Radio Interference: A Public Safety Perspective

Over the past several years, a radio interference problem has presented itself as a dangerous situation for all public safety agencies that are using or are migrating to 800 MHz radio systems.


Over the past several years, a radio interference problem has presented itself as a dangerous situation for all public safety agencies that are using or are migrating to 800 MHz radio systems.

WHAT IS THE PROBLEM?

The problem is that 800 MHz public safety radio systems (even those that have gone through full acceptance testing), suddenly and WITHOUT WARNING no longer work in specific areas.

NOTE: Public radio systems outside of the 800 MHz frequency spectrum are NOT affected.

HOW SERIOUS IS THE PROBLEM?

The problem is very serious because interference will continue to occur as new 800 MHz public safety radio systems are installed and as more commercial wireless systems expand. To date, over 1,000 reports of interference have been documented. In 2003, over 350 locations reported interference problems (the highest number of reports in any given year).

At a Public Safety event in Washington, D.C., International Association of Fire Chiefs President Ernest Mitchell stated, "The problem of radio interference at the 800 megahertz level is serious. Thankfully, by our measure no one has died as a result of this interference. However, it's only a matter of time since - and probability that some fire fighter or police officer will have a radio go dead at a critical moment and his or her life will hang in the balance." Additionally, Harlin McEwen a representative for the International Association of Chiefs of Police said, "These interference incidents are growing. We now have over 1,000 incidents reported in over 34 states and we know that the problem is not getting any better. Dropped calls and garbled radio messages, blocked messages are serious. When a police officer can't call for help, or a fire fighter needs help and can't get his radio to work, that's a serious business and we want people to understand that this is not something to take lightly."

WHAT CAUSES THE INTERFERENCE?

Unfortunately it has occurred by unforeseen technological characteristics and resulted even though every company involved is doing everything required by law. In simple terms, interference is caused by an intermodulation problem between commercial wireless vendors and 800 MHz public safety radio systems both of which are within the 800 MHz spectrum.

EXPLANATION OF INTERFERENCE

First, the co-location of commercial wireless vendors and public safety radio systems within the 800 MHz radio spectrum is where the problem begins. Often the two systems frequencies are too closely positioned within the 800 MHz radio spectrum. Unfortunately, there is no where to move public safety radio frequencies as the spectrum available to public safety has been exhausted.

To broaden the explanation, public safety radio systems traditionally work off of high power transmitters on high towers on mountaintops (high power, high tower) while commercial wireless vendors operate on low power transmitters on low towers (low power, low tower). The new problem occurs as the signal from our high towers converges upon the new low tower signals from the commercial wireless vendors (which sit between the public safety high towers and its units). At this point, it changes the public safety frequency to a range that will no longer work in a specific (unknown) geographic area. This can happen when there is a change to the commercial wireless system (added frequencies) or when a new commercial tower is built (as systems expand).

WHAT IS THE SOLUTION?

A couple of years ago, a plan was brought forth that recommended a rebanding of 800 MHz frequencies called the Consensus Plan (CP). In a cooperative effort, many of the nation's largest public safety organizations have expressed their support including the International Association of Fire Chiefs (IAFC), the International Association of Chiefs of Police (IACP), National Sheriffs Association (NSA), Association of Public Safety Communications Officers-International (APCO) and many more.

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