Public-Safety Entities Outline 800 MHz Interference Concerns in FCC Filings
Tuesday, January 05, 2016 | Comments

Several recent FCC filings are detailing problems with interference between 800 MHz public-safety systems and commercial networks, but public-safety entities offered different suggestions on the best way to address the problems.

In November 2014, the FCC released a report and order and further notice of proposed rulemaking that amended the commission’s rules for cellular services. The FCC also proposed to supplement its existing effective radiated power (ERP) limits for the cellular service with alternative limits based on power spectral density (PSD). The commission also proposed to increase cellular power levels in rural areas and to potentially incorporate power flux density (PFD) limits near cellular base stations.

The FCC requested comment, and the comment deadline of Feb. 20, 2015, is long past, but the FCC has not issued final rules. However, numerous ex parte filings specific to the WT 12-40 docket have been filed since the comment deadline, and several in recent months.

Specifically, during a November 2015 meeting with the FCC, Association of Public-Safety Communications Officials (APCO) International officials described reports of harmful interference to 800 MHz National Public Safety Planning Advisory Committee (NPSPAC) channels received from local government public-safety licensees. APCO said the interference appears to be originating from commercial cellular operations overloading the front-end portion of public-safety radios that remained following 800 MHz rebanding.

APCO told the FCC representatives that interference will continue to get worse with additional cellular deployments, including Long Term Evolution (LTE) technology, and that few radios are available that can address the overloading issue.

Regardless, in early December, the FCC granted a waiver request filed by AT&T to operate under a PSD model with increased power levels in certain Missouri counties covering four cellular markets. Last April, the National Public Safety Telecommunications Council (NPSTC) submitted comments to the FCC regarding AT&T’s waiver request and recommended the FCC address changes to cellular service power limit rules through the rulemaking proceeding and not through waiver requests.

In a December 2015 ex parte filing, Morton Leifer said Clarkstown, New York, has received a spate of new applications for LTE sites. Leifer, electronic communications specialist for the town of Clarkstown, also handles frequency coordination for Atlantic APCO.

He said the LTE expansions include 2,000 watts per sector per site ERP values. He said bench tests shows that the much stronger broadband signal would likely mask the town’s single-site public-safety signal completely.

The filing said that there are more than 30 cellular sites within the town of Clarkstown, many located near large public buildings. Each LTE permit issued by the town is likely to produce an additional dead spot for first responders. Leifer proposed a remedy.

He said an 800 MHz public-safety radio system’s ERP is limited and determined by its interference contours relative to the service contours of nearby incumbent co-channel and adjacent channel service contours. Therefore, the on-channel signal level of a two-way radio at an LTE site might likely be anywhere from -70 dBm to -100 dBm. The weaker the signal received, the more susceptible a mobile radio will be to the desense caused by the nearby strong LTE signal.

By measuring the received signal strength indication (RSSI) of the two-way radio, which may be displayed on its screen or available at its accessory plug, and performing an off-on test on the LTE sector, the sector parameters can be adjusted until it is determined that the two-way radio can reliably receive its dispatcher at that LTE site.

“The test is accomplished in the field with a minimum of test equipment and time required,” the filing said. “It would allow the LTE system to operate at the maximum power consistent with proper operation of the subscriber LMR.”

In a separate December filing from engineering firm Pericle Communications and Shulman Rogers said the issue is with PFD limits in the 800 MHz cellular band. A PFD limit should be verified through field measurement, when necessary.

Jay Jacobsmeyer, president of Pericle, said that while PSD is specified in units of bandwidth (e.g., watts per hertz), PFD is different. The purpose of specifying a PFD limit is to protect public-safety receivers from harmful interference in the 800 MHz bands. This harmful interference is created in the front end of the receiver where there is typically no selectivity over the entire ESMR band (862 –869 MHz) and very little selectivity over the 800 MHz cellular band (869 – 894 MHz). Jacobsmeyer said the dominant contributor to harmful intermodulation (IM) interference is the total power of the interferer, not its PSD. In other words, interferers with equal PSD do not cause equal harm.

Jacobsmeyer said there is no simple formula that would grant a wideband interferer — LTE — a higher allowed PFD than a narrowband interferer — GSM — without either compromising the public-safety receiver or applying such a conservative standard that it would be impractical for the wireless provider to implement.

“Creating complex tables showing the allowed PFD for every combination of interferers and every make and model of public-safety receiver is impractical and would prove impossible to enforce,” the filing said.

Instead, Jacobsmeyer proposed amended regulatory language that should satisfy the majority of interference scenarios, prove easy to implement and enforce, and minimize complaints of harmful interference. The language is available here.

In a February 2015 filing, NPSTC said the FCC should adopt a PFD limit to help control the ground-level signal in the vicinity of cellular towers, but it didn’t recommend a specific PFD value.

A December filing by Gogo, a provider of in-flight broadband to airline passengers using the 800 MHz band air-to-ground (ATG) spectrum, said the FCC should consider and mitigate the risk of significantly increased interference that adoption of higher power levels through PSD limits in the cellular band could pose to the neighboring 800 MHz ATG service.

At least two more public-safety entities are expected to file notices of ex parte in this docket in coming weeks.

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