Spectrum Access System (SAS)

The SAS enforces a strict priority-based access model defined by 47 CFR Part 96:

• Incumbent Access (Tier 1): The highest priority, reserved for federal radar systems (e.g., shipborne SPN-43) and Fixed Satellite Service (FSS) earth stations. The SAS must guarantee absolute protection from interference.
• Priority Access (Tier 2): Licensed users, such as enterprises or mobile network operators, who acquire Priority Access Licenses (PALs) via FCC auction for defined geographic census tracts. PALs receive interference protection from General Authorized Access users.
• General Authorized Access (Tier 3): Unlicensed, opportunistic use open to any FCC-compliant device (CBSD). GAA users must not cause interference to higher tiers and must accept interference from PALs and incumbents.

Every CBSD must report its precise geolocation, antenna height, and orientation to the SAS. The SAS cross-references this data against a regulatory Geolocation Database containing the protected contours of Tier 1 incumbents. It then applies sophisticated Propagation Modeling (e.g., Irregular Terrain Model or Longley-Rice) to calculate path loss and aggregate interference. The SAS grants a transmission authorization only if the calculated interference at the incumbent's receiver remains below a strict regulatory threshold (e.g., -109 dBm/MHz for FSS).

A CBSD interacts with the SAS through two primary procedures:

• Spectrum Inquiry: A preliminary request where a CBSD asks the SAS for a list of potentially available frequencies before committing to a transmission. The SAS returns a list of channels and maximum permissible power levels.
• Heartbeat Protocol: A mandatory, periodic keep-alive signal. An authorized CBSD must send a heartbeat request to the SAS at least every 240 seconds. If the SAS fails to respond or revokes the grant (due to an incumbent appearing), the CBSD must cease transmission within 60 seconds. This ensures rapid reclamation of spectrum.

The SAS applies different regulatory rules based on the CBSD device class:

• Category A CBSD: A lower-power device with a maximum EIRP of 30 dBm/10 MHz. It requires no professional installation and is subject to less stringent out-of-band emission limits.
• Category B CBSD: A higher-power device with a maximum EIRP of 47 dBm/10 MHz. It must be professionally installed with a precisely registered antenna height and orientation. The SAS applies tighter protection constraints to Category B devices to prevent aggregate interference to incumbents.

A dedicated network of highly sensitive RF sensors deployed to detect federal incumbent radar systems—particularly naval shipborne radars—operating in the 3.5 GHz band. When an ESC sensor detects incumbent activity, it triggers an immediate spectrum evacuation order to the SAS, which then instructs all Citizens Broadband Radio Service Devices (CBSDs) in the affected area to vacate the channel within seconds.

• Detection threshold: -109 dBm/MHz for shipborne radars
• Response time: Sub-60 second reconfiguration mandate
• Deployment: Coastal ESC networks protect Navy operations near port areas

A regulatory-approved, queryable data repository containing the protected contours, operational parameters, antenna heights, and interference protection criteria of licensed incumbent users. The SAS queries this database to determine exclusion zones and protection areas before authorizing any secondary transmission.

• Stores FCC-licensed incumbents: Fixed Satellite Service earth stations, grandfathered wireless broadband licensees
• Defines protection contours using propagation models and terrain data
• Enables dynamic protection zones that activate only when incumbents are present

The mathematical prediction of radio wave path loss used by the SAS to calculate interference protection contours around incumbent receivers. The SAS employs terrain-sensitive models to determine whether a proposed CBSD transmission would exceed the permitted interference threshold at the incumbent's location.

• ITU-R P.452 and Longley-Rice (ITM) models commonly used
• Accounts for terrain diffraction, atmospheric ducting, and clutter loss
• Generates exclusion zones where secondary access is prohibited to guarantee zero harmful interference

Every Citizens Broadband Radio Service Device must register with the SAS before transmitting, providing its geolocation, antenna height, EIRP, and requested frequency range. The SAS validates the request against incumbent protection rules and grants or denies a spectrum grant. Registered CBSDs must maintain a periodic heartbeat signal—typically every 60 to 300 seconds—to confirm continued operation.

• Grant types: PAL (Priority Access License) and GAA (General Authorized Access)
• Heartbeat timeout: If heartbeat ceases, the SAS revokes the grant and frees the channel
• Interference coordination: SAS recalculates grants when new CBSDs register nearby

A defined geographic area surrounding a high-priority incumbent receiver where secondary transmissions are strictly prohibited to guarantee a zero-interference protection contour. The SAS dynamically activates and deactivates exclusion zones based on incumbent presence detected by ESC sensors or scheduled operations from the geolocation database.

• Dynamic nature: Naval radar exclusion zones shift with vessel movement
• Granularity: Zones can be as small as a single census tract or span coastal regions
• Enforcement: SAS revokes all CBSD grants within an activated exclusion zone

The standardized messaging interface defined by the WInnForum (Wireless Innovation Forum) that governs all communication between CBSDs and the SAS. The protocol defines the JSON-based message formats for registration, spectrum inquiry, grant requests, heartbeat, and relinquishment.

• WINNF-TS-0016: Technical specification for SAS-CBSD protocol
• RESTful HTTPS interface with mutual TLS authentication
• Multi-step handshake: Registration → Spectrum Inquiry → Grant Request → Heartbeat → Relinquishment