Automated Frequency Coordination (AFC)

An AFC system relies on a regulatory-mandated geolocation database containing the precise technical parameters of all registered incumbent fixed services. Before transmitting, a standard-power unlicensed device must provide its geographic coordinates and antenna height to the AFC system via a secure interface. The AFC queries this database to identify all incumbent receivers within a calculated protection radius, forming the foundational input for the subsequent interference analysis.

The AFC system applies deterministic propagation models (such as Irregular Terrain Model or Longley-Rice) to calculate the path loss between each candidate unlicensed device and every identified incumbent receiver. Key inputs include:

• Terrain elevation data and clutter classification
• Antenna radiation patterns for both transmitter and receiver
• Frequency-specific atmospheric absorption This modeling predicts the precise signal attenuation over the specific geographic path, moving beyond simple distance-based estimates.

A single unlicensed device may not cause harmful interference, but the cumulative effect of multiple devices can. The AFC system calculates the Aggregate Interference Margin by summing the predicted received power from all authorized unlicensed transmitters at each incumbent receiver's location. The system must ensure that this total aggregate interference remains below the incumbent's protection criteria, typically defined as an I/N (Interference-to-Noise) ratio of -6 dB or a specific power flux density limit.

Based on the interference analysis, the AFC system generates a dynamic exclusion zone around each incumbent receiver. If a standard-power device is located inside this zone on a specific channel, it is denied operation. If outside, the AFC calculates the maximum permissible Effective Isotropic Radiated Power (EIRP) for that device on a per-channel basis. The response to the device is a list of available frequencies and their associated power limits, valid for a limited time interval.

AFC authorizations are not permanent. A device receives a time-limited grant (typically 24 hours) and must re-query the AFC system before it expires. This mechanism ensures that:

• Newly registered incumbent links are protected
• Devices that have moved are re-evaluated
• The system can adapt to changes in the operational environment If a device fails to revalidate, it must cease transmission on the standard-power channels immediately.

The AFC system's calculations are governed by strict regulatory protection criteria defined by the FCC. For fixed microwave incumbents in the 6 GHz band, the primary metric is an I/N ratio of -6 dB, meaning the aggregate interference power must be at least 6 dB below the incumbent receiver's noise floor. The AFC translates this into a power flux density limit at the incumbent's antenna, ensuring that the operational threshold for harmful interference is never exceeded.