DFS (Dynamic Frequency Selection)

Dynamic Frequency Selection (DFS) is a regulatory mechanism that enables radio equipment operating in specific sub-bands of the 5 GHz spectrum to detect radar signals and automatically switch channels to avoid interference. It is mandatory in many regions to ensure the protection of weather, aviation, and defense radar systems.

Regulatory Requirements

Devices operating in DFS-regulated frequency ranges must comply with national or regional requirements:

• European Union: EN 301 893 (harmonized under the Radio Equipment Directive)

• United States: FCC Part 15 Subpart E (§15.407)

• Canada: ISED RSS-247

• Japan: MIC technical standards for U-NII bands

These rules apply primarily to devices using the 5.25–5.35 GHz and 5.47–5.725 GHz sub-bands and are intended to prevent harmful interference with incumbent radar services.

DFS Functionality

A compliant DFS implementation includes the following key functions:

• Radar Detection: Identification of radar-specific pulse patterns

• Channel Availability Check (CAC): Passive listening before initial transmission

• In-Service Monitoring: Ongoing detection of radar signals during normal operation

• Channel Move and Non-Occupancy: Immediate channel switch and enforced avoidance of radar-affected channels for a defined period

These steps are required to ensure that devices vacate the channel quickly upon radar detection.

Testing and Certification

DFS testing must follow detailed procedures specified by the respective regulatory authorities. Key aspects include:

• Injection of standardized radar waveforms in controlled laboratory conditions

• Measurement of detection times and channel switch latency

• Validation against waveform types and timing constraints (e.g., ETSI EN 301 893 Annex D, FCC KDB 905462)

Testing is typically conducted in semi-anechoic environments using programmable signal generators and spectrum analyzers.

Application in Wireless Devices

DFS is mandatory for unlicensed transmitters operating in DFS-designated 5 GHz bands, such as:

• Wi-Fi access points and routers (IEEE 802.11a/n/ac/ax)

• Enterprise WLAN systems

• Outdoor wireless installations

Devices that do not support DFS must either avoid DFS-regulated bands entirely or restrict operation to DFS-exempt sub-bands, such as UNII-1 in the U.S.

Looking for comprehensive Wi-Fi testing beyond DFS?
Learn more about regulatory and performance testing for WLAN devices across 2.4 GHz, 5 GHz, and 6 GHz bands on our Wi-Fi Testing overview.