As you could read in the article about the legal regulations, there are multiple frequency bands that are license free. This means that the regulator is not involved in regulating the frequencies/channel that everybody uses and, therefore, the regulator is not involved in the interference that is caused by this either. This is exactly the disadvantage of an unlicensed band: everyone can use it freely.
The 5GHz band is not just used by wireless applications such as Wi-Fi and radiation connections but also by radar systems or, rather, weather radar. These radar systems use radio waves to detect precipitation. The radar sends an energy pulse for this during a fraction of a second and it, subsequently, “listens” to the signal that is returned by the reverberation on the precipitation particles in the precipitation area. The correct location of the precipitation is determined based on the time difference between the transmitted and received signal. The radar, therefore, scans the atmosphere in all directions at different elevations and can display a 3D image of the precipitation in the atmosphere. With double polarisation you can even find out whether this concerns raindrops or even hale or snow.
These radar systems are characterised by a pulsating signal and are difficult to classify.
It should, however, be noted that weather radar systems are not just located at an airport. They are distributed all over the country. The scope of one radar is approximately 80 to 100 km for qualitative measurements. The effective range of the signal is approximately 240 km. The most important conclusions are, therefore, that you can still detect radar up to 240 km from the tower and that several weather radars have been installed in even a small country such as Belgium.
Dynamic frequency selection
It was initially determined in Europe and later in the United States that dynamic frequency selection (DFS) had to be used to utilise the 5 GHz spectrum. These DFS regulations are different per region. For example, currently, channels 120-124-128 are still blacklisted in the United States because of possible interference with the thermal Doppler radar systems.
Specifically, DFS means that your system will plan a channel itself and, therefore, will itself select the channels per AP. Before a channel can be selected, first at least 60 seconds must be spent listening on a channel before the decision can be taken that this is a suitable channel. If this channel is already being used or there is radar detection, you go to the next channel. If there is radar detection, the channel will be blacklisted for at least 30 minutes. You must scan for 10 minutes and not 60 seconds on some channels before they can be declared suitable.
DFS, therefore, makes starting a system slower because scanning must take place passively and much time is spent before arriving at a conclusion.
DFS will, however, continue during the functional phase because you must always be “courteous” with regard to radar. If radar presence is detected in a functional system, the radio must leave this channel as soon as possible. Concretely, the AP is given another 10 seconds before the channel is free. Next, the selection of a new channel starts, which, therefore, can also take some time and definitely in a larger network. While the AP searches for a new channel, it can, of course, not serve any clients. After the initial 10 seconds, all connections are broken and clients must search for another AP if this is possible.
Although clients may not, in principle, perform an active scan on these frequencies, we have noticed that the behaviour of clients can trigger the DFS. Most radar alarms are, for that matter, false positives.
To DFS or not to DFS
The “abundance” of 5 GHz channels offers opportunities and certainly with regard to high-density deployments. Whether these channels will be used depends on several factors:
- Required bandwidth at one location
- The presence of other networks
- Possible coverage using non-DFS channels
- The presence of radar systems
- Number of simultaneous client connections
As a minimum, we recommend in any case to perform a DFS site survey so that it becomes clear how sensitive the location is for radar alarms. DFS channels can never be deployed with a 100% reliability.