David Schoenberger

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Cellular Phone Interference and 800 MHz Public Safety Radio Systems: An Unforeseen and Dangerous Problem

Introduction

As a follow-up to last year's Monroe Project, which compared the public safety radio systems of major East Coast cities, I have decided to explore the problem of cellular phone interference to 800 MHz public safety radio systems. Analog cellular phones, still in wide use in the United States, operate in the 800 MHz frequency band. Many of the newest public safety radio systems operate in the 800 MHz band as well. Unfortunately, the proximity of these two services to each other causes them to interfere with each other in many cases. Neither the cellular telephone operators nor the public safety operators are operating in an improper or illegal manner. The interference problem stems from the fact that the Federal Communications Commission failed to anticipate in the 1980s the rise of the cellular telephone industry, and assigned the newest public safety frequencies to a band adjacent to the cellular frequencies.

This project will document cases of cellular telephone interference to 800 MHz public safety systems around the country, and show how this problem has caused dangerous situations, and has the potential to cause dangerous situations in the future. Links to many articles discussing the problem are listed below. In the near future, interviews with public safety officials in the Hampton Roads, Virginia area will be conducted to assess the problem of cellular interference in this region. Interviews may also be conducted in the Richmond and Northern Virginia regions. Many public safety agencies in all three of these regions use 800 MHz radio systems, or are planning to migrate to these systems soon. Therefore, the information obtained from communications managers should provide excellent insight into the problem of cellular interference.

Does cellular interference to 800 MHz public safety radio systems exist?

According to data compiled by the Association of Public-Safety Communications Officials (APCO), many jurisdictions report interference to their 800 MHz radio systems from cellular transmissions.  The reports of the causes of the interference vary widely: some jurisdictions simply list the interference source as "cellular" or "Nextel," while other jurisdictions have pinpointed the exact tower where the offending interference originates.

APCO Survey Results

Why does cellular interference to 800 MHz public safety radio systems occur?

The interference occurs because of the different design characteristics of cellular systems and public safety systems.  Because governments are often financially limited, public safety radio systems are designed to minimize the number of towers that must be erected.  These systems are designed to maximize coverage from a centrally-located base station.  Cellular system designers, usually equipped with better financial resources and desiring guaranteed continuous coverage for roaming cellular phone users, place many towers in close proximity to each other.  When a cellular user approaches a closer "cell" the phone call is handed-off.  The cellular provider can reuse frequencies, which allows for high-capacity systems.

When a public safety radio user is far away from the centrally-located base station, and is near a cellular site, the cellular site may overwhelm the public safety radio.  This phenomenon causes the interference.

What is being done about the cellular interference?

Some jurisdictions are working to solve the interference problems themselves.  These jurisdictions might take steps such as increasing power output or realigning antennas.  Undoubtedly, some communities have chosen not to upgrade to the 800 MHz band because of anticipated problems with cellular interference.  Nextel, commonly cited as one of the principal agents behind the interference recognizes that there is a problem with cellular and other commercial radio service in the 800 MHz band and is working actively with some communities to resolve the interference issues.  Most communities, however, seem to be doing little about the interference problem.

APCO has identified cellular telephone interference to 800 MHz public safety radio systems as a major nationwide problem.  As such, the organization has devoted research and financial resources to the issue.  APCO's effort to examine interference issues and resolve them is known as Project 39.  Because the issue has been assigned "Project" status, financial resources are available for the study.

Project 39

Motorola has published a guide explaining the technical details behind the interference.

Motorola's Interference Technical Appendix (PDF)

Has the interference been resolved?

In some cases, the interference problem has been resolved, most often through close cooperation between the public safety agency affected and the cellular or commercial mobile radio service provider.  But in the vast majority of jurisdictions which are reporting interference, the issue has not been resolved.

Future Research

Articles

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Copyright 2020 David Schoenberger