airFiber - Guide to Planning & Deployment Using DFS Channels

Readers will learn how to set up an airFiber5 link using Dynamic Frequency Selection (DFS) bands.

We recommend performing the install temporarily using 5.8GHz non-DFS frequencies, even if your 5.8GHz spectrum is loaded. While testing the AF5 link, turn off or minimize the channel of a nearby production 5.8GHz link to open up a 10MHz wide 5.8GHz channel. Using 5.8GHz for initial alignment is important to avoid potential radar detection, since this can disrupt the aiming process.

  1. Introduction
  2. Countering DFS Radar Hits
  3. Channel Selection and DFS Channel Progression
  4. Modulation Tuning
  5. Using airView


As stated in the note above, it is important to temporarily switch to 5.8GHz non-DFS frequencies to perform the install. Remember you might be aiming and aligning for two minutes, only to have to wait for 30 minutes while the the DFS unavailable to time out before you can resume aiming for another two minutes.

Additionally, 5.8GHz allows for higher potential EIRP, making the initial aiming easier. Aim using 5.8GHz, and in the final aiming, set the TX power to 30dBm on both units to simulate actual DFS requirements. Also aim using 10MHz wide channels in HDD mode (half-duplex state), aiming one side at a time for the strongest remote reading.

Countering DFS Radar Hits

Operating the AF5 too close to the ground or with reflective surfaces nearby may cause radar hits because the AF5 can see strong reflections as potential radar signatures. In fact, these reflections can stem from sources outside the fresnel zone. In close-to-ground settings or where reflective surfaces are nearby, achieving a successful DFS link at low power may be difficult. Remember to use 5.8GHz for testing and aiming.

Even higher off the ground, nearby foliage or reflective surfaces can cause radar hits. If your signal is strong, try turning down the transmit power on the side experiencing the radar hits. Lower transmit power means less potential for strong reflections and can make a significant difference when avoiding constant, erroneous radar hits.

Other 5GHz devices that frequency hop onto the DFS channel used by the AF5 link can cause false radar signatures. Remember to use airView to find a clean 5GHz channel, but even with a low use channel, there are devices that hop over the entire UN-II band and may occasionally hop onto the DFS channel for a short period, causing a radar hit. This at first can be very frustrating. But the solution to the problem is patience. The longer you use the channel, the less often the frequency hopping device will try to occasionally hop onto it. The trick is staying on the channel long enough to win the battle and convince the hopping device the channel is a waste of time. Of course make sure you are dealing with a hopping device and a clean channel. Don't use this strategy with someone trying to serve customers in non-hopping mode on the channel you are trying to use.

There are several interesting strategies to winning the battle against entire band-hopping devices given the 30 minute radar-off channel wait. One is patience and that often works. The other is to assign a new channel very near to the previous channel used; the second channel should overlap the first. After a 30 second non-occupancy check, you're back to using the frequency. Eventually, these devices that hop over the entire UN-II band will flag your channel as not useable and stop hopping onto it, diminishing the potential for radar hits.

Channel Selection and DFS Channel Progression

The way AF5 moves to the next channel after a radar hit is not always intuitive, until you understand the logic. Whether in FDD (full duplex, frequency division) or HDD (half duplex, hybrid division) mode, only the side detecting the radar will move to the next channel. The other side will continue to use the existing channel. This means that if you are using half duplex on a single channel, you will switch to half duplex on TWO different channels after a radar hit. You will instantly double your use of the spectrum. For this reason, using half duplex where radar hits are possible may not be a good strategy. In such cases, it may be better to use two narrower channels in full duplex.

The next important thing to understand is that the neat list of frequency pairs listed on the Wireless tab are NOT really pairs! Since each side of the link progresses to the next channel on its own, they both go down that list independently from one another following a radar hit. Any frequency listed on the TX radio frequency list could potentially be paired with any other frequency on the RX radio frequency list. In this way, it can be difficult to keep sufficient channel space between TX and RX channels, should a sloppy combination occur.

Note: The frequencies on the list are based on priority. It will always try for a frequency at the top of the list if available before going further in the list.

Modulation Tuning

Once your link is successfully up and running, observe the constellations (modulations rates) the airFiber unit uses by paying close attention to unsuccessful attempts at higher modulation rates (unsuccessful attempts may be cause for a brief dip in the throughput). If you see this regularly occurring or it never succeeds at that constellation (modulation rate), set it to a lower modulation rate to avoid those dips.

Using airView

Run the airView tool for a long time, on both sides. The scan rate on the AF5 seems lower than on airMAX and will miss low use hits if you don't give it time. And true radar and frequency hopping devices need to exactly coincide with a AF5 scan rate to show up. Give the radios plenty of time to collect all energy signatures in the area.

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