Function and the history of development

For the DFS method (Dynamic Frequency Selection) required for 5 GHz WLANs, an unused frequency is automatically selected, for example, to avoid interference from radar systems. Occasionally, however, signals from weather radar stations cannot be identified reliably.

For this reason the European Commission is extending the requirements of the standards ETSI EN 301 893 V1.3.1 and ETSI EN 301 893 V1.4.1 to additionally avoid the use of three channels (120, 124 and 128) in subband 2 of the 5 GHz band, and not to allow use of these bands for automatic channel selection until a process to auto-detect weather radar station signals is made available. The versions EN 301 893 V1.3 and EN 301 893 V1.4 are referred to as "DFS2"

In the middle of 2010 the new version ETSI EN 301 893 V1.5.1 came into force, which was accompanied by changes in the usage of WLAN frequencies in the ranges 5.25 to 5.35 GHz and 5.47 to 5.725 GHz. The new Version 1.5.1 regulates the DFS (Dynamic Frequency Selection) method for the protection of radar stations from WLAN systems working in this frequency range. By using DFS to detect certain patterns in the radio signals received, it is now possible to detect active radar stations, and WLAN systems can automatically switch their operating channel. To differentiate from previous regulations, the new standard EN 301 893-V1.5 for the updated DFS is referred to as "DFS3".

A pulse pattern can generally be described in terms of its pulse rate, pulse width and the number of pulses. Former DFS technology was only able to detect fixed radar patterns as defined by the various combinations of pulse rates and pulse widths which were stored in the WLAN device. According to DFS3, the device is now able to recognize changing pulse rates and pulse widths as radar patterns. Furthermore, two or three different pulse rates may be used within a radar signal.

The version ETSI EN 301 893 V1.5.1 (DFS-3) expires on 01/01/2013. The new version ETSI EN 301 893 v1.6.1 (known as "DFS4"), which also detects shorter radar pulses, applies thereafter.

Note: The recognition of weather radar stations (channels 120, 124 and 128 in the 5.6 to 5.65 MHz frequency range) is subject to special conditions. The DFS implementation in LCOS does not support the more stringent recognition conditions. Therefore, these three channels will be omitted from newer versions of LCOS.