Cooperating to solve spectrum interference
Last winter, the Washington County (OR) Consolidated Communications Agency (WCCCA) began experiencing interference with its radio system, putting officers from the Tigard Police Department in potentially dangerous situations. “If the officers were in the parking lot of their own police station, they couldn’t receive any transmissions at all,” says RoxAnn Brown, director of WCCCA. “If they were transporting a prisoner and had trouble, we couldn’t talk to them and they couldn’t talk to us. [The interference obstructed] their data transmission and their verbal transmission.”
When the department determined that a recently constructed cellular tower was causing the interference, the agency worked with the cellular carrier to reposition the antenna. Unfortunately, that caused more problems. “They tweaked the antennas at such an angle that it caused problems for a fire station,” Brown says. “That’s when we knew we had bigger problems.”
Situations like the one in Washington County occur across the country every day; however, spectrum interference in public safety used to be rare. Spectrum had few users before the growth of cell phone use, wireless data transfer, advanced paging and fleet location tracking. It also was more segregated by bandwidth than it is now. Although new technology — such as better receivers, stabler transmitters and mobile radio equipment — is developing to help public safety agencies use spectrum more efficiently, the agencies are increasingly experiencing problems with communications interference.
Most spectrum interference stems from the use of cellular telephones because many cellular carriers are licensed to use spectrum bandwidth that is close to the frequencies assigned to public safety. “There is a general lack of spectrum frequencies available for all users,” says Ron Haraseth, director of the Association of Public Safety Communications Officials International (APCO), Daytona Beach, FL. “Early regulations for the 800 band placed both types of users in close proximity [sic] to each other.” For example, WCCCA has an 800MHz trunked radio system that regularly competes with cell companies that are in the same band, Brown says.
Public safety departments often have large jurisdictions, encompassing both heavily populated urban areas and sparsely populated rural areas. To cover all areas of their jurisdictions as equally as possible, they must spread equipment out, rather than concentrate it in urban areas. “Trunked radio system technology, as used in [public safety agencies], is very expensive,” Haraseth says. “As a result, system planners attempt to cover the maximum amount of area and associated mobile vehicle and handheld units from each site.”
That was the case in Washington County. “We have a typical public safety mountain-top radio system [that was designed] to use few sites and cover a large area,” Brown says. “What happens is that [cellular companies] have a frequency, and we have a frequency; the frequencies come together and mirror each other, and our frequency is dead because [they have more power].”
Generally, wireless carriers have more money to invest in infrastructure and more discretion about where to concentrate it than do public safety agencies. Wireless carriers are able to concentrate a large amount of spectrum in urban areas, where public agencies have cut back. That leaves public safety responders unable to hear a signal coming from five miles away because of a stronger, competing wireless signal coming from a distance of mere blocks. And some systems were designed before the need for resistance to spectrum interference became so acute.
No one in the situation causes interference intentionally. Usually, both the manufacturers and the carriers are in compliance with the specifications and license limitations set for them by the Federal Communications Commission. For its part, the FCC, charged with making public safety a priority, has repeatedly shown its commitment to stopping spectrum interference.
The problem is that no one could have predicted that wireless technologies would take off the way they have. No one knew that some equipment regulations, specifications and other related practices would fall short when confronted with the unprecedented and growing demand for wireless technology. After all, the regulations were certainly capable of meeting the needs of a pre-wireless society.
“Part of the spectrum management issue comes about because the cellular industry was not required to go through the same sort of coordination effort that everybody else had to go through. [That effort] required that if you were going to license a radio or expand it, you had to coordinate those frequencies and make sure that they were not going to interfere with someone else,” Brown says. “With the cell industry, the FCC did not make them do that. [The FCC] allowed [the industry] carte blanche to go out and gather as many frequencies and build their systems as quickly as they could.”
Solving the problem
Although cellular carriers may be at the root of the spectrum interference problem, they often are helpful in correcting it. “Everyone wants to clear it up,” Brown says. “Most of the [cellular carriers] are pretty cooperative. They understand that this is a life safety issue for us, and … it is to their advantage from a business perspective to cooperate and try to make the problem go away.”
