What is co-channel and adjacent channel interference?

27 March 2024

When you’ve got a Wireless Network, it’s not all about signal strength, you can have a great signal strength and still have a less-than-ideal connection, one of the key metrics is the RF environment, and contention plays a big part of that, contention is directly related to channel interference.

Wireless is a half-duplex medium, what this means simply is that wireless radios are not capable of transmitting and receiving at the same time, simply put, they can’t multi-task! This can be incredibly problematic, if two radios broadcast data at the same time, at the same frequency, the communication will collide and fail, this leads to retries and all sorts of bad issues we want to avoid, thankfully we have something in place, called the Clear Channel Assessment.

What is the Clear Channel Assessment?

802.11 has a system in place to prevent this from happening, it uses something called a Clear Channel Assessment (CCA), and there are two Clear Channel Assessment mechanisms, Signal Detect and Energy Detect.

To maintain fairness, before a Wireless Device can transmit, it listens and carries out the Clear Channel Assessment, if it hears something, it will back off and wait a random time before attempting again, if it is still busy, it will again back off and wait a random time – the more times it is unsuccessful, the higher the chance of waiting longer. Whilst this happens before every transmission, it only takes roughly four microseconds.

Signal Detection (SD)

Signal Detection listens out for RF Transmissions that are 802.11, effectively, it is listening out for 802.11 preamble transmissions from another wireless radio (whether an Access Point or a Station/Client). If it detects a preamble, the signal detection check will fail and prevent the transmission.

The Signal Detection threshold is 4dB above the noise floor, so whilst we want a good secondary signal strength to promote roaming, channel interference can prove to be a real problem.

This isn’t the only mechanism though, because after all, we share the same frequency space with other technologies, and transmissions alongside these technologies can breach regulatory requirements or cause problems for both technologies, for this, we have our Energy Detection (ED).

Energy Detection (ED)

The key difference between Energy Detection and Signal Detection is that ED doesn’t discriminate between the type of technology, so whilst SD is only worried about 802.11, ED simply doesn’t even try to work out what the interferer is.

Another difference is whilst Signal Detection happens roughly 4dB above the noise floor, Energy Detection happens 20dB above this.

To put this into perspective, if we had a non-Wi-Fi interferer, say, a microwave oven, generating RF Interference, and the client had a noise floor of around -95dB (for easy math, mainly), the SD threshold would be -91dB, but the Energy Detection threshold would be -71dB, this makes it very easy for a failure.

So now we understand what goes on behind the scenes, it is clear as to why we want to avoid any interference in the channel, after all, interference equals a slower wireless network.

What is Co-Channel Interference (CCI)?

Co-Channel Interference is fairly self-explanatory, it’s interference that is happening within the same channel, some of this is unavoidable, for example when a Station is talking to an Access Point, and another Station also wants to transmit, it will detect co-channel interference, however, sometimes, co-channel interference can be caused by two access points operating on the same channel, within the listening distance of the station or access point conducting the CCA.

When we’re using 2.4GHz for Wi-Fi, Co-Channel Interference is almost unavoidable, we only have three channels (Channels 1, 6 & 11) to use that don’t overlap (more on that later…), this leads to channel reuse, which means we’ll see Channel 1, 6 & 11 used several times in close proximity. Coupled with the fact 2.4GHz penetrates materials greater than that of 5GHz or 6GHz, you can see why its a challenge.

2.4GHz Wireless Frequency Chart showing Co-Channel Interference
2.4GHz Wireless Frequency Chart showing Co-Channel Interference

In this image, the grey lines mark out each access point. We can see the three 2.4GHz channels I mentioned firmly in the centre of each of the grey areas, Channels 1, 6 & 11 are heavily reused, and this will lead to client devices, when communicating with an access point on Channel 1, also potentially failing their Clear Channel Assessment checks due to other stations talking.

And as I touched on, each time this CCA fails, they back off, for a longer time (you can see more here).

However, it’s not just access points on the same channel that can cause bother, we also have Adjacent Channel Interference.

What is Adjacent Channel Interference?

Adjacent Channel Interference is very similar to Co-Channel Interference, except it is when the interference happens not on the same channel, but one that is overlapping the channel that the transmission is happening on.

Much the same as Co-Channel Interference, if the interference is detected during a Clear Channel Assessment check, it causes the wireless station to back off and wait, leading to a less-than-ideal client device experience.

I mentioned when discussing Co-Channel Interference that we really only have three 2.4GHz channels… some people don’t think rules affect them and without a fundamental understanding of Wi-Fi, they chose to break them, lets look at the image below from a survey we conducted for a customer who was struggling with a poorly performing wireless network…

2.4GHz Wireless Frequency Chart showing Adjacent Channel Interference
2.4GHz Wireless Frequency Chart showing Adjacent Channel Interference

As you can see here, we have three access points, AP-1 on Channel 6 (cool), AP-2 on Channel 7 and AP-3 on Channel 5. The orange we see, is a Wireless transmission happening on Channel 7 (not cool), and as demonstrated, this is talking right over any would-be transmission set to happen on Channel 5 & 6, so what’s happening here is, a device associated with either AP-1 or AP-3 (Channels 5 & 6) wanting to transmit, is having their CCA fail because of poor Wi-Fi config.

In this example, what we would like to see is AP-1 on Channel 6, AP-2 on Channel 1 and AP-3 on Channel 11, giving us a nice 20MHz separation which happens to be the Channel Width.

This is a fundamental example of Adjacent Channel Interference (ACI).

Conclusion

There are far more interferes than just Wi-Fi, but within the Wi-Fi world, we can make our lives a lot easier but adapting the wireless network to work within the confines of what we have.

Having a good foundation understanding on the Clear Channel Assessment and the mechanisms in place can ensure that you work to deploy the best channel design you can. It’s a struggle within the 2.4GHz frequency, but it is possible in some environments, if we stick to 20MHz channel widths in 5GHz, we get 25 Channels, so that really helps with avoiding Co-Channel Interference.

We always recommend that when deploying a wireless network, starting with a good design is key, and after deployment, a Verification Survey is undertaken to validate that the network is configured optimally.

If you’re about to deploy a wireless network, why not get in touch with one of our experts and see how we can help, from Wireless Design & Consultancy through to our experienced Wi-Fi Installation Teams.

About the Author

Joshua Passmore
Joshua Passmore

Joshua Passmore

Technical Director

Joshua Passmore is our Technical Director, he’s been part of the NDNS Team since 2015. He heads up our Technical Solutions and has a real soft spot for Wireless Technologies.