February 14, 2022
National Spectrum Policy: Interference Issues in the 5G Context
Radio Spectrum as a National Resource
The President has the authority to assign frequencies for
Most wireless technologies, including mobile
federal use. Since 1978, this power has been delegated to
communications, radionavigation systems (e.g., the Global
the National Telecommunications and Information
Positioning System, GPS), radar systems, satellites, and
Administration (NTIA), an agency in the Department of
radio and television broadcasting, rely on a range of
Commerce. Federal users must obtain frequency
frequencies of electromagnetic radiation to transmit signals
assignments and comply with NTIA regulations and
and data. Radio spectrum is the continuum of frequencies
guidance. Its policy priority is to ensure a balance between
allocated for various radiocommunication services.
vital federal operations and commercial use.
Spectrum is an invisible, vital, and limited resource for the
NTIA has the statutory responsibility to present to the FCC
nation’s economic well-being, security, science, and safety.
the views of the executive branch on telecommunications. It
The Code of Federal Regulations specifies frequency
chairs and receives advice from the Interdepartment Radio
allocations between 8.3 kilohertz (kHz) and 275 gigahertz
Advisory Committee, which consists of representatives of
(GHz). This portion of the spectrum is divided into about
20 departments and agencies that use spectrum.
800 bands (or groups of frequencies) for over 30 types of
communication services, based on frequency
Coordination of Spectrum Policy
characteristics. Some frequencies can travel long distances
The FCC and NTIA coordinate spectrum allocations, which
and penetrate seawater, making them useful for land-to-
are not perpetual and may be reassigned. By statute (47
submarine communications. Other frequencies can travel a
U.S.C. §922), the agencies must meet regularly to conduct
few miles and penetrate obstacles (e.g., buildings), making
joint spectrum planning. They maintain a memorandum of
them useful for mobile communications.
understanding (MOU) setting terms of coordination.
Moreover, a frequency allocated primarily for federal use
Radio communication involves the transmission and
may be used for nonfederal purposes on a mixed-use basis
reception of signals by a radio system. Intended signals may
(subject to conditions). Over 90% of U.S. radio spectrum is
be interfered with by signals transmitted on the same
shared between federal and nonfederal users. The FCC and
frequency at the same time as well as by spill-over from
NTIA coordinate this sharing to avoid harmful interference
signals transmitted on adjacent frequencies. Spectrum use is
and resolve technical, procedural, and policy differences.
regulated and coordinated to minimize interference and
maximize its efficient and productive use.
Spectrum Repurposing
According to a 2011 Government Accountability Office
Regulatory Framework for Spectrum
report, over 60 federal users hold about 240,000 frequency
For cross-border radio operations and frequency
assignments for a variety of purposes, including air
allocations, the International Telecommunication Union
navigation and traffic control communications, national
(ITU), an agency of the United Nations, issues a collection
defense, land management, and law enforcement. To
of Radio Regulations (RR), an international agreement
address the growing demand for spectrum for wireless
reached by member nations, including the United States.
broadband technologies, including fifth-generation (5G)
Nations present and discuss spectrum allocation and use
telecommunications, Congress has directed NTIA to
positions at international meetings, including the ITU’s
identify federal frequencies that can be reallocated to the
World Radiocommunication Conference that revises and
FCC for nonfederal or shared use. As of 2020, NTIA and
adopts the RR. Nations also align their domestic regulations
the FCC had made a total of 1,131 megahertz of mid-band
with ITU regulations, but may adopt different policies that
(1-10 GHz) spectrum available for commercial 5G services.
align with their national interests and priorities.
Interference Issues
In the United States, the Communications Act of 1934, as
An emission that falls outside its intended frequency and
amended, assigned joint jurisdiction over spectrum
spills over into adjacent frequencies is an out-of-band
regulation to the Federal Communications Commission
emission (OOBE) and a major source of interference.
(FCC) and the President. The FCC regulates nonfederal
Common methods to mitigate OOBE interference are to
spectrum use. Commercial entities and state and local
create a guard band of unused frequencies between the
governments generally obtain a license from the agency to
intended and adjacent frequencies and to reduce the power
transmit on an allocated frequency. During the licensing
level of transmitters in adjacent frequencies.
process, the FCC’s primary consideration is “whether the
public interest, convenience, and necessity will be served.”
The FCC and NTIA use the term “harmful interference” as
defined in the ITU’s RR. The term indicates interference at
a level that “endangers the functioning of a radionavigation
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National Spectrum Policy: Interference Issues in the 5G Context
service or of other safety services or seriously degrades,
Aviation Altimeter Interference
obstructs, or repeatedly interrupts a radiocommunication
Radio altimeters are critical aircraft instruments that
service.” A challenge addressing potential harmful
provide precise height-above-terrain/water information.
interference is that users do not always agree on mitigation
Low altitude operations and flight guidance systems rely on
methods and power levels. This issue has drawn
radio altimeter inputs. Globally, governments and civil
congressional attention as the FCC has issued licenses for
aviation use the 4.2-4.4 GHz band exclusively for radio
reallocated spectrum, in some instances despite objections
altimeters. The Federal Aviation Administration (FAA)
by NTIA and affected agencies. Several cases highlighted
raised concerns in 2015 that 5G signals transmitted on
their competing or even conflicting positions on spectrum.
