Dynamic Frequency Selection

DFS is a spectrum-sharing mechanism that allows wireless LANs (WLANs) to coexist with radar systems. It automatically selects a frequency that does not interfere with certain radar systems while operating in the 5 GHz band. DFS is a feature of ETSI BRAN HIPERLAN/2 and IEEE Standard 802.11h.

Overview

In some regions (predominantly Europe), it is important to ensure that WLAN equipment does not interfere with certain radar systems that are the primary users in the 5 GHz band. The DFS feature of the AP software searches for radar pulses in the frequency channel where it is operating, or during the autochannel scan. It constantly monitors errors in the received frames and analyzes the timing patterns for periodicity. If the pattern matches a radar signal, it instructs the WLAN equipment to discontinue operation on that channel as quickly as possible. The WLAN equipment reboots, then begins to operate on another frequency after checking that the new frequency is free of radar signals.

Operation

DFS detects radar interference and moves the wireless network to another frequency with no interference. It maintains a list of channels where radar has been detected in the NOL (non-occupancy list). The AP avoids using these channels for at least 30 minutes after detecting radar on them. When DFS is enabled, the AP:

• Looks for radar detection before securing a frequency channel.
• Scans continuously for radar signal patterns during normal operation.

Radar Signal

A conventional radar signal is a burst of pulses of a high frequency signal. When one burst is over, it is repeated after some duration, called sweep time. ITU has defined several different types of radar based on different frequencies, the number of pulses in a burst, and pulse repetition frequency within the burst. The sweep time is a result of radars that slowly rotate through 360 degrees, so that in a 30-second sweep period, the AP may be hit by the radar signal only for fraction of a second. During this hit, the AP sees a few pulses separated by a time interval (called the period), which is anywhere from 250 microseconds to 20 milliseconds. Each pulse can be 2 microseconds or smaller in width.

Atheros Radar Detection

The AR5210 chip does not have any special mode to listen for and detect radar signals. However, it indicates a PHY error when this type of interference hits the receiver while it is receiving the preamble and PLCP header of an 802.11a frame. It can result in indicating a zero length frame to the host if the interference activates the receiver. Any interference that hits the receiver during the payload of the frame causes a CRC error. On AR5210, zero length frame detection and promiscuous mode for receive filter are enabled in the hardware to indicate the PHY errors to the host. The AR5213/AR2313 chips generate a special type of PHY error code when a short width energy pulse is detected, typically generated by radar. On AR5213/AR2313, zero length frame detection and radar PHY error detection is enabled in the hardware to indicate the PHY errors to the host. By turning on only radar PHY errors, number of errors to be processed by the algorithm is lower and probability of false detects is reduced too. The hardware generates an interrupt to the host on every received frame with an error. The characteristics of these errors are classified and used in radar detection.