Citizens Broadband Radio Service (CBRS)

CBRS defines a strict priority structure for spectrum access, ensuring that higher-priority users are protected from interference by lower-priority ones:

• Incumbent Access (Tier 1): The highest priority, reserved for federal radar systems, Fixed Satellite Service earth stations, and grandfathered terrestrial wireless operations. These users receive absolute protection from all other tiers.
• Priority Access (Tier 2): Licensees who acquire spectrum via competitive bidding in specific geographic areas (PALs). They receive protection from General Authorized Access users but must not interfere with incumbents.
• General Authorized Access (Tier 3): Open, unlicensed access available to any FCC-certified device, similar to Wi-Fi. GAA users must accept interference from all higher tiers and coordinate among themselves.

The ESC is a dedicated network of RF sensors deployed along coastlines to detect the presence of incumbent federal shipborne radar systems operating in the 3550-3650 MHz sub-band.

• Real-Time Detection: ESCs continuously monitor for specific radar signatures and immediately notify the SAS when an incumbent is active.
• Dynamic Protection Zones: Upon detection, the SAS dynamically computes and activates a moving exclusion zone along the coast, temporarily suspending commercial operations on affected channels.
• Sensor Network: A network of professionally installed, calibrated sensors ensures reliable detection, as individual CBSDs cannot be trusted to reliably sense low-power radar signals.

PALs represent the middle tier of the CBRS framework, providing guaranteed, interference-protected access for entities requiring predictable quality of service.

• Auction-Based Allocation: PALs are assigned via FCC-administered competitive bidding within defined census tracts, with a maximum of 7 PALs available per area.
• 10 MHz Channels: Each PAL grants exclusive use of a single 10 MHz unpaired channel within the 3550-3650 MHz portion of the band.
• Renewable Terms: Licenses are granted for 10-year terms with a renewal expectancy, encouraging long-term investment in private network infrastructure.
• Secondary Market Trading: Licensees can lease or transfer their PALs in a secondary market, enabling flexible and efficient spectrum utilization.

A CBSD is any radio transmitter that operates in the CBRS band under the control of a SAS. They are categorized into two classes based on power and deployment:

• Category A CBSD: Lower-power devices for indoor or outdoor small cell deployments, limited to 30 dBm EIRP per 10 MHz. These are the workhorses of private LTE/5G networks.
• Category B CBSD: Higher-power devices for outdoor macrocell deployments, limited to 47 dBm EIRP per 10 MHz. They require professional installation and must report their precise geolocation to the SAS.
• SAS Client Protocol: All CBSDs must implement the WInnForum standardized protocol to communicate with the SAS, including registration, spectrum inquiry, grant request, and heartbeat procedures.

The CBRS framework employs a multi-layered defense-in-depth strategy to guarantee that federal incumbents, particularly naval radar systems, are never subjected to harmful interference:

• Static Exclusion Zones: Permanent geographic areas around known, fixed incumbent sites where commercial operations are prohibited or severely restricted.
• Dynamic Protection Areas (DPAs): Pre-defined coastal regions that are activated into temporary exclusion zones by the SAS upon ESC detection of shipborne radar.
• Propagation Modeling: The SAS uses sophisticated terrain-aware propagation models (e.g., Irregular Terrain Model) to calculate precise interference contours, rather than relying on simple distance-based rules.
• Grandfathered Wireless Protection: Existing 3650-3700 MHz licensees are protected for a defined transition period, after which they must migrate to the CBRS framework.

The hierarchical access model that defines CBRS. Incumbent Access users (federal radar, satellite) receive absolute protection. Priority Access licensees obtain interference-free spectrum via auction. General Authorized Access users opportunistically use remaining spectrum.

• Tier 1: Incumbent Access (highest priority)
• Tier 2: Priority Access Licenses (PALs)
• Tier 3: General Authorized Access (GAA, lowest priority)

A dedicated network of sensors that detects federal incumbent radar operations in coastal areas. The ESC triggers immediate evacuation of the 3550-3650 MHz band by lower-tier users.

• Latency: Must detect and notify SAS within seconds
• Deployment: Primarily along US coastlines to protect naval radar
• Alternative: Inland areas use propagation models and exclusion zones instead of physical sensors

The primary enterprise use case for CBRS GAA spectrum. Organizations deploy their own localized 5G networks without a mobile network operator license.

• Spectrum: Uses GAA tier for cost-free, localized coverage
• Applications: Industrial IoT, automated guided vehicles, smart warehouses
• Advantage: Deterministic latency and data sovereignty compared to Wi-Fi

CBRS represents a paradigm shift from exclusive, static spectrum assignments to dynamic, automated sharing. Traditional licensing grants a single entity permanent rights, while CBRS uses a dynamic protection zone model.

• Efficiency: Increases spectrum utilization by 10x compared to static allocation
• Risk: Lower-tier users have no interference protection guarantees
• Innovation: Lowers the barrier to entry for private network deployment