Public Safety Advocate: Radio Interference in the Public Safety World

There are many different types of interference that may have an impact on existing radio communications systems whether they are Land Mobile Radio (LMR), broadband, marine band, aviation and satellite or, of course, Wi-Fi systems. Deliberately causing interference might be considered as “hacking” wireless systems. Then there is the issue of the noise floor and its level being higher than it has ever been, which can also have a negative impact on all types of wireless communications. Radio transmitters either by themselves or in conjunction with other transmitters can cause major interference issues as well. While there are other types of interference, I will limit the discussion for this week’s Public Safety Advocate (PSA) to those mentioned above.

Malicious Interference

Malicious interference is often attributable to how easy it is to purchase cheap handheld radios on the land mobile radio channels and then program them to work on almost any radio channel in use. Most of these radios are made in China. Although these units have been causing interference for many years, the Federal Communications Commission (FCC) only recently began to crack down on their use. Once a device is programmed to, for example, a fire radio channel, the “radio hacker” can make calls, disrupt incidents, and otherwise cripple communications. In one series of events a teenager with a radio tuned to a fire dispatch channel in California caused mass confusion by re-routing engines to different locations as they were being dispatched.

Public safety radios are sometimes taken out of service and sold or given to others, but most departments wipe them clean of their programming information before handing them over, or the radios are simply destroyed. A stolen or lost radio can normally be silenced and taken off the network much like your cell phone if it is lost or stolen. With mobile units, when the microphone button is pushed it sometimes inadvertently sticks and stays on the air. Many departments require all these radios to have time-out-timers in them to limit the time of the transmission and release the channel. It is difficult to identify open transmissions or catch radio hackers since they only transmit for a few minutes at a time and may move around. Some departments have enlisted the assistance of the local ham radio community as many hams practice “transmitter hunts” and have become very good at tracking down radios that should not be on the air.

Noise Floor

The next issue is just as important as global warming is to the environment. The is the measure of all signals other than the one being monitored in a given portion of the radio spectrum. If you went to Idaho National Lab’s (INL) radio test range, which is huge, you would be able to measure very little noise, but shifting to New York, the noise floor is much higher. Why does this matter? Well, the more noise there is in the portion of spectrum you are using, the more difficult it is for a receiver to “hear” weak signals. While this is true in all portions of the spectrum, new technologies (e.g., broadband, Wi-Fi, GPS) are designed to listen for very low-level signals. If these signals are drowned out by noise, the radio systems including GPS don’t function nearly as well as they should. The noise floor in your piece of spectrum can also be raised by a neighbor’s radios in another part of the spectrum.

One example I have used before is one we have all experienced with our Wi-Fi access points, especially the 2.4-GHz variety. Over time, you probably have noticed that the range and speed of your Wi-Fi seem lower than when you first installed it. This is not due to the age of the access point but rather the noise floor of the 2.4-GHz band in and around your house or office. The higher the noise floor, the worse your access point will perform. There have been advances in antennas and technology to be sure, but there are a limited number of channels available to Wi-Fi users. For example, if your cable company ships you a router that includes Wi-Fi, it is usually set to channel 6, one of only three channels in the 2.4 GHz-band that don’t overlap with others. If too many units are set to the same channel, you will increase the noise floor and end up with interference between several units. Interference that you cannot see manifests itself in deterioration of performance. Most people solve this problem by simply adding more access points around their home. This seems to cure the problem—but for how long?

Radio Site Interference

The most difficult type of interference to discover and eliminate is interference that can be generated from a radio site or cell site that serves a number of radios operating in different portions of the spectrum, perhaps with different technologies such as analog, LTE, DMR, and others. Paging transmitters on the site can be a real headache because they are normally on the air 24/7, spewing out a lot of RF energy with no receiver to hear any interference they may cause. Much of this interference is referred to as “intermodulation” or “intermod.” Intermod is generated when two or more transmitters’ signals are mixed together and the output of the mix turns out to be on a receive frequency on the site. There are formulas for intermod and plenty of software for entering in many different radio channels and running calculations to determine whether adding a new user to an existing tower will introduce intermodulation.

The most common of all intermod is A+B=C+ or A-B=C-. This is when the signals from transmitter A and transmitter B mix and create third signal on C. If A and B are added, C is a higher frequency, if B is subtracted from A, the C frequency is lower. This is called a first order harmonic and can be mitigated in a number of ways that have been used effectively in the field for years. It is tougher when you have three, four, or more channels mixing and emitting errant signals on a number of different channels. There are companies today that specialize in being able to run multiple models of thirty or more channels on a tower and predicting if there will be interference.

