Citizen Broadband Radio Service (CBRS)

CBRS implements a strict, automated priority structure to manage interference:

• Incumbent Access (Tier 1): The highest priority, reserved for federal radar systems (e.g., U.S. Navy) and Fixed Satellite Service (FSS) earth stations. All lower tiers must vacate frequencies upon detection of an incumbent.
• Priority Access License (Tier 2): Licenses acquired via competitive bidding, granting interference protection from GAA users within a defined census tract. PALs are renewable, non-exclusive, and limited to 70 MHz of the band.
• General Authorized Access (Tier 3): Unlicensed, opportunistic access to any spectrum not assigned to higher tiers. GAA users receive no interference protection and must accept interference from other GAA users.

The SAS is the automated, cloud-based brain of the CBRS ecosystem. It is a mandatory intermediary that authorizes every transmission:

• Incumbent Protection: Maintains a secure Environmental Sensing Capability (ESC) network to detect federal radar activity and instantly commands reconfiguration of the band.
• Dynamic Channel Assignment: Calculates propagation models and aggregate interference margins to assign frequencies and maximum power levels to CBSDs (Citizens Broadband Radio Service Devices) on a per-request basis.
• Coexistence Management: Resolves interference conflicts between multiple GAA users within the same geographic area, ensuring fair spectrum access without human intervention.

Citizens Broadband Radio Service Devices (CBSDs) are the physical radios that operate in the band. They are categorized by power and deployment type:

• Category A CBSD: Lower-power indoor or outdoor small cells (max 30 dBm/10 MHz EIRP). These are the primary devices for enterprise private networks and neutral host deployments.
• Category B CBSD: Higher-power outdoor base stations (max 47 dBm/10 MHz EIRP) requiring professional installation. They must report geolocation and antenna parameters to the SAS for precise interference modeling.
• End User Devices (EUDs): Consumer handsets and IoT sensors controlled by an authorized CBSD. They do not communicate directly with the SAS.

DPAs are predefined geographic zones that protect federal incumbent radar systems from aggregate interference. When a naval radar is active:

• Activation: The SAS receives a trigger from the ESC network and activates the relevant DPAs along the coastline.
• Suspension: All CBSDs operating on frequencies overlapping the incumbent's mission within the DPA must cease transmission within a mandated timeframe (typically 300 seconds for coastal DPAs).
• Granularity: DPAs are not monolithic. The SAS calculates interference on a per-CBSD basis, potentially allowing some devices to continue operating at reduced power if they do not contribute to the aggregate interference budget.

Priority Access Licenses are the mechanism for guaranteed quality of service within the shared band. Key characteristics include:

• Geographic Granularity: PALs are defined by census tracts, allowing highly targeted deployments for industrial campuses or specific enterprise facilities.
• Auction Mechanics: The FCC conducted an auction (Auction 105) using a clock auction format, assigning 7 PALs per license area, each comprising a 10 MHz unpaired channel.
• Renewability and Build-Out: Licenses are renewable, but licensees must demonstrate substantial build-out to retain priority rights, preventing spectrum warehousing and ensuring productive use.

When multiple GAA users compete for the same spectrum, a Coexistence Manager (CxM) ensures fair and efficient sharing. The CxM is a logical function, often integrated within a SAS:

• Interference Resolution Group (IRG): A set of CBSDs that mutually interfere and must be jointly managed. The CxM forms IRGs based on propagation models and interference thresholds.
• Fairness Algorithms: Implements channel assignment strategies like proportional fairness scheduling to balance total network throughput against individual user starvation, preventing a single aggressive network from dominating the band.
• Multi-SAS Coordination: In areas served by multiple SAS administrators, CxMs must exchange information to manage inter-SAS interference, ensuring a consistent and coherent spectrum picture.

A renewable, non-exclusive license awarded through FCC Auction 105 that guarantees interference protection from GAA users within a defined census tract. Each PAL grants a 10 MHz channel for a 10-year term, with a maximum of 7 PALs per license area.

• Licensees may aggregate up to 40 MHz of PAL spectrum in any area
• Must vacate or reduce power when a Dynamic Protection Area (DPA) is activated
• Enables enterprise private 5G deployments with predictable quality of service

A predefined geographic zone activated by the SAS when a federal incumbent radar system is operating. Upon activation, all CBRS devices (CBSDs) within the DPA must cease transmission or reduce power within 60 seconds to prevent aggregate interference.

• DPAs cover coastal and inland military operational areas
• Activation is triggered by Environmental Sensing Capability (ESC) networks
• Represents the core mechanism protecting tier 1 incumbents from harmful interference

The unlicensed, opportunistic tier of CBRS that allows any FCC-certified device to transmit on available spectrum without interference protection. GAA users must yield to both incumbents and PAL holders.

• Managed by a Coexistence Manager (CxM) to resolve conflicts among GAA users
• Ideal for neutral host networks, industrial IoT, and fixed wireless access
• Access is granted dynamically based on real-time SAS calculations

A network of RF sensors deployed along coastlines that detect federal shipborne radar systems operating in the 3.5 GHz band. When radar signals exceed a detection threshold, the ESC notifies the SAS to activate the corresponding DPA.

• Provides the sensing layer for incumbent protection
• Complements the SAS geolocation database approach
• Operated by FCC-approved ESC operators under strict performance requirements

Any radio transmitter operating in the CBRS band under SAS authorization. CBSDs are categorized into two classes:

• Category A: Lower power (max 30 dBm/10 MHz EIRP) for indoor/small cell use
• Category B: Higher power (max 47 dBm/10 MHz EIRP) for outdoor deployments, requires professional installation and geolocation reporting

All CBSDs must register with a SAS and receive a grant before transmitting.