Spectrum Access System (SAS)

The SAS enforces a strict priority hierarchy to manage access to the 3.5 GHz band:

• Tier 1 - Incumbent Access: Absolute protection for federal radar systems (e.g., shipborne and ground-based) and Fixed Satellite Service (FSS) earth stations. The SAS must suspend all lower-tier operations within a Dynamic Protection Area (DPA) upon activation.
• Tier 2 - Priority Access License (PAL): Licensees who acquired spectrum via FCC auction receive interference protection from Tier 3 users within defined geographic boundaries. The SAS guarantees their Aggregate Interference Margin is never exceeded.
• Tier 3 - General Authorized Access (GAA): Opportunistic, unlicensed access for any FCC-certified device. GAA users receive no interference protection and must accept interference from PAL and incumbent users.

To protect dynamic federal incumbents that do not appear in static databases, the SAS integrates with a network of Environmental Sensing Capability (ESC) sensors deployed along coastlines. These highly sensitive RF sensors detect the specific signatures of naval radar systems in real-time. Upon detection, the ESC sends a trigger to the SAS, which calculates the affected Dynamic Protection Area (DPA) and immediately instructs all CBRS devices (CBSDs) within that zone to cease transmission on the contested channels, ensuring zero harmful interference to critical military operations.

The SAS functions as a centralized propagation modeling engine. It ingests detailed installation parameters from every Citizens Broadband Radio Service Device (CBSD), including geolocation, antenna height, and orientation. Using advanced terrain-aware propagation models like Irregular Terrain Model (ITM) or Longley-Rice, the SAS calculates the cumulative Aggregate Interference Margin at every protected contour. It then authorizes specific channels and maximum Effective Isotropic Radiated Power (EIRP) for each CBSD to ensure that the sum of all transmissions mathematically remains below the regulatory noise floor for incumbents.

Within the GAA tier, multiple network operators may compete for the same unlicensed spectrum. The SAS incorporates a logical Coexistence Manager (CxM) to ensure fair and efficient sharing. The CxM resolves conflicts between competing GAA users by applying algorithmic fairness doctrines such as Proportional Fairness Scheduling. This ensures that no single operator can monopolize the band, balancing total network throughput with a minimum guaranteed quality of service for all connected users within the same geographic interference neighborhood.

The SAS operates under a strict, multi-layered security protocol mandated by the FCC's Wireless Innovation Forum (WInnForum) standards. All communication between a CBSD and the SAS occurs over HTTPS with mutual TLS/SSL authentication using device-specific certificates. The SAS must verify the identity of every CBSD, validate its installation parameters against a professional installer database, and detect fraudulent or misconfigured devices. This chain of trust prevents rogue devices from spoofing locations or exceeding power limits to disrupt protected incumbents.

A CBSD cannot transmit autonomously; it must maintain a persistent, transactional relationship with the SAS. The process involves two key messages:

• Spectrum Inquiry: The CBSD requests a list of available frequencies and maximum permissible power levels based on its location.
• Grant Request: The CBSD requests formal authorization for a specific channel.
• Heartbeat: The CBSD must send a periodic heartbeat message (often every 60-240 seconds). If the heartbeat fails, the grant expires, and the device must immediately cease transmission. This ensures the SAS retains positive control over the spectrum at all times.

A network of dedicated RF sensors deployed along coastlines that detects federal incumbent radar emissions in real time. The ESC feeds detection events to the SAS, which then activates Dynamic Protection Areas (DPAs) to immediately suspend or power-down CBRS transmissions that could interfere with naval radar operations.

• Operates independently from the SAS but provides critical incumbent detection data
• Required because federal incumbents do not communicate their presence to the SAS directly
• Detection-to-suspension latency must meet strict regulatory timelines

A predefined geographic zone activated by the SAS upon receiving an incumbent detection notification from an ESC sensor. When a DPA is activated, all CBRS devices operating within its boundaries must cease transmission or reduce power within a mandated timeframe to protect federal radar systems from aggregate interference.

• DPAs are static in definition but dynamically activated and deactivated
• Multiple DPAs may be activated simultaneously across coastal regions
• The SAS calculates which specific CBSDs must suspend based on propagation models

A renewable, non-exclusive license granted via FCC auction within the CBRS framework. PAL holders receive guaranteed interference protection from GAA users within a census tract for a three-year term. The SAS enforces this protection by managing channel assignments and power levels for all CBSDs in the area.

• Each PAL grants a 10 MHz channel within the 3550–3650 MHz sub-band
• Up to 7 PALs may be issued per census tract
• PAL holders may use their spectrum or lease it through the SAS

A logical entity responsible for resolving interference conflicts among multiple GAA users operating within the same geographic area. The CxM implements fairness algorithms to allocate spectrum resources equitably when demand exceeds available bandwidth.

• May be integrated within the SAS or operate as a separate service
• Uses proportional fairness scheduling to balance throughput and equity
• Manages inter-network coexistence between different GAA operators

A calculated safety buffer representing the total allowable interference from all secondary users at an incumbent receiver. The SAS uses propagation models and device registration data to ensure that the cumulative emissions from all active CBSDs do not exceed this threshold at any protected location.

• Expressed as an I/N ratio (Interference-to-Noise) at the incumbent receiver
• The FCC specifies the maximum aggregate interference margin for each incumbent type
• Drives power allocation and channel exclusion decisions across the entire SAS coverage area