Citizens Broadband Radio Service (CBRS)

The Spectrum Access System is the centralized, automated frequency coordination engine mandated by the FCC to manage the CBRS band. It acts as the policy enforcer and interference guardian.

Key operational responsibilities include:

• Incumbent Protection: Ingests data from Environmental Sensing Capability (ESC) networks to detect federal radar activity and dynamically reallocates spectrum away from affected channels.
• Channel Assignment: Processes spectrum inquiries from Citizens Broadband Radio Service Devices (CBSDs) and assigns available frequency channels based on tier priority and geographic separation.
• Interference Management: Calculates propagation models and enforces power limits to ensure co-channel and adjacent-channel interference thresholds are never exceeded.
• Database Coordination: Multiple SAS administrators (e.g., Google, Federated Wireless) must synchronize their databases to maintain a consistent view of spectrum occupancy across the entire band.

The automated frequency coordination engine mandated by the FCC to manage the CBRS band. The SAS dynamically assigns channels and enforces the three-tiered hierarchy by:

• Maintaining a database of incumbent federal and FSS earth station protection zones
• Authorizing Priority Access License (PAL) holders within their assigned geographic areas
• Granting opportunistic access to General Authorized Access (GAA) devices on a non-interfering basis
• Calculating interference protection contours using propagation models like Irregular Terrain Model (ITM)

The SAS re-evaluates spectrum assignments every 300 seconds to respond to dynamic incumbent activity, such as naval radar operations along coastlines.

The foundational regulatory architecture of CBRS that prioritizes spectrum access across three distinct tiers:

• Incumbent Access (Tier 1): The highest priority, reserved for federal radar systems (e.g., U.S. Navy shipborne and ground-based radars), Fixed Satellite Service (FSS) earth stations, and grandfathered wireless broadband licensees operating in the 3650-3700 MHz range for a limited time
• Priority Access (Tier 2): Licensees who acquire PALs via FCC auction (Auction 105) receive interference protection from GAA users within defined geographic census tracts for 10-year renewable terms, with up to 7 PALs per area (each 10 MHz)
• General Authorized Access (Tier 3): Opportunistic, unlicensed-like access available to any FCC-certified device on any channel not assigned to a higher tier, enabling private LTE/5G deployments without spectrum acquisition costs

A network of dedicated RF sensors deployed along U.S. coastlines that detects federal incumbent radar operations in the 3550-3650 MHz band in near real-time. Key operational characteristics:

• Detects specific radar signatures and communicates incumbent presence to the SAS within 5 seconds
• Triggers immediate channel reassignment, forcing PAL and GAA devices to vacate affected frequencies
• Enables dynamic protection zones that shrink and expand based on actual radar activity rather than static exclusion zones
• Operated by certified ESC operators (e.g., Federated Wireless, Google) who must meet stringent FCC detection probability and false alarm rate requirements

The ESC is the sensory layer that makes dynamic sharing with high-power naval radars technically feasible.

The industry consortium (now part of the OnGo Alliance, formerly CBRS Alliance) that established the OnGo certification program to ensure device interoperability and compliance with FCC Part 96 rules. The certification covers:

• CBSD (Citizens Broadband Radio Service Device) registration and SAS communication protocol conformance
• Adherence to CPC (CBSD Professional Installation) requirements for Category B devices with higher EIRP limits (up to 47 dBm/10 MHz)
• End-to-end network performance and coexistence testing in shared spectrum environments
• OnGo Certified devices are pre-validated for enterprise private network deployments, reducing integration risk for system integrators and end-users deploying private LTE/5G solutions in the 3.5 GHz band.

The FCC defines two device classes for CBRS transmitters, each with distinct power and deployment rules:

• Category A CBSD: Lower-power indoor or outdoor small cells with a maximum EIRP of 30 dBm/10 MHz (1 Watt per 10 MHz). No professional installation required. Suitable for enterprise indoor deployments.
• Category B CBSD: Higher-power outdoor base stations with a maximum EIRP of 47 dBm/10 MHz (50 Watts per 10 MHz). Requires professional installation (CPI) and the installer must register geographic coordinates and antenna parameters with the SAS.

Both categories must comply with SAS-directed frequency and power assignments and must cease transmission within 60 seconds of losing SAS connectivity.

Advanced research applying multi-agent reinforcement learning (MARL) to optimize GAA channel selection within the CBRS framework. Key approaches include:

• Deep Q-Networks (DQN) trained on historical SAS occupancy data to predict channel availability and proactively select frequencies with the longest expected idle periods
• Centralized Training Decentralized Execution (CTDE) architectures where agents learn cooperative spectrum sharing policies in simulation but execute independently using only local SAS grant information
• Safe RL constraints that prevent agents from exploring actions that would cause interference to Tier 1 incumbents or PAL holders
• Integration with spectrum occupancy prediction models using LSTMs to forecast naval radar activation patterns along coastal ESC sensor networks

This enables GAA devices to maximize throughput while maintaining strict non-interference guarantees.