28 January 2020

IWLAN – related to interference

It’s part four of the blog post series and it might feel a bit like everything is said:
OK, WLAN is not easy in an industrial environment.
OK, standard WLAN is not good enough for many use cases.
OK, there are special protocols (like iPCF) to get automation working.
Now I get to where many wireless blogs are starting: Physics. As everything is set up I am going to answer the questions:
How do wireless waves get propagated?
Can they live with each other?
What should be done to take full advantage of the setups mentioned in the earlier blog posts?

What’s attenuation, reflection, scattering, …?

Putting it simple: A wireless signal can be compared to a wave in a lake.

The wave gets smaller with distance.

It gets reflected and maybe even scattered at obstacles. And there might be places where the wave signal gets extinguished by itself through the reflection. Everything depending on the environment, read which material and obstacles there are, how much of it and where it is.

Wave propagation

If you want to dig deeper I can recommend the following to start from:

RF behaviours

At the end, the physical propagation leads to a point where you can see effects of moving just half a meter and all of a sudden ending up with a very bad signal on your mobile wireless device.

Techniques like antenna diversity, spatial time block coding (STBC) and maximum ratio combining (MRC) are helping out by making the best use of at least two antennas. Well positioned a wireless application is getting a great illumination of RF signal for every critical WLAN client.

By the way: MIMO is in such circumstances not one of those relevant techniques, as it does not make the signal more robust and is ‘just’ aiming for a higher data rate by sending one data stream per antenna.

But what exactly can be done for handling interference? Have a look at the chapter “What to do to get a stable application?”.

What’s interference?

To keep it simple, with ‘interference’ I am referring to influences from other Wireless LAN devices (like co-channel interference, CCI) and also from other wireless technologies.

As soon as you do not have full control over ‘your’ frequencies (which is called either frequency or channel management), you might end up with wireless applications disturbing each other.

Decoupling of Wireless applications by frequency

So plan ahead. Above you see a decoupling of applications where not just another channel is used, but even a different band. This still is an option for less critical communication, despite some Wi-Fi people saying that 2,4 GHz is dead because of Bluetooth and too many other WLAN devices.

Another topic of interference: There are dedicated channels where WLAN is no primary user, but radar is it. This means a wireless LAN device seeing radar on that channel needs to move, inevitably loosing from milliseconds to seconds on the go.

Radar pattern making a Wireless LAN AP moving to a different channel

What to do to get a stable application?

Easiest way: Get a trusted partner on your site
https://new.siemens.com/global/en/products/automation/topic-areas/industrial-communication-networks/professional-services.html

At the end, it comes down to three things from my point of view: Experience, planning and testing.
Sometimes the requirement for experience can be mitigated by studies and planning can be mitigated by experience. But till now I did not find a way to mitigate the requirement of testing to prove the requested KPIs of your application.

How to do an Industrial Wireless LAN project if you are not deciding to give it out of your hand (simplified):

  1. Define KPIs
    (which communication will get transmitted, which number of clients, which area needs to be covered, …)
  2. Make a rough planning
  3. Do a fine planning by including an on-site-survey
    (‘AP on a stick’ is best practice in complex environments)
  4. Install the Wireless LAN devices
  5. Do the fine tuning
  6. Verify KPIs by testing your application

And in case you are delivering a wireless application as a solution provider to a customer: Do not forget the wireless aspect within your contract (e.g. which channels are reserved for me) !

More to come …

Thanks for going through this article. I hope you enjoyed the very rough overview on a few basics of Wireless LAN.

Within my small blog series there is more to come, and at the moment I am struggling whether to move directly to some sentences about Wi-Fi 6 or firstly to my understanding of the so called IIoT. What’s your opinion?

  • IWLAN – Building block for IIoT (WLAN)
  • IWLAN – Raising the standards (WLAN and iPCF)

As always: Feel free to give feedback, ask questions and approach me directly!

Teaser Photo by Omar Flores on Unsplash

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