Failure to communicate
When police officers cannot use their 800MHz radios to call for back-up and firefighters are unable to use their radios to communicate across short distances separating them, it brings attention to shortcomings in radio communications.
Sometimes the communications failures can be traced to inadequate system design. But when public safety radio communications fail because of interference from other radio communications systems, including commercial digital radio systems and cellular systems, interference resolution becomes a high priority.
“Commercial digital systems” usually means cellular-style networks operated by Nextel Communications, Reston, VA, and “cellular systems” are those operated on the A and B frequency bands above the 800MHz public safety frequency bands.
In April 2001, the Association of Public-Safety Communications Officials — International formed the Project 39 Committee with the mission to provide multiple solutions at six-month, 12-month and 18-month benchmarks. The committee first met in August 2001 at the APCO national conference. In December 2001, the committee filed an interim report with the FCC that detailed short-term solutions that have proven effective.
Long-term solution
Project 39 and Nextel are also seeking a long-term solution. In November 2001, Nextel asked the FCC to initiate a six-month proceeding to adopt new regulations that would reallocate radio spectrum among various users in a way that the company said could resolve the interference. Project 39’s goals include cataloging current interference within six months and identifying short-term solutions within 12 months and long-term solutions within 18 months.
The interim report detailed interference in 28 states. The chairperson, RoxAnn Brown, said that the committee is attempting to verify each and every interference report.
Brown consults the Washington County Consolidated Communications Agency in Beaverton, OR, where she served as the agency’s director for more than six years before resigning in December 2001. WCCCA has extensive experience with interference from Nextel. Its technical systems manager, Joe Kuran, can be credited with first identifying Nextel signals as an interference source.
Brown has praised Nextel for implementing an effective short-term solution for the benefit of WCCCA. Kuran has detailed in the interim report a modification to the Motorola MTS2000 radio that reduces its internally generated intermodulation interference.
Brown said that Nextel has reduced its cell site transmitter power in some locations by as much as 70% and has selectively removed channels it uses to eliminate interference to WCCCA system users. She said that WCCCA was surprised to learn that sideband noise from Nextel transmitters was not the primary cause of interference, nor was receiver overload. Receiver-generated intermodulation products account for most of the interference — although when it is eliminated, remaining sideband noise and receiver desensitization are sometimes evident.
Receiver-generated intermod
In the interim report, Kuran described the classic use of antenna input attenuation to confirm receiver-generated intermodulation products. Nextel has characterized a solution involving power reduction and channel use limitations as “short-term.” Kuran reported that Nextel limited its operations in this way at 200 sites in the WCCCA service area.
With Nextel’s exclusive contribution to receiver-generated intermodulation interference under control, WCCCA found that Nextel and A-band AT&T Wireless signals from cell sites within a quarter mile of each other mix in the public safety receivers to cause the same problem.
Brown said that AT&T Wireless initially seemed reluctant to negotiate a solution. But when she told the AT&T Wireless representative that Nextel’s connection with public safety radio interference had been profiled on the front page of USA Today and that “AT&T Wireless could be next; all of a sudden, they came to the table,” she said.
Kuran also found that substituting a seven-cent capacitor for a PIN diode in the MTS2000 reduces the receiver-generated interference by 7dB to 10dB, a significant improvement.
Yet receiver improvements do not seem to hold the key to resolving the interference. Kuran pointed out that the receiver meets high-quality standards with an intermodulation specification of 70dB. A higher specification would be impractical unless a user would submit to carrying an external battery pack to power a high-power injection oscillator in the receiver.
The prospect of shortened battery life overrules sharper filters that might narrow a front end designed to receive 851MHz-869MHz. The filter response doesn’t roll off much below 880MHz, leaving it more open to A-band cellular signals.
“The better the filter, the shorter the battery life. Public safety likes to see a 10-hour battery life,” Kuran said in the interim report.
Brown at PSWN
Speaking to an audience at the Public Safety Wireless Network conference in Charleston, SC, on Jan. 30, Brown identified some additional points involving the interference problem.
