FCC PUBLIC SAFETY PANEL STUDIES INTERFERENCE AT 700 MHZ
Proposal that policymakers examine possible changes in public safety equipment to better withstand interference from commercial users at 700 MHz got mixed reception at FCC’s Public Safety National Coordination Committee (NCC) meeting Fri. Glen Nash, pres. of Assn. of Public-Safety Communications Officials (APCO) and chmn. of NCC’s technology subcommittee, outlined scenario in which public safety users at 700 MHz could increase minimum desired signal level from 40 dBu that’s typical today to design signal level of around 50 dBu. Nash said at 2-day NCC meeting last week that he raised potential option for mitigating interference to public safety at 700 MHz from adjacent band users at request of FCC. Possible solution that would entail public safety operators’ raising their signals appeared designed to head off same kinds of interference problems that have beset those users at 800 MHz. But attendees raised concerns ranging from disproportionate cost burden that public safety would bear under that type of plan to how local govts. could address siting from increased radio sites. “This is not a simple answer, there are many parts of the equation,” Nash said.
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NCC agreed to hand off issue to Telecom Industry Assn. to conduct technical analysis of trade-offs to be made relative to increased signal level, Nash said. Answer could be that number between 40 and 50 dBu was most appropriate, he said. “We don’t know at this point.” Possible trade-offs include whether as signal level increases, operator must raises power output, which could create additional interference, he said. Other considerations include need to increase number of radio sites in some areas, Nash said. In past, NCC has addressed interference issues by making recommendations to commercial users to keep their signals into public safety spectrum “at very low levels,” Nash said.
At May 30 subcommittee meeting, one audience participant voiced concern about rural areas where additional site build- out could be particularly costly. “I think there are a lot of negative aspects to this, not the least of which is the cost to put in interference limited systems,” said Robert Schlieman, project manager in N.Y. State Office for Technology. “The fact that the commercial people do this and receive revenue to offset the cost of it doesn’t imply that government or public safety systems have those kinds of resources.”
FCC in March approved notice of proposed rulemaking examining ways to remedy interference problems for public safety at 800 MHz, including plan floated by Nextel that would reconfigure users at 700, 800, 900 MHz and 2.1 GHz bands. At 700 MHz, FCC already has allocated 24 MHz for public safety, although its availability is subject to same band-clearing efforts as commercial operators face in that spectrum concerning analog broadcasters who haven’t yet made DTV transition. At NCC technology subcommittee meeting Thurs., Nash said that to mitigate potential interference, either interfering signal of commercial operator can be reduced or so-called desired signal of public safety operator can be increased. FCC asked NCC to participants to examine idea of increasing desired signal to 50 or 52 dBu, he said. One issue that commercial users have raised in past is that public safety operators don’t always design systems with adequate signals to provide interference protection, Nash said. But he and others noted that one issue that still must be examined is potential benefits of raising signals from public safety equipment by 10 dBu if effect is simply to raise overall noise floor. “You are looking at increased signal levels, but in order to do that you're also looking at increased adjacent noise levels by that same amount,” Nash said at subcommittee meeting. “If everybody ramps up 10 dB nothing is accomplished. So potentially the advantage might be relative to the adjacent band users given some additional protection, but that’s kind of an unknown at this point.”
Wayne Leland, Motorola vp-dir. spectrum & standards, said possibility of raising signal strength of public safety systems came up at recent meeting at FCC at which company had proposed reducing out-of-band emissions of commercial carriers to level that would cause 3 dB rise in noise floor. Signal and receiver standards were discussed as other potential alternatives, he told subcommittee. “The Commission is looking for something in addition or instead of limiting commercial carriers,” Leland told subcommittee. “You can talk about the 10 dB noise floor but as I see it, it’s going to be there.” He said depending on what rules come out of FCC, public safety operators may have few other choices.
Concerns raised by public safety representatives at Thurs. subcommittee meeting and full NCC meeting Fri. included difficulty of siting additional transmitters and how to pay for changes. “It seems like it’s the cart before the horse,” said NCC Implementation Subcommittee Chmn. Ted Dempsey, formerly of N.Y.C. Police Dept. “Public safety again is going to be asked to improve their equipment, spend more money, build out more sites and better their systems because a commercial entity has caused interference to us,” he said. In N.Y.C., police department worked out interference problems experienced with Nextel on radios that police used at 470 MHz, Dempsey said. Problem was that Nextel’s emissions mask was greater than N.Y.P.D.’s receivers could handle, he said. “If we are asked to survive in the New York City RF environment and now we have to make a better portable radio,” cost would increase beyond $2,000 per conventional analog radio that Dept. pays now, Dempsey said. “I agree with having TIA look at it but I don’t agree with public safety having to make adjustments to our equipment to accommodate a commercial provider.”