The number of 800 MHz interference cases reported has decreased significantly since 2011 because of 800 MHz reconfiguration. However, new rules may result in the resurgence of some interference in the spectrum environment. On March 24, 2017, the FCC released rule changes that allow commercial cellular system licensees to use higher overall power levels and employ power spectral density (PSD) metrics to assess those power levels.

A PSD approach describes the amount of effective radiated power (ERP) allowed per unit of bandwidth. Therefore, new broadband technologies that have a wider spectrum bandwidth could deploy more power than 2G and 3G cellular deployments. Commercial wireless carriers requested these rule changes for the 800 MHz cellular band to facilitate the transition to broadband technology. Rules in other more recently allocated wireless commercial spectrum bands such as 700 MHz already incorporated such technical provisions to support broadband technology deployments from the outset.

Cellular Rule Changes
The cellular rule changes can result in a higher signal at ground level near an 800 MHz cellular site, especially at relatively low height sites. When an officer, firefighter or EMS professional is operating a mobile or portable radio in close proximity to an 800 MHz cellular site, interference could impact reception of the desired communications. Most often, any such interference is the result of the higher power at the ground level and the characteristics of the public-safety receivers. Receivers can generate intermodulation (IM) products — signals created from a mix of other signals. A receiver’s immunity to generating such IM products is known as its intermodulation rejection (IMR).

The FCC held a multistakeholder forum Nov. 6 to examine the potential interference issues and determine if there are steps that could improve spectrum sharing among public-safety entities and commercial carriers in the 800 MHz spectrum. One speaker at the forum presented measurements that showed current land mobile receivers exhibit different IMR characteristics when in the presence of strong undesired signals than when in the presence of weaker undesired signals. The current standard IMR measurement procedure was developed for weaker signal level environments normally found in LMR implementations. Given the more complex spectrum environment that has emerged, the IMR measurement procedure needs to be updated to include a strong signal intermodulation measurement (SSIM) methodology. SSIM is a key consideration when assessing radio performance in an environment that has become increasingly prone to higher cellular power levels.

In its March 2017 decision, the FCC maintained Section 22.970 – 22.973 of the rules that address resolution of any interference occurring between cellular and public-safety licensees. However, the FCC declined to adopt public-safety recommendations that commercial carriers found to have caused interference be required to reimburse public-safety entities for reasonable costs required to locate and mitigate the interference. As part of implementing these new broadband power levels at 800 MHz, some carriers voluntarily offered to conduct testing with public-safety agencies.

Mitigating Interference
Public-safety organizations should consider a few things when dealing with potential 800 MHz interference.

If selected for testing, what testing approaches should a public-safety agency request of an 800 MHz cellular carrier to assess the potential for interference from broadband cellular deployments?

Each public-safety licensee must determine its individual requirements. Following are some basic recommendations. First, ensure testing is conducted in fringe LMR coverage areas as well as strong LMR coverage areas. Cellular carriers will need to install Long Term Evolution (LTE) sites in both types of areas. Second, ensure testing is conducted near an LTE cell site, as well as at the fringe of an LTE cell site coverage area. Third, testing should be conducted at the maximum power level allowed. There is no use in testing LTE at lower power levels than those likely to be used in implementing the system. Fourth, test multiple LMR subscriber options, including mobiles and portables, subscribers from different vendors, both current and older subscribers still in operation, and subscribers with RF automatic gain control (AGC) turned on and turned off. Finally, document the test approaches and the findings.

If an 800 MHz licensee experiences interference to its operations that appears to be the result of nearby cellular signals, what should it do?

First, it is extremely important that the licensee report any interference to http://www.publicsafety800mhzinterference.com/CTIAWeb/, which was established to notify carriers of interference experienced. Public-safety, critical infrastructure industries (CII) and business/industrial land transportation (B/ILT) 800 MHz licensees can use this site to submit a report to local wireless carriers about a particular interference incident. Under the provisions of Section 22.972 of the FCC rules, such reporting starts a deadline clock.

Cellular licensees must respond no later than 24 hours after receipt of notification from a Part 90 public safety/CII licensee or 48 hours after receipt from other Part 90 licensees, provided affected communications on these systems are not safety related. According to the CTIA site, carriers operating within 5,000 feet of the reported interference will investigate, and those responsible for any harmful interference will take action to resolve it. Be aware that some interference could be the result of signals from multiple carriers, not just a single cellular system. Finally, recognize that the provisions of Sections 22.970 – 22.973 define parameters for unacceptable interference and minimum technical requirements for LMR receivers, as well as for carrier responsibilities regarding investigation and mitigation of the problem.

From a practical standpoint, investigating and resolving any 800 MHz cellular/LMR conflicts require a joint cooperative effort among the parties involved. Going forward, increased FCC and industry interest in spectrum sharing is likely to lead to the need for similar cooperation in other bands, so the spectrum road most traveled can be used effectively by all involved.

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Dave Buchanan, retired from San Bernardino County, California, serves as chair of the Southern California Region 5 700 MHz regional planning committee. He is also a local frequency advisor for the Association of Public-Safety Communications Officials (APCO) International. Buchanan has 47 years of experience in public-safety radio systems.

Stu Overby has 44 years of experience in spectrum management and communications. He serves as a member of the National Public Safety Telecommunications Council (NPSTC) support office. Email feedback to editor@RRMediaGroup.com.