She acknowledged Nextel’s white paper that proposes a wholesale reallocation of public safety, commercial, business, industrial/land transportation and mobile satellite frequencies. She described APCO as “cautiously optimistic” and discounted published articles that said a public safety coalition supports the reallocation proposed in the white paper.
Brown said the public safety community has sought contiguous spectrum since the 1980s. She said the plan embodied in the white paper offers contiguous spectrum, but not necessarily in the right way or in the only way.
“We want the FCC to issue a Notice of Proposed Rulemaking, and then we can beat the heck out of it. As long as we merely write letters to the FCC about the interference, what do we get repaired? Not much,” she said.
Brown said that the APCO Web page that has been used to collect data related to the interference problem doesn’t ask all of the questions for which the Project 39 committee needs answers.
Ron Hasareth, APCO’s staff liaison to the committee, confirmed that lack of funding has hampered efforts to construct a new Web page. Brown said that APCO has spent about $52,000 so far on Project 39, despite the fact that APCO had not budgeted for the committee’s expenses.
“We’re off to a good start, and we’ve accomplished a lot in short time. We can do it in two years, but we have to fund it,” Brown said. “We don’t want this thing to drag on.”
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Positions on 800MHz interference resolution
Nextel Communications would move its systems to the NPSPAC frequencies; move public safety systems to the lower end of the 800MHz band, double its allocation, and pay $500 million of its expenses; change the status of business, industrial/land transportation and non-cellular SMR systems at 800MHz to “secondary” while offering them replacement spectrum at 700MHz and 900MHz with relocation at their own expense — and Nextel would receive 10MHz in the 2.1GHz mobile satellite band reallocated for cellular use.
Seven trade associations representing public safety organizations characterize Nextel’s proposal as a “major step in the right direction” and call on the FCC to give it “serious and expedited consideration.”
Daigneault Communications and the Licensing Assistance Office ask that the FCC put the Nextel proposal on public notice and then issue a Notice of Inquiry seeking comment. About Nextel’s desire to have a reallocation of frequencies codified in the FCC rules within six months, Daigneault and LAO suggest: “It is not clear, however, that such a hasty implementation would serve the public interest.”
Washington attorney Dennis C. Brown suggests that all 800MHz systems, including public safety, be reclassified as “commercial,” which would allow Nextel the opportunity to negotiate buy-outs, frequency swaps or other alternatives at prices acceptable to incumbents.
The National Association of Manufacturers and the Manufacturers Radio Frequency Advisory Committee prefer that public safety be given the lower end of the 800MHz band, business users the middle of the band and Nextel the upper end, to minimize frequency relocation expense.
Milbank Communications, Milbank, SD, urges the FCC to reject Nextel’s proposal, noting that Nextel has yet to complete a previous system relocation plan. Gene Johnsen asks that any solution “not result in the wholesale eviction of private wireless systems from the 800MHz band.”
United Airlines estimates it would cost $2 million or more to relocate its Denver International Airport radio system to the 700MHz or 900MHz band and points out that no suitable equipment is available for the 700MHz and that the 900MHz band is overcrowded.
Kay Communications, Sulphur, LA, says it would have to go out of business if faced with the $1 million expense of replacing its petrochemical and public safety customers’ radios, and questions Nextel’s ability to supply replacement spectrum in the 700MHz and 900MHz bands in the first place. “Nextel’s current licensing assignments in these bands are not available on a nationwide basis,” Kay’s letter to the FCC reads.
Taxi Equipment Company, Gardena, CA, echos the comments of Milbank, United Airlines and Kay, and places its system relocation expense at $2 million.
Pegasus Guard Band LLC, licensee of 34 guard band authorizations in the 700MHz band, despite having no discernable plan for using its frequencies, protests that Nextel’s plan would alter the equipment supply for the guard band and the demand for services.
Boeing puts its relocation cost at $50 million and, as a mobile satellite licensee, objects to a reallocation of mobile satellite frequencies to Nextel to use for cellular service. Boeing supports the NAM/MRFAC plan and forecasts the submission of other acceptable alternatives.
Motient, Reston, VA, says: “Incredibly, Nextel’s plan fails to address Motient and how the needs of its 250,000 customers will be met while essentially requiring Motient to pay the price — a very steep price — for the interference sins of others.”
