Citizens Broadband Radio Service (CBRS)

CBRS defines a strict priority-based hierarchy for spectrum access in the 3550-3700 MHz band:

• Incumbent Access (Tier 1): The highest priority, reserved for authorized federal users (primarily U.S. Navy radar operations) and Fixed Satellite Service (FSS) earth stations. These users are protected from all interference.
• Priority Access (Tier 2): Licensed users who acquire Priority Access Licenses (PALs) through FCC auctions. Each PAL grants a 10 MHz channel within a census tract for a 10-year term, with up to 7 PALs available per area.
• General Authorized Access (Tier 3): Unlicensed, opportunistic access similar to Wi-Fi, available to any FCC-certified device on a non-interfering basis. GAA users share whatever spectrum remains after Incumbent and PAL allocations.

The Spectrum Access System is the automated, cloud-based brain of the CBRS ecosystem. It is a centralized spectrum coordinator that:

• Ingests real-time data from Environmental Sensing Capability (ESC) networks to detect incumbent federal radar activity along coastlines.
• Authorizes and revokes spectrum grants to Citizens Broadband Radio Service Devices (CBSDs) on a per-second basis.
• Enforces the hierarchy by dynamically reallocating spectrum: when a Navy radar is detected, the SAS immediately instructs PAL and GAA users to vacate the affected channels.
• Calculates interference protection contours using propagation models to ensure PAL licensees receive interference protection from GAA and other PAL users.

Multiple SAS administrators (e.g., Google, Federated Wireless, Amdocs) compete commercially but must interoperate through a standardized information-sharing protocol.

The Environmental Sensing Capability is a network of dedicated RF sensors deployed primarily along U.S. coastlines to detect federal incumbent radar operations in the 3550-3650 MHz band.

• Detection Requirement: ESCs must reliably detect radar signals at levels as low as -109 dBm/MHz at the sensor location.
• Coastal Focus: Because the primary incumbents are Navy shipborne radars, ESC sensors are concentrated near naval bases and littoral operating areas.
• SAS Integration: When an ESC network detects a radar signature, it sends an alert to the SAS, which triggers immediate channel revocation for CBSDs operating within the defined Dynamic Protection Area (DPA).
• Redundancy: Multiple ESC operators provide overlapping coverage to ensure no single point of failure in incumbent protection.

Citizens Broadband Radio Service Devices (CBSDs) are classified into two categories based on their maximum transmit power and deployment characteristics:

• Category A CBSD: Lower-power indoor or outdoor small cells with a maximum EIRP of 30 dBm/10 MHz (1 Watt). These are typical for enterprise private LTE/5G networks and indoor coverage. Category A devices are not required to be professionally installed.
• Category B CBSD: Higher-power outdoor infrastructure with a maximum EIRP of 47 dBm/10 MHz (50 Watts). These require professional installation and must report their precise geographic coordinates and antenna height to the SAS. Category B devices are subject to more stringent interference management rules.

Both categories must be FCC-certified and capable of receiving and executing SAS commands to change frequency or cease transmission.

Priority Access Licenses (PALs) are the commercial licensing mechanism within CBRS Tier 2, auctioned by the FCC on a county-by-county basis:

• Channel Size: Each PAL is a 10 MHz unpaired channel within the 3550-3650 MHz band.
• License Term: PALs are granted for a 10-year term with renewal expectancy.
• Census Tract Granularity: Licenses are defined at the census tract level, enabling highly localized spectrum rights. The U.S. contains approximately 74,000 census tracts.
• Cap Per Market: A maximum of 7 PALs (70 MHz total) can be awarded in any given license area, ensuring 80 MHz remains available for GAA use.
• Secondary Market: PAL holders may lease or transfer their spectrum rights through the SAS, enabling dynamic spectrum marketplaces.

Dynamic Protection Areas are pre-defined geographic zones along U.S. coastlines where federal incumbent radar operations may occur. DPAs are the operational mechanism for protecting Tier 1 users:

• Pre-Configured Zones: The FCC has defined approximately 72 DPAs covering coastal areas and select inland military operating zones.
• Activation Logic: When an ESC network detects radar activity, the SAS activates the corresponding DPA. All CBSDs operating within that DPA on the affected frequencies must vacate within 60 seconds.
• Granularity: A DPA may be partially activated (e.g., only certain channels) depending on the detected radar parameters, allowing non-interfering spectrum to remain available.
• Re-Activation Cooldown: After a DPA is deactivated, a mandatory waiting period prevents immediate reclamation to avoid thrashing and ensure stable network operation.

The automated frequency coordinator mandated by the FCC to manage the CBRS three-tier hierarchy. The SAS dynamically assigns channels and enforces protection rules by ingesting data from Environmental Sensing Capability sensors and incumbent databases. It calculates interference protection contours and authorizes or suspends transmissions from Citizens Broadband Radio Service Devices (CBSDs) in near real-time.

• Key Functions: Interference protection, channel assignment, power authorization
• Regulatory Basis: FCC Part 96 rules
• Operational Requirement: Must re-evaluate spectrum assignments every 300 seconds

A network of dedicated RF sensors deployed along coastlines that detect federal incumbent radar operations in the 3550-3650 MHz band. ESCs communicate real-time detection events to the SAS, which immediately commands all CBRS devices operating on affected channels to cease transmission. This sensor network is the critical mechanism that enables commercial sharing with high-power naval radar systems.

• Primary Mission: Protect DoD shipborne radar from interference
• Detection Latency: Sub-second notification to SAS
• Deployment: Concentrated in coastal geographic areas

The middle tier of the CBRS hierarchy, consisting of county-based, 10 MHz channel licenses auctioned by the FCC. PAL holders receive interference protection from General Authorized Access users but must yield to Incumbent Access operations. Licenses are awarded for 10-year terms and are renewable, providing a balance between licensed certainty and shared access.

• Spectrum: 70 MHz total available for PAL (3550-3650 MHz)
• License Unit: County-level geographic area
• Cap: Maximum 4 PALs per licensee per county

The lowest tier of the CBRS framework, providing unlicensed-like, opportunistic access to any spectrum not assigned to Incumbent or PAL users. GAA users operate under a listen-before-talk paradigm managed by the SAS and receive no interference protection. This tier enables innovation for private LTE/5G networks, industrial IoT, and neutral host deployments without the cost of licensed spectrum.

• Access Model: Opportunistic, unlicensed-like
• Protection: None from PAL or Incumbent users
• Use Cases: Private 5G, neutral host, industrial IoT

The physical radio transmitter operating in the CBRS band, classified as either Category A (lower power, indoor/outdoor) or Category B (higher power, outdoor only, professional installation). Every CBSD must register with a SAS, providing its geolocation, antenna height, and power class before receiving authorization to transmit. Category B devices require professional installation to ensure accurate location data.

• Category A: 30 dBm/10 MHz EIRP max
• Category B: 47 dBm/10 MHz EIRP max
• Requirement: GPS-disciplined geolocation reporting

The highest priority tier in the CBRS hierarchy, reserved for federal radar systems (primarily Navy shipborne SPN-43 and SPN-46 radars), Fixed Satellite Service earth stations, and grandfathered wireless broadband licensees. These users receive absolute interference protection. The entire sharing framework is architected around dynamically creating exclusion zones whenever and wherever these incumbents are actively transmitting.

• Protected Entities: DoD radar, FSS earth stations, grandfathered licensees
• Protection Mechanism: Dynamic Protection Areas (DPAs) computed by SAS
• Precedence: Absolute priority over all PAL and GAA operations