Correcting the problem takes time and effort from public safety agencies and private companies. WCCCA has worked with a local carrier for the past seven months to correct the problems experienced by its police and fire agencies. The carrier has reduced its power about 75% and reduced its channels from 14 to four or five, Brown says. “They can’t do that long-term and stay in business. We know that,” she notes. “We have to figure out the next step and move forward. Is it going to be fixed tomorrow? No, but we certainly are committed to [fixing the problem], and I can tell you that [the company] is committed. My hope is that the rest of the providers are committed to working with us as well.”
In an effort to alleviate interference, several agencies have developed a guide. “The ‘Best Practices Guide’ was a joint effort to better quantify the problem, [determine] the scope of the problem and provide a basic look at the mechanics, solutions and manifestations of the problem,” Haraseth says. “The [Best Practices Guide] was a direct result of a meeting called by the FCC [to address] complaints coming from public safety systems.” The guide identifies the following steps that public safety agencies and local wireless carriers can take to minimize interference.
Re-tune channels. Local wireless carriers can modify their channel deployment and/or channel reuse plans to re-tune commercial channels farther away from public safety operators’ channels. Increasing the channel separation to 1.5 MHz or more has been effective. “Since spectrum is so congested, the most likely method to increase frequency separation would require wholesale movement of many users, both public safety and commercial,” Haraseth says. “That would take close cooperation by public safety, the commercial users, as well as the regulatory sector — the FCC and possibly Congress.”
Modify power levels or antennas. Carriers can modify their transmission power levels, antenna heights or other antenna characteristics at sites that may have contributed to spectrum interference.
Increase base-station maintenance. Poorly operating equipment has been shown to exacerbate interference.
Improve the signal strength of the public safety communications system. That can be accomplished by adding base stations, increasing the power output of existing stations, improving antennas or upgrading mobile and handheld equipment to more interference-resistant models.
To finance those upgrades, agencies need to look at several solutions. “There is a general movement to consolidated systems,” Haraseth says. “New systems are capable of handling all users [like police, fire, EMS] in a shared system, reducing the total cost. There is also a trend to [use] more atypical financing such as creating independent agencies that are isolated and modeled along the lines of ‘joint powers of authority,’ much like [with] airports or public housing. Another trend is towards leased service systems where a service provider finances the system, installs and operates the system for a fee.”
Add filtration to the transmission site. If “sideband noise” (a by-product of transmission in which some energy is produced on frequencies above or below the licensed frequency) is contributing to the interference problem, filtration can be added at the transmission site to suppress sideband emission and reduce interference. “Filtering in the radio frequency (RF) environment is much like putting blinders on a horse,” Haraseth says. “It removes the annoying adjacent information.
“Filtering in RF is highly specific [to each] channel. Any change in one system will require a new filter or at least a re-tuning of existing equipment. Commercial systems tend to change their channel assignments to match changing dynamic customer demands. That can make solutions [to the spectrum interference problem] a changing target.”
Segregate public safety and commercial spectrum assignments. Receiving FCC permission to swap or reassign channels so that public safety frequencies are not co-located with commercial frequencies can often mitigate interference problems.
Encourage comprehensive advanced planning and frequency coordination. As in most things, prevention is better than cure.
Buy interference-resistant equipment. New public safety radio equipment should incorporate the resistance necessary for the equipment to function properly in RF-intense environments.
Make sure system design fits the environment. Public safety systems intended to function in RF-intense environments must be designed to a high degree of intensity. The design should include increased signal-strength generation throughout the service area. Not only will it compensate for adjacent-spectrum commercial entities, but it also will help deal with other sources of interference.
For more information about decreasing spectrum interference, consult “Best Practices Guide: Avoiding Interference Between Public Safety Wireless Communications Systems and Commercial Wireless Communications Systems at 800 MHz.” The report was published by APCO; Schaumburg, IL-based Motorola; Reston, VA-based Nextel Communications; Washington, DC-based Cellular Telecommunications and Internet Association; and Fairfax, VA-based Public Safety Wireless Network. The entire “Best Practices” report is available at APCO’s Web site, www.apco911.org.
Lyle Gallagher is a past president of APCO International, Daytona Beach, Fla.