nearby frequencies could potentially interfere with radio
altimeters, particularly if high-powered 5G base stations
Weather Satellite Interference
were placed close to airport runways. In October 2020,
In May 2019, the FCC auctioned licenses for commercial
RTCA, an aviation technical advisory group, cautioned of
5G deployment. The auction made 2,909 licenses available
likely interference to current generation altimeters from 5G
in the 24 GHz band and raised more than $2 billion.
signals in the 3.7-3.98 GHz band. See CRS In Focus
Throughout the proceeding, the National Oceanic and
IF12028, Aviation Concerns Regarding the Rollout of 5G
Atmospheric Administration and the National Aeronautics
Wireless Telecommunications Networks, by Bart Elias.
and Space Administration raised concerns that 5G services
could cause harmful interference to the passive sensors
In February 2020, the FCC found the OOBE limits it set
onboard satellites that the agencies operate in the adjacent
and the 220 megahertz separation between 5G services in
23.8 GHz band for weather forecasting and earth science
the 3.7-3.98 GHz band and radio altimeter operations in the
research. Based on technical studies by the agencies, NTIA
4.2-4.4 GHz band should mitigate any significant
advocated for more stringent OOBE limits than those
interference. It concluded a spectrum auction in February
adopted by the FCC. The ITU later updated its standard
2021, making 5,684 licenses available and raising over $81
with more stringent OOBE limits in the 24 GHz band,
billion. The FCC asserted the auction was critical for
which the FCC subsequently was considering to adopt. For
implementing its 5G FAST Plan, which emphases quick
more information of the issue, see CRS Report R46416,
mid-band 5G buildout, and fulfilled its mandate under the
Forecasting Tropical Cyclones: Overview and Issues for
MOBILE NOW Act (P.L. 115-141, Division P, Title VI) to
Congress, by Eva Lipiec.
identify 100 megahertz below 6 GHz for 5G use. In January
2022, 5G carriers agreed to establish buffer zones near
GPS Interference
airports identified by FAA, where wireless carriers would
In April 2020, the FCC authorized Ligado Networks LLC
turn off transmitters for six months while FAA assesses
to deploy a terrestrial wireless network operating in a
altimeter performance in the 5G environment.
segment of the L-band (i.e., 1.5-1.6 GHz). The national
security community and GPS equipment makers raised
Improving Interagency Coordination
concerns that Ligado’s operations would interfere with GPS
The aforementioned interference disputes have relied on
signals in the 1559-1610 MHz band. Based on technical
technical studies submitted by interested parties and
analysis by its engineers, the FCC concluded that the
agencies, which the FCC analyzed before making decisions.
conditions in place (e.g., power levels, guard band, and
Affected agencies contend that the FCC has dismissed their
coordination requirements) would protect against
technical findings and their interference concerns.
interference, promote efficient use of spectrum, and support
5G deployment, and therefore the authorization was “in the
Congress may consider whether the current interagency
public interest.” See CRS In Focus IF11558, Spectrum
process is adequate for identifying potential harmful
Interference Issues: Ligado, the L-Band, and GPS, by Jill
interference and resolving differences. It may also consider
C. Gallagher, Alyssa K. King, and Clare Y. Cho and CRS
whether the process is effective to balance public and
Insight IN11400, DOD Concerns About the FCC-Approved
private sector interests in accelerating 5G deployment with
Ligado Network, by Kelley M. Sayler and John R. Hoehn.
protection of critical national functions. Several bills have
been introduced addressing spectrum coordination between
NTIA argued that Ligado’s network would interfere with
the FCC and NTIA. For example, S. 1472, the Improving
critical national security and public safety GPS use and that
Spectrum Coordination Act of 2021, would require the
the FCC failed to adequately weigh agency concerns. NTIA
agencies to update their MOU periodically and report to
disagreed with the FCC’s interference evaluation methods,
Congress annually on joint spectrum planning activities.
and countered that the FCC should use Department of
The update would (1) outline processes for addressing
Transportation (DOT) methods, which found a level of
differences in frequency allocation; (2) clarify NTIA’s role
interference high enough to endanger the reliability of GPS
in managing federal spectrum use; (3) take into account
devices. Pursuant to P.L. 116-283, the Department of
scientific analyses and implications of spectrum policy in
Defense contracted with the National Academies of
decisionmaking; and (4) ensure the efficient use of
Sciences, Engineering, and Medicine (NASEM) to conduct
frequencies assigned to the federal government and the
an independent technical review to compare FCC and DOT
reallocation of those frequencies not required for federal
methods and recommend which most effectively evaluates
use to nonfederal users. Another bill, H.R. 2501, the
interference risks to GPS devices. NASEM started the
Spectrum Coordination Act, would also require the FCC
review process in September 2021 and projected the
and NTIA to update the MOU to “improve the process for
duration of the review to be 15 months.
resolving frequency allocation disputes in shared or
adjacent bands ... expeditiously and efficiently.”
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National Spectrum Policy: Interference Issues in the 5G Context

IF12046
Ling Zhu, Analyst in Telecommunications Policy


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