Radar detection with Linux

In the US, at least, the term "radar detection" is usually associated with devices designed to warn heavy-footed drivers about police officers lurking in the vicinity. As far as your editor knows, none of those devices run Linux. Radar detection may become important for Linux in another context, though: wireless networking - especially in a base station mode - will require it. Some early work is now afoot to give that capability to the Linux kernel.

Most wireless networking happens in the 2.4 GHz frequency band; as many users will have noticed, that band tends to get crowded and noisy in places. For this reason, both 802.11a and 802.11n specify a number of channels in the 5GHz band as well. The relative lack of traffic at 5GHz makes it attractive for this use, even though the effective range of an access point is reduced somewhat. Pushing more wireless traffic to 5GHz will greatly increase the total bandwidth available.

Naturally, there is a catch. While other uses of that frequency range are few, among them are counted air traffic control and weather radars. Interfering with these radars will be frowned upon by regulators who have strange notions about how aviation safety should take priority over that post-lunch Twitter update. These regulators typically show a distinct lack of humor toward anybody who doesn't pay attention to their rules; once again we see how wireless networking often tends to be the leading edge of encounters between Linux and the regulatory environment.

To make the 5GHz band available for wireless networking in a safe manner, various agencies have laid out specifications for how a wireless device selects an operating channel. This scheme, called "dynamic frequency selection" (DFS), requires that a "master" station listen to a channel for a minimum period of time to ensure that no radars are operating there before transmitting. Thereafter, the station must continue to listen for radars; should one happen to move into the neighborhood, the station must shut down all communications and move to a different channel. In essence, wireless devices operating in the 5GHz band must actively avoid transmitting on channels where radars are operating.

Most Linux systems will not have to concern themselves directly with radar detection. A "slave" device, as might be found in a typical laptop, need only follow the master device's instructions with regard to where it can transmit. But any device which wants to function as a master - including access points and anything running in ad hoc mode - must notice radars and react accordingly.

Wireless adapters, having radio receivers tuned to the frequency range of interest, can help with this process. Should a blast of RF energy hit the antenna, the adapter can return an error to the host system indicating that a radar-like patch of interference was encountered. It's not quite that simple, though: random interference is far from unknown in the wireless world. If a wireless device bailed out of a channel every time it received some unexpected interference, communication would be painful at best. So something a little smarter needs to be done.

That something, of course, is to look for the specific patterns of interference that will be generated by a radar. Radars emit short bursts of RF radiation, followed by longer periods of listening for the returns. The good news is that these patterns are fairly well defined in terms of the radar's pulse width, pulse repetition interval, and frequency. The bad news is that these parameters vary from one regulatory domain to the next. So while the US has specified a specific set of patterns that a device must recognize, the European Union has defined something different, and Japan has a variant of its own. So radar detection must be specific to the environment in which the device is operating.

A group of developers, mostly representing wireless hardware companies has started a project to implement DFS for Linux. A preliminary patch set has been posted by Zefir Kurtisi to how how DFS might be done. These patches add a simple function to the ieee80211 API:

    void ieee80211_add_radar_pulse(u16 freq, u64 ts, u8 rssi, u8 width);

The hardware driver can use this function to inform the 802.11 core whenever the interface reports the detection of a radar pulse. These events will be tracked; if, over time, they match the pattern for radars defined by the regulatory environment, the code will conclude that a radar is operating and that evasive action is called for. If the hardware can do full radar detection directly, the driver can report the existence of a radar with:

    void ieee80211_radar_detected(u16 freq);

The current patch is only able to detect one variety of European radar; it is meant as a sort of proof of concept. The means by which parameters will be loaded to describe radars in different jurisdictions is yet to be worked out; one assumes that the existing regulatory compliance mechanism will be used, but alternatives are being considered. One way or the other, Linux should be able to coexist with radars in the 5GHz band in the near future. A version which helps in the avoidance of speeding tickets may take a little longer.

---

(Log in to post comments)