Introduction and Overview

This report discusses the potential to expand the concept of indoor-only rights to use available radio frequency spectrum on a non-interfering basis. More specifically, we explore how indoor-only underlays of unlicensed (or licensed by rule) use rights could promote spectrum access and innovation in many additional bands, thereby providing massive additional bandwidth for private networks and other forms of wireless innovation to a wide variety of enterprises and institutions. This will be increasingly important as spectrum grows more congested and as the overwhelming need for wireless data connectivity is indoors. In this respect, we suggest that the traditional policy of not distinguishing between outdoor and indoor spectrum use unnecessarily limits public access to the airwaves and fails to maximize the benefits of more intensive spectrum sharing and connectivity.

This concept is particularly relevant in the immediate future as the National Telecommunications and Information Administration (NTIA) and Federal Communications Commission (FCC) collaborate to implement the National Spectrum Strategy, which is premised on studying alternative means to expand private sector use of underutilized federal bands, particularly bands occupied by U.S. military systems.¹ The precedent of a low-power, indoor-only (LPI) underlay across the entire 6 GHz and the success of dynamic sharing at low power with military radar in Citizens Broadband Radio Service (CBRS) suggest that an indoor-only allocation should be at the forefront of the band studies that are just getting underway as part of the U.S National Spectrum Strategy. For example, in its initial 2023 study and report to Congress on the feasibility of sharing the lower 3 GHz band with the private sector, the Department of Defense stated that “even with stringent adherence to a coordination framework and associated conditions, spectrum sharing between federal and non-federal users in the 3100–3450 MHz band [would] remain challenging.”²

Similarly, large portions of the 7 GHz band currently used by sensitive military systems in the United States and in Europe (e.g., NATO operations in 7250–7750 MHz) may be far more conducive to private sector use indoors in locations, or at power levels, that will not be feasible for commercial use outdoors (and especially not for very wide area mobile networks). With these constraints in mind, LPI authorizations provide an additional way to expand on the “use it or share it” approach to spectrum sharing pioneered in CBRS, but without the need for control by a geolocation database (which requires scale, is costly, and is limited today to just a few frequency bands). Five federal bands that may be prime candidates for indoor-only authorizations—1,675 megahertz in total—are discussed in the concluding chapter.

Most spectrum licenses (including federal spectrum assignments) confer exclusive or at least primary rights to transmit on designated frequency bands at specific locations or over defined geographic areas, including inside factories, office buildings, and other facilities owned or controlled by others. This traditional form of licensing precludes property owners from using licensed spectrum inside their own facility even if the transmissions are contained and would not cause harmful interference to the primary licensee’s operations. Currently, only frequency bands designated for unlicensed use, under Part 15 of the FCC’s rules, allow the public to transmit inside buildings under their control. This leaves enormous wireless capacity fallow indoors, which is precisely where the need and demand for wireless connectivity is predominant and growing most rapidly.

Of course, the concept of an indoor-only underlay has a recent and powerful precedent: The FCC’s 2020 Report and Order authorized LPI use of 1,200 megahertz across the entire 6 GHz band (5925–7125 MHz) on an unlicensed basis.³ While the FCC also authorized unlicensed operations outdoors up to the maximum (standard) Part 15 power level, such use is subject to obtaining a list of available channels and permissible power levels from an FCC-certified Automated Frequency Coordination system designed to protect incumbent microwave fixed links from harmful interference. As described further in the next chapter, the FCC determined that by imposing low-power and form-factor restrictions on LPI access points (e.g., no battery power, no weatherized container, and no external antennas) it could reasonably ensure that indoor-only transmissions posed no significant risk of harmful interference to the higher-power fixed microwave, fixed satellite, and broadcast auxiliary services that operate almost entirely outdoors in one or more of the four 6 GHz sub-bands.⁴ Later in 2020, the FCC adopted the same indoor-only restrictions for the lower portion of the 5.9 GHz band (5850–5895 MHz) reallocated from the Intelligent Transportation Service to unlicensed use, along with a Further Notice of Proposed Rulemaking proposing to add an authorization of outdoor use at a later date.⁵

As the FCC ultimately concluded in the 6 GHz Order, at a sufficiently low-power level, incumbent licensed services operating outdoors in 6 GHz would be protected from harmful interference “due to significant building attenuation and clutter losses for transmissions originating from indoor devices.”⁶ Although the FCC’s focus was on protecting the outdoor operations of licensed incumbents from harmful interference, in practice, the LPI rights it authorized belong to whoever controls the venue. While most of us rarely stop to think about it, the transmission of wireless broadband connectivity indoors effectively requires permission to use the site, access to fixed backhaul (typically high-capacity wireline service), and electrical power. This is particularly true for LPI use of the 6 GHz band since the FCC requires that even client devices (e.g., smartphones) must operate at an even lower power and be under the control of a conforming indoor-only access point (router).

A related consideration is that building owners (or their lessees) are already completely free to use building materials, including the increasingly common energy-efficient windows (Low-E glass) that block (intentionally or not) most ordinary radio transmissions from entering (or escaping) the structure.⁷ A well-known example is the massive Amazon fulfillment centers, which run their army of autonomous robots to track inventory and do other tasks.⁸ Since they are forced to rely on unlicensed spectrum, Amazon’s motivation to turn its fulfillment centers into the enterprise equivalent of Faraday cages is to protect its own operations from external transmissions; yet under different rules, it could use additional bands while completely avoiding interference with other authorized services outside its walls.

