Introduction

The Federal Communications Commission (FCC) has recently expanded the use of the 6 GHz band to include unlicensed devices, such as wireless routers [1] [2] [3] [4]. This decision aims to support the growing demand for wireless connectivity and the development of new technologies. However, this expansion has raised concerns about potential interference with licensed point-to-point microwave systems, which are critical for various applications, including telecommunications, public safety, and broadcasting.

The 6 GHz Band and Its Uses

The 6 GHz band, ranging from 5.925 to 7.125 GHz, is a valuable spectrum resource used for both licensed and unlicensed applications. Licensed point-to-point microwave systems operate within this band, providing reliable, high-capacity links for data transmission over long distances. These systems are essential for backhaul networks, connecting cell towers, and supporting emergency services.

Unlicensed Devices and Their Potential for Interference

Unlicensed devices, such as Wi-Fi 6E/7 routers, are now permitted to operate in the 6 GHz band under specific conditions set by the FCC. These devices are categorized into three types: standard power, low power indoor (LPI), and very low power (VLP) devices. Each type has different power limits and operational requirements to minimize interference with licensed services[5].  Low power indoor units are now available from a plethora of vendors, and can be found online for less than $200.

Standard power units can be operated outdoors strictly in conjunction with Automatic Frequency Coordination (AFC).  Low power indoor units do not require AFC and are similar in power, but much larger in bandwidth than older wireless routers.   Examples of VLP devices include wearables, smart home devices, IoT and portable media. 

Mechanism of Interference

Interference occurs when the signal from an unlicensed device disrupts the operation of a licensed microwave link.  Co-channel Interference occurs when an unlicensed device operates on the same frequency as a licensed microwave link, it can cause direct interference, leading to signal degradation.

Regulatory Mitigation Strategies

To address the potential for interference, the FCC has implemented several mitigation strategies:

1. Automated Frequency Coordination (AFC): Unlicensed devices must use AFC systems to dynamically select frequencies that minimize interference with licensed services. AFC systems rely on databases of licensed operations to ensure that unlicensed devices avoid occupied frequencies[5].
2. Power Limits: The FCC has set strict power limits for unlicensed devices to reduce their potential for interference. For example, VLP devices are limited to a maximum effective isotropic radiated power (EIRP) of 14 dBm[6].
3. Operational Restrictions: Unlicensed devices are subject to operational restrictions, such as indoor-only use for LPI devices and prohibition of fixed outdoor infrastructure for VLP devices[7].

Unlicensed devices can emit signals that overlap with the frequencies used by licensed point-to-point microwave systems. This overlap can cause signal degradation, reduced data transmission rates, and even complete communication failures in some cases[6].

Assuming the AFC implementations are compatible with licensed operating systems they will pose little risk to existing licensees.  VLP devices are also of limited risk to any specific systems but will increase the overall noise floor in the 6 GHz band.  Indoor low power units, however, given proximity to a licensed path, pose a real risk to licensed 6 GHz operators.

Licensed Operator Mitigation Strategies

The first step in dealing with potential interference from unlicensed sources is to assess where the risk of path degradation exists for all paths in a given network.  AT&T took the FCC to court [5] claiming the FCC did not adequately protect licensed 6 GHz fixed wireless receivers.  The Commission acknowledged that full compliance “will not prevent harmful interference under all circumstances”.  However, the Commission repeatedly explained that the Order makes the “potential for harmful interference to incumbent services operating in the 6 GHz band . . . insignificant.”  The degree of insignificance is very much in the eye of the beholder.

All licensed 6 GHz users must determine for themselves the risk unlicensed transmitters pose to their networks.  A study of the impact of the interference on a given network can be performed by Spatial Datalyst, and is accomplished through a combination of geospatial data, analytics and RF engineering principles. The results indicate which sites in a 6 GHz network are susceptible to interference and provide an estimate of the fade margin reduction at each location.  This allows the network operator to make an informed decision about implementation of a mitigation strategy.

Conclusion

The expansion of unlicensed use in the 6 GHz band presents both opportunities and challenges. While it supports the growing demand for wireless connectivity and innovation, it also raises concerns about interference with critical licensed point-to-point microwave systems. By implementing mitigation strategies such as AFC, power limits, and operational restrictions, the FCC aims to balance the needs of both unlicensed and licensed users, ensuring that the 6 GHz band can be used effectively and efficiently for all. The extent to which the licensed community is adequately protected by the FCC’s rules must be determined by the licensed user.

References

[1]: [FCC] REPORT AND ORDER AND FURTHER NOTICE OF PROPOSED RULEMAKING, Unlicensed Use of the 6 GHz Band, ET Docket No. 18-295, Expanding Flexible Use in Mid-Band Spectrum Between 3.7 and 24 GHz, GN Docket No. 17-183, Released: April 24, 2020.

2]: [FCC] SECOND REPORT AND ORDER, SECOND FURTHER NOTICE OF PROPOSED RULEMAKING, AND MEMORANDUM OPINION AND ORDER ON REMAND, Unlicensed Use of the 6 GHz Band, ET Docket No. 18-295, Expanding Flexible Use in Mid-Band Spectrum Between 3.7 and 24 GHz, GN Docket No. 17-183, Released: November 1, 2023.

[3]: [FCC] SECOND REPORT AND ORDER, SECOND FURTHER NOTICE OF PROPOSED RULEMAKING, AND MEMORANDUM OPINION AND ORDER ON REMAND, ERRATUM, Unlicensed Use of the 6 GHz Band, ET Docket No. 18-295, Expanding Flexible Use in Mid-Band Spectrum Between 3.7 and 24 GHz, GN Docket No. 17-183, Released: January 30, 2024

[4]: [FCC] THIRD REPORT AND ORDER, Unlicensed Use of the 6 GHz Band, ET Docket No. 18-295, Expanding Flexible Use in Mid-Band Spectrum Between 3.7 and 24 GHz, GN Docket No. 17-183, Released: December 13, 2024.

[5]: AT&T SERVICES, INC., vs FEDERAL COMMUNICATIONS COMMISSION AND UNITED STATES OF AMERICA, On Petitions for Review of an Order of the Federal Communications Commission, Argued before the United States Court of Appeals for the District of Columbia Circuit, No. 20-1190, page 9, Decided December 28, 2021

[6]: Federal Communications Commission. "Unlicensed Use of the 6 GHz Band; and Expanding Flexible Use in Mid-Band Spectrum Between 3.7 and 24 GHz." Federal Register, 2024.

[7]: Federal Communications Commission. "FCC FACT SHEET Unlicensed Use of the 6 GHz Band Third Report and Order." 2024.