Spectrum Access System (SAS)

The SAS enforces a strict, three-tiered priority model for spectrum access, a foundational characteristic that differentiates CBRS from other sharing frameworks.

• Tier 1 - Incumbent Access: The highest priority, reserved for federal radar systems and Fixed Satellite Service (FSS) earth stations. The SAS must guarantee absolute protection from interference.
• Tier 2 - Priority Access License (PAL): Licensed users who purchase spectrum at auction for defined geographic areas. They receive protection from Tier 3 users and are protected from other PALs within their area.
• Tier 3 - General Authorized Access (GAA): The lowest priority, open to any FCC-authorized device. GAA users receive no interference protection and must opportunistically use any spectrum not claimed by higher tiers.

To protect critical Tier 1 federal incumbents, particularly naval radar systems operating along coastlines, the SAS integrates with a network of Environmental Sensing Capability (ESC) sensors.

• Function: ESCs are highly sensitive, dedicated RF sensors that detect the presence of federal radar signals in real-time.
• Mechanism: Upon detection, the ESC sends an alert to the SAS, which immediately calculates a Dynamic Protection Area (DPA) and instructs all lower-tier devices operating on the affected channels to vacate within seconds.
• Significance: This sensor-based approach is a core characteristic, enabling dynamic incumbent protection without requiring federal systems to share classified operational data.

The SAS functions as a centralized, cloud-based computation engine that models and manages co-channel interference across the entire band.

• Propagation Modeling: It uses advanced propagation models (e.g., Irregular Terrain Model) combined with high-resolution terrain data to predict signal path loss between all registered devices.
• Interference Analysis: Before authorizing a transmission, the SAS calculates the aggregate interference a new device would cause to all protected, higher-tier receivers.
• Power Authorization: Instead of a simple on/off grant, the SAS issues a specific maximum Effective Isotropic Radiated Power (EIRP) to each device, dynamically controlling its interference footprint.

The CBRS ecosystem is designed to support multiple, competing SAS vendors (e.g., Google, Federated Wireless, Amdocs). A core characteristic is the standardized protocol for inter-SAS communication.

• Purpose: To ensure a consistent, holistic view of the spectrum environment even when devices are managed by different SAS administrators.
• Mechanism: SAS instances periodically exchange information on registered devices, spectrum assignments, and interference protection requests using the SAS-SAS Protocol defined by the WInnForum standards.
• Outcome: This prevents a 'tragedy of the commons' scenario where one SAS's allocations could inadvertently cause interference to devices managed by another, ensuring a unified and fair coordination ecosystem.

All spectrum access grants are predicated on a device's precise location. The SAS operates fundamentally as a sophisticated, policy-enforcing geolocation database.

• Registration: Every CBSD (Citizens Broadband Radio Service Device) must register with the SAS, providing its exact geographic coordinates, antenna height, and equipment class.
• Policy Enforcement: The SAS cross-references the device's location against static exclusion zones (e.g., FSS earth station protection zones) and dynamic protection areas triggered by ESC sensors.
• Grant Process: A spectrum grant is only issued if the device's location and requested power level are determined to not cause harmful interference to any protected entity in that specific geography.

The SAS's role is not a one-time authorization but a continuous, closed-loop management cycle for every connected device.

• Heartbeat Mechanism: CBSDs must maintain a periodic heartbeat with the SAS, confirming their continued operation and location. A missed heartbeat immediately invalidates the spectrum grant.
• Spectrum Relinquishment: Upon receiving an incumbent detection alert, the SAS sends a relinquishment command, and the CBSD must cease transmission on the specified channel within a strict timeframe (e.g., 60 seconds).
• Grant Renewal: Grants have a finite lifetime (e.g., 300 seconds) and must be periodically renewed, allowing the SAS to continuously re-optimize the spectrum allocation based on the latest environmental and usage data.