Despite all this practical control over wireless within their structures, prior to the 6 GHz Order venue, owners were limited to using only those frequency bands authorized for unlicensed public use outdoors and indoors. There were few significant indoor/outdoor distinctions in rules governing spectrum use.⁹ Not even the 6 GHz Order explicitly recognizes that the authorization for indoor-only use of a band already licensed for other uses could be defined in relation to control of the structure and compliance with whatever restrictions are necessary to protect the primary operations beyond its four walls. In the 6 GHz Order, the FCC wisely took the more established and inclusive approach of authorizing LPI under the Part 15 rules for use by anyone, but with restrictions tailored to keep the devices (and potentially interfering transmissions) strictly indoors.

The comparison to other real property rights is made here not to advocate private “ownership” of spectrum rights indoors (which would, in any case, require an unlikely amendment to the Communications Act),¹⁰ but to highlight that the distinction between rights for indoor- and outdoor-shared use can be extended and generalized to many other bands, in part because of the ability to control indoor emissions. Whether the authorization is unlicensed under Part 15 (viz., available to anyone meeting specified technical requirements, as LPI is), adopted as part of a licensed-by-rule framework, or licensed exclusively to select categories of facilities (such as factories and schools), an indoor underlay of unlicensed (or licensed by rule) spectrum could promote spectrum access and innovation in many additional bands.

More immediately, the FCC recently requested comment on the option of conferring some form of priority indoor-only rights as part of its update of the rules for the Citizens Broadband Radio Service (CBRS). Before the Commission first adopted CBRS in 2015, it proposed giving priority indoor-only use rights to specific categories of what it called “Contained Access Facilities” (e.g., hospitals, critical infrastructure, and public safety users), but it ultimately did not include this approach. In its new Notice of Proposed Rulemaking—released August 16, 2024—the FCC is again seeking comment on the Contained Access Facility (CAF) concept, stating that “growing industry interest in private wireless networks may support a reassessment of how the Part 96 rules [that govern CBRS] treat low-power indoor operations.”¹¹ The agency asks whether it “should allow operators to reserve some amount of GAA [General Authorized Access] spectrum for private, low-power indoor operations—akin to the CAF approach.”¹² We discuss this further in a later chapter, where we question why any “reservation” in the GAA portion of the band is needed, particularly if the indoor-only user can certify that its out-of-building emissions are below a level that adequately protects primary U.S. Navy and Priority Access licensees.

As described by the FCC, the CAF concept also highlights how indoor-only use rights might vary from the unlicensed LPI approach authorized across the entire 6 GHz band. LPI use is predicated on power level and device restrictions (e.g., no battery power) that focus solely on protecting licensed operators outdoors.¹³ In contrast, the FCC’s concept of a CAF in CBRS seems predicated on identifying indoor-only use cases that could receive a degree of protection from other general authorized access users operating outside its walls. This is presumably possible in CBRS because all commercial users must receive permission to transmit by a geolocation database (Spectrum Access System) that knows the user’s precise location. But it also suggests a possible model where indoor use in certain bands is limited to users that meet specified qualifications (e.g., certified professional installation), register their location, and meet other requirements for non-interfering indoor use. This is discussed further in the key policy considerations chapter.

Because indoor-only use rights can and likely will vary based on a variety of factors, this chapter describes some of the key considerations that regulators should consider as they seek the right balance between opening additional spectrum for property holders and protecting other co-frequency users beyond those four walls from harmful interference. For example, controlling the spectrum inside a facility is more practical at higher frequencies, where transmissions do not readily penetrate building materials or travel long distances. Other considerations include the degree of signal attenuation (building and entry loss), device and operational restrictions, whether coordination by a geolocation database is appropriate, and a potential requirement to certify that the indoor-only transmissions do not exceed a particular threshold outdoors.

Finally, it is important to think about indoor-only authorizations as part of a larger trend toward enabling what we call a “use it or share it” approach to unlocking wireless communications capacity in underutilized bands. In a paper presented at the Research Conference on Communications, Information, and Internet Policy (TPRC) in 2021, one of this report’s authors documented how the FCC has increasingly taken this approach to both increase the intensity of spectrum use and to increase the availability of prime spectrum, at lower power, for a very diverse range of local users and use cases.¹⁴ In some cases, use-or-share authorizations have been enabled by dynamic spectrum coordination systems, most notably the CBRS three-tier spectrum sharing framework enabled by FCC-certified Spectrum Access Systems.¹⁵ In CBRS, not only do Priority Access License (PAL) and GAA users operate when and where the U.S. Navy is not operating its radar systems in this formerly exclusive military band, but GAA users can also operate on vacant PAL spectrum in local areas where the licensees have not commenced operations.

In contrast, LPI represents a different “use or share” approach that effectively unlocks all of the nation’s indoor 6 GHz spectrum for low-power use, a capacity that the long-time licensees in the band were not using. We believe that whether or not geolocation database coordination is appropriate, there is enormous untapped bandwidth inside all of our homes, businesses, schools, and other structures.