Dynamic Frequency Selection (DFS)

DFS mandates that unlicensed devices continuously monitor their operating channel for radar pulse patterns before and during transmission. Detection algorithms must identify specific waveforms defined by regulatory bodies—including FCC, ETSI, and MIC—with varying pulse widths, repetition rates, and burst lengths. A device must reliably detect radar signals at sensitivity levels as low as -62 dBm to -64 dBm, depending on the regulatory domain.

Before initiating any transmission on a DFS channel, a master device must perform a Channel Availability Check—a continuous listening period typically lasting 60 seconds (FCC) or 60-600 seconds (ETSI). During CAC, the device must confirm the absence of radar signals. If radar is detected, the channel is marked as unavailable and the device must remain silent on that frequency for the Non-Occupancy Period, usually 30 minutes.

After a channel is occupied, the device must perform In-Service Monitoring—continuous real-time radar detection during normal operation. Upon detecting a radar signature, the device must:

• Cease all transmissions on the channel
• Complete channel vacation within the Channel Move Time (typically 10 seconds)
• Limit remaining aggregate transmission to the Channel Closing Transmission Time (typically 200 ms to 1 second) This ensures near-instantaneous protection of incumbent radar systems.

To prevent devices from clustering on a small subset of available channels, FCC regulations require uniform spreading across the entire DFS frequency range. This means a device's channel selection algorithm must distribute its operating frequency pseudo-randomly across all available DFS channels, ensuring that no single channel bears a disproportionate interference load. This is critical in dense deployments like enterprise Wi-Fi networks.

DFS functionality is asymmetrically distributed in network architectures:

• Master devices (access points, mesh roots) perform radar detection, CAC, and channel switching decisions
• Slave devices (clients, stations) rely on the master for DFS compliance and must follow channel change announcements A slave may not transmit on a DFS channel until it receives explicit authorization from a master that has completed CAC.

DFS parameters differ significantly across jurisdictions:

• FCC (US): 5.25-5.35 GHz and 5.47-5.725 GHz, 60-second CAC, 10-second channel move time
• ETSI (EU): 5.25-5.35 GHz and 5.47-5.725 GHz, 60-second to 10-minute CAC, 1-second channel move time
• MIC (Japan): 5.25-5.35 GHz and 5.47-5.725 GHz, 60-second CAC, unique radar test patterns Devices must be certified against the specific radar pulse library of their target regulatory domain.

The minimum signal power level at which a DFS mechanism must reliably identify a radar pulse and trigger a channel vacation. Regulatory bodies define specific thresholds, typically measured in dBm, that vary by frequency band and regulatory domain. ETSI mandates a detection threshold of -62 dBm for devices with less than 23 dBm EIRP in the 5 GHz band, while FCC requirements differ slightly. The threshold must balance sensitivity to distant radar with immunity to false triggers from noise. Advanced implementations use adaptive thresholding that dynamically adjusts based on the noise floor to maintain a constant false alarm rate.

A mandatory monitoring period during which a DFS-enabled device must listen for radar signals on a channel before initiating any transmission. The CAC duration is specified by regulators—typically 60 seconds under FCC rules and 60 seconds or 10 minutes under ETSI depending on the channel and device type. During this window, the device passively scans for radar pulse patterns. If any radar signal is detected, the channel is marked as unavailable, and the device must select an alternative frequency. The CAC ensures that a newly powered-on access point does not blindly interfere with an active radar system operating on the same channel.

The continuous background radar detection process that runs for the entire duration of a device's operation on a DFS channel. Unlike the one-time Channel Availability Check, ISM constantly evaluates the spectrum for radar signatures. Upon detection, the device must cease all transmissions and vacate the channel within the Channel Move Time (typically 10 seconds under FCC rules). The aggregate transmission time during the channel closing period is limited to Channel Closing Transmission Time (usually ≤ 1 second). ISM algorithms must distinguish true radar pulses from bursty Wi-Fi traffic and other non-radar impulsive noise.

A mandatory quiet period during which a DFS channel that has been vacated due to radar detection cannot be used again. The FCC specifies a 30-minute non-occupancy period, while ETSI requires 30 minutes for most devices. After the NOP expires, the device must perform a fresh Channel Availability Check before resuming operations on that frequency. The NOP prevents a device from repeatedly re-entering a channel where an intermittent or rotating radar beam may still be active. This parameter is critical for protecting terminal Doppler weather radars and military surveillance systems that may not emit continuously.

A regulatory requirement that DFS-enabled devices distribute their channel selections uniformly across the available spectrum rather than clustering on a few preferred frequencies. This prevents a scenario where all devices converge on the same non-DFS channels, creating congestion while DFS channels remain underutilized. Under FCC Part 15.407(h)(2), devices must implement a channel selection mechanism that ensures uniform loading. Practical implementations often use a pseudo-random channel selection algorithm weighted by channel availability and quality metrics. Uniform spreading is essential for maximizing aggregate spectrum efficiency in dense enterprise Wi-Fi deployments.