Key Technical Considerations for Indoor-Only Authorizations

In authorizing low-power, indoor-only (LPI) unlicensed use in 6 GHz, as well as in Citizens Broadband Radio Service (CBRS) and some other proceedings, the Federal Communications Commission (FCC) has had to weigh assumptions and trade-offs inherent in a number of factors impacting propagation and, ultimately, the risk of harmful interference to other licensees or band incumbents (e.g., U.S. Navy radar in CBRS). These factors are typically interactive and include, most notably, maximum power levels, propagation loss (particularly building entry loss, or BEL, and clutter loss), antenna characteristics, height above average terrain, device form factors, and other operational restrictions. The FCC’s analysis in the 6 GHz Order considered a few additional factors (e.g., bandwidth mismatch) that added to or subtracted from the risk of an LPI signal causing harmful interference to a fixed microwave receiver. The primary considerations are discussed further in this chapter.

More generally, it’s important to realize that because propagation varies widely by frequency band, all of these factors will be of greater or lesser concern in different bands. As a generalization, the higher the frequency (e.g., above 24 GHz compared to below 3 GHz), the less likely it is that a signal will penetrate into (or out of) buildings or propagate over long distances (as even air and water molecules attenuate millimeter wave transmissions).

As the outcome and ongoing debate over the maximum allowable LPI power level in 6 GHz suggests, the nature of the primary outdoor use—in particular the incumbent’s vulnerability to interference—is the starting point. For example, if the primary (protected) use is satellite uplink, it’s very likely that the maximum power level for indoor-only use can be quite a bit higher than the FCC adopted to protect terrestrial fixed link receivers (which can be almost anywhere) from LPI unlicensed use.

In the 6 GHz Order, after examining studies submitted by both proponents and opponents of LPI,¹ the FCC relied primarily on a probabilistic analysis (based on data taken from 500,000 Wi-Fi access points) demonstrating that at a power level of 8 dBm power spectral density, LPI Wi-Fi access points would rarely emit outdoors at an interference threshold (signal to noise ratio) that exceeds “the conservative -6 dB I/N threshold” suggested by the fixed link incumbents.² Notably, the FCC stated that it was “not making a determination that any signal received with an I/N greater than -6 dB would constitute ‘harmful interference.’”³ Rather, it simply concluded that the LPI power limit it adopted would clearly protect all incumbents in the band. The DC Circuit upheld this approach.⁴ And, perhaps understandably, the Commission took extra caution because many of the incumbent fixed links are used for public safety and critical infrastructure.

The allowed power level factors in the degree to which the indoor-only signal is attenuated by the structure, called the building entry loss (BEL), as well as a potential allowance for ground clutter (see below). The Monte Carlo simulation analysis the FCC relied on to authorize LPI used the traditional International Telecommunication Union (ITU) value for BEL, which is between 10 dB and 30 dB, weighted based on data indicating an assumed 70/30 percent distribution of traditionally constructed buildings and thermally efficient buildings.⁵ A recent academic study of actual outdoor signal leakage from 16,000 LPI hotspots at the University of Michigan found that this BEL is, if anything, overly conservative.⁶ Overall, the study concluded that only about 5 percent of LPI access points were detectable at all outside (detectable mainly near glass doors), but none at a level that seemed strong enough to risk harmful interference to incumbent fixed links.⁷

Clutter models take surrounding structures, foliage, and topography into account. A CableLabs simulation study assumed a statistically average amount of signal loss from ground clutter, based in part on assumptions about the typical location of access points in a building (e.g., they are rarely mounted in front of a window).⁸ In the 6 GHz Order, the FCC accepted this ground clutter loss as part of the probabilistic risk analysis and rejected “AT&T’s use of a free-space propagation model [that] ignores clutter that often surrounds the transmitter and receiver sites (and that may significantly reduce the risk of harmful interference).”⁹ However, it also did not accept as much loss from clutter as the technology company coalition proposed.¹⁰

More recently, ground clutter played a far more dramatic role when the National Telecommunications and Information Administration (NTIA) and Department of Defense (DoD) agreed that rather than assume line of sight between CBRS devices and Navy ships, the Spectrum Access Systems (SASs) could use a standard clutter model and assume that lower-power CBRS devices below 6 meters posed no risk of harmful interference to Navy radar operations. Accordingly, CBRS devices very close to ground level would never need to interrupt operations unless they were within a kilometer or two of the coastline. Along with other changes, the NTIA, DoD, and FCC announced in June 2024 that “CBRS 2.0” would allow far more commercial use due to a radical reduction in the size of the Dynamic Protection Areas that determine if CBRS devices must temporarily vacate one or more channels to avoid interfering with incumbent operations.¹¹

As described in a previous chapter, the authorization of LPI ultimately heavily relied on the FCC’s confidence that Wi-Fi access points would remain indoors and rarely transmit on a line-of-sight basis outdoors. Accordingly, the Commission imposed the three primary equipment-related restrictions on LPI access points that, together, ensure this intended outcome. These are (1) a prohibition on battery power, (2) a prohibition on weather-resistant containers or casings, and (3) a prohibition on external antennas or “the capability of connecting other antennas to the devices, which will prevent substituting higher gain directional antennas.”¹² Moreover, the FCC noted that “the non-continuous nature of the transmissions of the most widely used unlicensed systems today, like Wi-Fi, makes the occurrence of harmful interference even less likely.”¹³ That is, because Wi-Fi in the band is required to employ the Institute of Electrical and Electronics Engineers (IEEE) 802.11 contention-based protocol—which is a non-continuous, listen-before-talk protocol designed for sharing—the unlicensed devices will not transmit continuously (and generally have a very low activity factor).¹⁴

Implementation of dynamic sharing in the CBRS band introduced a professional installer program that could be an option in bands (like CBRS) where the FCC sees a heightened need to ensure indoor signals are not exceeding a particular interference threshold outdoors. As part of the CBRS ecosystem, more than 5,600 certified installers help to ensure the protection of incumbents in the band, which adds “belts and suspenders” to a license-by-rule framework coordinated by certified SASs.¹⁵