Why can’t we talk? It’s the interference
Under the current FCC requirement, public safety mobiles and portables used as close as 350 feet from a wireless base station antenna could lose communications with the public safety system as much as 30% of the time.
See the towers on the ridge in the distance?
Notice the silhouette of the short wireless tower in the foreground?
To cover a large area, public safety radio communications systems use a limited number of base station antennas placed in locations with commanding views, such as tall towers, skyscrapers and ridges. Wireless telephone systems use base station antennas placed on short towers, rooftops and other locations with limited views, and they use a great number of them to cover a large area.
Therein lies a conflict.
Public safety radios used at a distance from their base station antennas have to be sensitive enough to pick up comparatively weak signals and selective enough to reject interference on nearby frequencies. As selective as some of these radios may be, they cannot reject unwanted signals that come in directly on public safety frequencies.
Some wireless base station antennas emit radio signals directly on these public safety frequencies. Even though the signals may be small, when the public safety radios are used in close proximity to the wireless base station antennas and at a distance from their own base station antennas, the wireless signals block reception on the public safety radios.
The Phoenix Police Department is among those affected by interference from wireless base stations. The department’s mobile data terminals sometimes don’t work when they are close to wireless base station antennas.
Key problem areas include street intersections where officers often aid victims of vehicle collisions and where wireless carriers commonly place base station antennas to serve the driving public.
“When the mobile data terminals don’t work, the officers drive on, and when they can, they retry,” said Melvin G. Weimeister, police telecommunications superintendent for the department’s Computer Services Bureau. “The way we determine that they’re not working is by driving through an intersection and sending a message. If the terminals don’t receive an acknowledgement after making three tries automatically, it lets you know.”
Priority radio calls
The mobile data terminals carry the department’s “Priority Two” information about crime and accident victims and “Priority Three” information about crime history. The city’s antiquated voice radio system handles “Priority One” calls about crimes in progress.
“We’ll voice other calls, too, when it’s necessary to make other officers aware,” Weimeister said. “We make unit-specific calls on the mobile data terminals.”
The Phoenix voice radio system is unaffected by interference of the same kind because it uses a different frequency that is widely separated from those used by wireless base stations. In the face of the city’s growth, the voice system has enough additional problems and limitations to require replacement.
But the department can’t risk moving it to frequencies that might be plagued with wireless base station interference, even though other frequencies would allow an enormous expansion of capacity and features.
Many public safety agencies, including the Phoenix police department, have their eyes on frequencies in the ranges of 764MHz-776MHz and 794MHz-806MHz.
That plan fit fairly well with the federal government’s plans to assign nearby frequencies for more wireless services, as long as the wireless services would use the frequencies below 764MHz for transmitting and the frequencies below 794MHz for receiving. That’s an oversimplification. But basically, it’s a workable plan because it separates the wireless base station transmitting frequencies from the public safety base station receiving frequencies widely enough to allow the use of relatively inexpensive technology to control interference with a high expectation of success.
In a nasty turn of events for public safety agencies, the government changed its assignment plan for wireless services in a way that allows base station transmitters to use frequencies near those planned for receiving public safety radio signals. Public safety communications has been left with nowhere to run and nowhere to hide when it comes to avoiding wireless system interference in the new 700MHz band.
A solution does exist.
If wireless system base station transmitters were regulated to reduce the strength of signals they emit on the public safety radio frequencies to a sufficiently low level, they wouldn’t interfere. What’s required?
• First is a requirement to limit wireless base station interference in the 794MHz-806MHz frequency range where public safety base station receivers will operate.
Motorola has suggested that the wireless base-to-base interference should be limited to a level of -80dBm in a 6.25kHz channel. If so, the wireless base station antenna could be placed as close as 250 feet to the public safety base station, and it would only reduce the reception coverage of the public safety base station by 5%.
• Second is a requirement to limit wireless base station interference in the 764MHz-776MHz frequency range where public safety mobiles and portables will operate.
Base-to-mobile
Motorola has suggested that the wireless base-to-mobile interference should be limited to a level of -63dBm in a 6.25kHz channel. For mobiles and portables operated as close as 350 feet from a wireless base station antenna, reception would be interrupted only 1% of the time.
Here’s the contrast:
Under the current FCC requirement, a wireless base station antenna within 1,000 feet of a public safety base station could reduce the coverage of that base station receiver by as much as 95%. Even if the base stations were 2.3 miles apart, the public safety system could lose 33% of its coverage.
Under the current FCC requirement, public safety mobiles and portables used as close as 350 feet from a wireless base station antenna could lose communications with the public safety system as much as 30% of the time.
Motorola is in a special position when it comes to making recommendations about interference. The Phoenix police department and many other public safety agencies have interference problems involving Nextel Communications wireless base stations. Motorola makes Nextel’s equipment, and it supplies many of the agencies that have interference. Even when everyone’s equipment meets specifications and is operated within current FCC limits, interference can result.
The FCC has yet to place limits that would protect public safety radio communications in the 700MHz band. In fact, last year it took steps in the name of “technical flexibility” that brought the potential for destructive interference to a virtual certainty. It was in the name of flexibility that the FCC changed the original 700MHz plan to allow wireless base station transmitters to be used in the range below 794MHz instead of limiting them to the range below 764MHz.
The FCC seems to have been responding to requests from wireless system operators that technical flexibility is necessary to allow wireless systems to avoid potential interference from channel 59 TV stations.
Steve Sharkey, Motorola’s director of telecommunications regulation, put it this way in a letter to the FCC: “With respect to interference from broadcast stations, we believe the proper solution is to address the television incumbency issue directly. The incumbency issue should not be sidestepped by establishing inadvisable technical rules that would continue to cause harmful interference to public safety and other commercial carriers long after television stations are removed from the band.”
Technical flexibility for commercial operators to locate high-powered base stations in either frequency range may result in one carrier’s base transmitter operating in spectrum directly adjacent to another carrier’s base receiver.
“This possibility should not be ignored by carriers preparing to bid millions in the upcoming auction,” Sharkey’s letter reads.
It means that what’s bad for public safety could be bad for the wireless carriers themselves.
‘Best Practices Guide’
Motorola, the Association of Public-Safety Communications Officials-International; Nextel, the Cellular Telecommunications & Internet Association and the Public Safety Wireless Network worked together to develop a “Best Practices Guide” to help those experiencing interference between wireless and public safety radio systems.
The guide provides a broad overview of practices that can be used to identify and alleviate interference between public safety systems and wireless systems, but it stops short of the recommendations made individually by Motorola.
A copy of the guide is available on the APCO Web site at www.apco911.org. Additional technical background can be found at: www.Motorola.com/cgiss/NA/contact/Interference%20Technical%20Appendix.pdf.
The Phoenix chief of police, Harold Hurtt, has circulated a letter asking for help in persuading the FCC and Congress to adopt Motorola’s recommendation. A copy of the letter appears on page 96.
The public safety community’s ability to protect life and property will suffer if it cannot use the 700MHz band as effectively as possible.
Bishop is editorial director.