Many tower owners will not accept a new client on the tower until it has been determined that the new client will not create problems for existing occupants of the tower. One thing that can make it difficult to manage interference on a tower is that those using the tower must be willing to cooperate with each other and the tower owner to help solve any issues that might arise. Transmitters do mix all the time without incident. However, as in the case of the tower I owned in the Santa Cruz Mountains, one customer called us about interference to his receiver at the site. We spent a lot of time troubleshooting, analyzing the issue, and trying to find a fix. Finally, we found that a rusted bolt on the tower was causing the problem. It would mix the two signals and then transmit another signal on the receive channel of another client. We had all the tower bolts replaced and then made sure the fence and barbed wired were not rusted because that too could have caused a problem.

Finding interference is sometimes easy, as in the A+B=C formula, but it can also be the most difficult thing to identify, determine its cause, and then correct. And who knows what the next interference issue will be?


Moving off the radio site for a moment, you might remember when Nextel began having interference issues with LMR radios carried by public safety personnel operating in the same 800-MHz band but it only occurred when a public safety radio came close to a Nextel radio site. Nextel built low-level sites that were basically on the ground as opposed to cellular high-level sites and this was the cause of the interference. There was a long and expensive fix that is still to be completed by Sprint in a few areas. It recently became an issue in Oakland, Calif. between the LMR system for police and local cell phone providers. Fortunately, this was resolved as well but it demonstrated that while very stringent about operating specifications for transmitters, the FCC basically ignored receivers. As a result of this lack of oversight, receivers used in the public safety community have been designed to operate over a very wide portion of spectrum, meaning their ability to reject unwanted signals is greatly diminished.

Are We Good Stewards of the Spectrum?

A year or so ago I gave a lecture to a group of Auburn University students studying wireless. My remarks were about spectrum, interference, noise, and more, and I challenged them to become the next stewards of the radio spectrum. Are we good stewards today? Some political organizations seem more intent on auctioning spectrum than taking better care of it. Most elected officials don’t understand the Radio Frequency (RF) spectrum and that it is a limited resource absolutely necessary if we are to succeed in business, work, play, serving communities, and making the best possible choices for use of the spectrum we have.

We need more trained people in the field who understand RF spectrum and related issues that have to be addressed. We need more oversight before a group of commissioners votes on sharing, dividing, or reordering who has access to which spectrum for what purposes. We need to return to the days when the engineer-in-charge of the FCC technical group was called on for advice and council before decisions were made. Today, experts are called after the FCC decisions when faced with interference issues that occur because the decisions were not based on history or the results of research.

Vendors, spectrum users, and those who advise spectrum users all need to pay more attention to what we need to do to keep the spectrum noise floor from changing the entire way communications systems work. We need to support new technologies and investments in technologies that can be used to augment what we know and how we manage spectrum today. In short, we have to treat wireless spectrum as a finite resource that needs to be nurtured and managed.

The wireless industry needs more technical types who know how to identify interference issues and more engineers who know how to deal with them. We need to train people who have an interest in communications. We cannot survive using “technicians” who are called out to a cell site and told to “go to rack 4 and change out card number 6” and walk away without understanding what the problem was and if he/she fixed it. It has been many years since I met a real “tech” at a cellular radio site or public safety system site though I know there are some out there. A real tech is someone who knows the ins and outs of radio and knows how to use test equipment to help find and troubleshot a problem. We don’t seem to be producing new techs today. Remember what happened to Quasar, a Motorola TV that came into the market in 1967? It was heralded as the “works in a drawer” and Motorola had a policy for TV techs. Pull and replace modules until the TV is working, then send the pulled modules back to us and we will replace them—no technical expertise required. Quasar is only a memory, and a blot on Motorola’s record, since it turned out to be a total economic failure. If we try to turn wireless into a Quasar TV, both the spectrum and our wireless futures are in trouble.

Andrew M. Seybold
©2018, Andrew Seybold, Inc.


Be the first to comment on "Public Safety Advocate: Radio Interference in the Public Safety World"

Leave a comment

Your email address will not be published.


This site uses Akismet to reduce spam. Learn how your comment data is processed.