Even as 800 MHz rebanding finally winds down, 800 MHz interference issues continue to plague public-safety communications officials. New cellular carrier rules in the band will likely affect public-safety networks nationwide.

The importance of public-safety 800 MHz interference led the FCC to hold a full-day forum Nov. 6 on cellular and public safety 800 MHz coexistence. Earlier this year, the FCC released new rules for 800 MHz cellular services, adopting power spectral density (PSD) limits, an advance notification requirement at specified higher PSD levels, and a power flux density (PFD) limit that will apply for a seven-year transition period if the cellular licensee operates at PSD limits that exceed a certain threshold.

PSD describes the amount of effective radiated power (ERP) that would be allowed per unit of bandwidth from a base station antenna, such as 100 watts per megahertz. PFD describes the strength of signals at ground level in a given location.

The FCC said the rules take steps to protect public safety and other systems in the 800 MHz band from potential increased interference. As part of those steps, AT&T and Verizon agreed to test the rules’ new power levels in six small markets with public safety. If there are no interference issues, the carriers will roll out PSD in 10 more markets while also working with public safety, said David Pollard, AT&T principal radio access network (RAN) engineer.

Verizon has identified test markets, and it is contacting local public-safety officials for coordination of the tests, said Scott Townley, a technology fellow with Verizon Network Technolgy Strategy. The executive said the carrier would announce the markets after the FCC releases rules around the testing, which is expected soon, FCC officials said.

Most Long Term Evolution (LTE) services are deployed at 700 MHz. But as carriers roll out LTE services on 800 MHz frequencies, potentially increasing power levels, public-safety communications networks could be affected. Verizon has started rolling out 800 MHz LTE primarily in the Southeast with both A and B block licenses.

In 2016, AT&T and Verizon offered a plan to mitigate interference in markets where they transition to PSD operations. The carriers said they would provide adequate notice to public-safety agencies before transitioning any cellular market to PSD operations and would work closely with public safety after a PSD transition to quickly address any interference concerns.

John Kay from US Cellular said the carrier has deployed CDMA and LTE across its 800 MHz licenses but also said US Cellular has not had any interference complaints from public-safety agencies through the reporting system. AT&T’s Pollard said the 800 MHz band is not one of its top spectrum blocks for LTE deployment. “We’re not going to push LTE into 850 MHz,” he said.

Another factor in mitigating interference involves a major change during the past decade in two-way radio design, said Dennis Martinez, Harris chief technology officer (CTO). He said multiband radios brought a redesign of the radio front end with tunable switched preselector filters.

Jay Jacobsmeyer, president of engineering and consulting firm Pericle Communications, has tested many radios to determine how each product reduces interference. He said one interference solution is a band pass filter in the front end of the receiver.

“Only recently, as rebanding is completed, have vendors been able to add this filter,” he said. “Regardless of the mitigation methods used, manufacturers should publish their strong signal intermodulation performance in their product databases."

He also highlighted nonfilter interference solutions such as more robust low-noise amplifiers (LNA) and attenuators for automated gain control (AGC).

In fact, Jacobsmeyer and others said an interference test should be added to Telecommunications Industry Association (TIA)-603 standards. He said the test should not be pass/fail; users will want to see the data. “Some radios have great performance in this area, but the public-safety agencies don’t even know the manufacturer is offering it,” he said.

Dave Buchanan, representing the National Public Safety Telecommunications Council (NPSTC) and the Association of Public-Safety Communications Officials (APCO) International, said the groups could facilitate such standards work. “It’s hard for a public-safety agency — in absence of a standard — to put that into an RFP (request for proposals),” Buchanan said. “If you don’t have a standard to reference, it gets hard to do it and keep the purchase process clean.”

Harris’ Martinez suggested the Project 25 (P25) Compliance Assessment Program (CAP) might be the best method for the feature certification.

Both carrier and public-safety officials agreed that system design is also a key part of eliminating interference. “Let’s design a system to be interference tolerant, not just the equipment, but how the system is deployed,” Martinez said. “There is an education process that starts with the consulting community to help them design systems.”

Sprint engineer Keith Mathers said the carrier trains its engineers to be aware of public-safety systems and reduce power on sites closer to the ground, among other techniques.

Public-safety agencies should report any interference through the website established under the 800 MHz rebanding rules at http://www.publicsafety800mhzinterference.com.

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