, Savant and EMIT. If you would like to learn more about ANSYS HFSS and Savant please join us on Wednesday, November 4th for our webinar entitled,
The Best of Both Worlds: Applying ANSYS HFSS and Savant to Simulate Installed Antenna Performance.