The Citizens Broadband Radio Service (CBRS) is a shared spectrum in the 3.5 GHz band (3550-3700 MHz) in the United States, designed to enable efficient use of mid-band spectrum for wireless broadband and other applications. Learn More.
The CBRS framework allows for dynamic, hierarchical spectrum sharing among three levels of users to maximize spectrum access and minimize interference. Here’s an overview of the frequency allocation in CBRS and the role of the heartbeat in maintaining the system’s functionality:
1. CBRS Frequency Allocation and Access Tiers
The CBRS band operates with three tiers of users:
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Incumbent Access: This tier includes federal/government users, such as the U.S. Navy, and other pre-existing users, like satellite ground stations, who have priority access to the band. They can operate without interference from other users, and the SAS protects this tier by dynamically managing other users when incumbents are active.
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Priority Access License (PAL): Licensed to specific users (such as wireless carriers) for exclusive use in specific geographic areas. PAL licenses are auctioned by the FCC, providing license holders with access to 10 MHz channels within the 3550-3650 MHz portion of the CBRS band. PALs must still defer to Incumbent users, ensuring they don’t interfere with Incumbent operations.
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General Authorized Access (GAA): This tier provides open access to any CBRS-compliant device (CBSD) without the need for a license. GAA users can operate in both the 3550-3650 MHz (if unassigned by PALs) and 3650-3700 MHz bands, but they must avoid interfering with PAL and Incumbent users.
2. Spectrum Access System (SAS)
The SAS is a cloud-based coordination and control system that manages access to the CBRS band across these tiers. The SAS dynamically assigns frequencies and power levels to CBSDs based on current spectrum conditions, ensuring non-interference with higher-priority users. Each CBSD must register with the SAS, which then authorizes it to transmit based on available frequencies and geographic location.
3. The Heartbeat Process
To maintain real-time management of CBRS frequencies and ensure compliance with Incumbent and PAL protections, the SAS and CBSDs engage in a heartbeat exchange. Here’s how the heartbeat works in CBRS:
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Periodic Authorization Check: The CBSD sends a heartbeat signal to the SAS at a defined interval (typically every 5-60 seconds), confirming its continued authorization to operate on a specific frequency and at a particular power level. This keeps the SAS informed of active devices and ensures they are not creating harmful interference.
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Dynamic Reallocation: If a higher-priority user, like an Incumbent, begins using the band within the CBSD’s operating area, the SAS will respond to the next heartbeat with instructions for the CBSD to change frequency, lower power, or cease operations entirely to protect the Incumbent’s use.
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Disconnection Detection: If the SAS stops receiving heartbeats from a CBSD, it assumes the device has disconnected or ceased transmission, and the frequency it was using can be reallocated. This minimizes unused spectrum and prevents interference with other devices.
The heartbeat system is central to the CBRS’s ability to share spectrum dynamically while protecting priority users and optimizing spectrum usage across all tiers.
4. Benefits of the CBRS Framework
- Efficient Spectrum Use: The tiered, SAS-coordinated approach allows multiple users to share the same frequencies effectively, maximizing the use of the valuable mid-band spectrum.
- Interference Management: Through the heartbeat mechanism and SAS oversight, the CBRS framework ensures priority users are protected from interference while allowing flexible access for others.
- Cost-Effective Expansion of Wireless Services: By allowing GAA access, the CBRS band enables smaller operators, enterprises, and new entrants to utilize high-quality spectrum for applications like private LTE/5G networks without having to buy spectrum at auction.
Contact us for help with spectrum allocation in Part 96 (CBRS) or any other FCC Radio Service, and for other wireless telecom legal issues.