CBRS
Understanding CBRS
The Three-Tiered Access Framework
In the United States, the Federal Communications Commission (FCC) designated the 3.5 GHz band (3550 MHz to 3700 MHz) as Citizens Broadband Radio Service (CBRS). This band represents a significant departure from traditional spectrum management, which relies either on exclusive licensing (like mobile carriers) or unregulated open access (like Wi-Fi). Instead, CBRS utilizes a dynamic, three-tiered spectrum-sharing model coordinated by a centralized cloud database:
- Tier 1: Incumbent Access: Reserved for primary users, including federal military radar systems, particularly US Navy shipborne radars, and fixed satellite service earth stations. When these incumbents require spectrum, all lower-tier users must immediately vacate the channels.
- Tier 2: Priority Access License (PAL): Licenses are auctioned to commercial operators for 10 MHz channels in specific counties. PAL holders receive protection from lower-tier interference when active.
- Tier 3: General Authorized Access (GAA): Unlicensed commercial access that is free to use. GAA users must accept interference from Tier 1 and Tier 2, but are protected from other GAA users.
Spectrum Access System (SAS) Coordination
The core of CBRS coordination is the Spectrum Access System (SAS). The SAS is a dynamic cloud database that monitors all active CBRS base stations (called Citizens Broadband Radio Service Devices, or CBSDs). When a CBSD wishes to transmit, it must contact the SAS to request a channel allocation and maximum allowable transmit power. The SAS checks coordinates, terrain, and environmental databases, including Environmental Sensing Capability (ESC) sensors along the coasts, to ensure the transmission will not cause interference to military radars or PAL licensees.
Key Mathematical Relations
Technical Specifications Comparison
| Spectrum Tier | User Class | Priority Level | Interference Protection | Licensing Method |
|---|---|---|---|---|
| Tier 1: Incumbent | US Navy radar, Satellite Earth stations | Highest (1st) | Protected from all other users | Government allocation |
| Tier 2: PAL | Mobile carriers, utility companies, enterprises | Medium (2nd) | Protected from Tier 3; must protect Tier 1 | FCC Auction (10 MHz blocks) |
| Tier 3: GAA | Private networks, public hotspots, local WISP | Lowest (3rd) | No protection; must protect Tiers 1 and 2 | Open registration (no fee) |
Frequently Asked Questions
What is the role of the Environmental Sensing Capability (ESC) in CBRS?
The ESC is a network of physical RF sensors deployed along the US coastlines. These sensors detect military radar transmissions and alert the SAS, which immediately instructs nearby commercial CBSDs to vacate the active channels.
What are the 3GPP bands defined for CBRS?
In 3GPP standards, the CBRS band is designated as Band 48 for LTE and Band n48 for 5G NR, covering the full 3550 MHz to 3700 MHz range.
Can a CBRS GAA user obtain reliable service without a PAL license?
Yes. In areas away from military coasts and where no PAL licenses were purchased, GAA users can access up to 150 MHz of clean, SAS-coordinated spectrum, making it ideal for private enterprise networks.