2.4GHz spectrum

[cedd]

A recent thread included several suggestions for comms and walkie talkie systems, some of which were 2.4GHz. I've also recently been trying to get Arduino's to talk to each other via wifi to build a wireless cue light system. It all got me thinking, just how much space do we actually have in the 2.4GHz spectrum and has anybody begun to see problems?

 

I own 4 channels of Line6 2.4GHz radio mics. They're really quite good and have been very reliable from an RF point of view. They have a very robust comms protocal and seem reasonably immune to interference, often at the expense of other systems though which struggle or slow down in their presence.

 

I started listing potential uses of 2.4Ghz and I'm sure there are more;

 

Radio mics

Show comms

Wireless DMX

Tablet - mixing desk remote control

Tablet - lighting desk remote control

Video senders

General venue wifi

Personal hotspots in the audience

Remote control devices (cars, boats, planes) - I certainly have used them in the past for props

 

 

You get the idea, there's a fair bit of stuff competing for space.

 

I just wondered what other people's thoughts are, if others have seen problems and what, if anything, should be done about it.

Personally I've just bought a 5GHz hub to do my desk remote control duties to try and free up some space.

[pjb304]

I was at the European Wireless Sensor Networks Conference a couple of weeks ago, and one of the papers there was to do with a similar issue. Unfortunately I don't think the proceedings have been put online yet but for those with academic paper access the title is "Coexistence Aware Clear Channel Assessment: Implementation of a cross technology CCA on an FPGA SDR platform" Peter De Valck (Ghent University, Belgium); Lieven Tytgat (University Ghent, Belgium); Ingrid Moerman (Ghent University - IBBT, Belgium); Piet Demeester (Ghent University, Belgium). For those who can't get at the paper, or don't want to read it! I'll summarise it (from my memory of the presentation - I'm not a WiFi or zigbee expert so I might have got some of the details wrong)

 

Zigbee (IEEE802.15.4) occupies the 2.4GHz spectrum and so competes with WiFi for bandwidth. WiFi will typically have a much higher utilisation as it can be used for transfering large files and so on. When a certain threshold is reached there is so much WiFi traffic that there are no time gaps for the zigbee traffic to fit in and the sensor data can't get through. In order to combat this they proposed making WiFi APs aware of zigbee traffic so that they can detect when a zigbee transmission is occuring and wait until the zigbee traffic has stopped. The idea is that the zigbee traffic will be a single sensor reading and so doesn't take long to transmit. Whilst the idea of different wireless protocols cooperating with each other to reduce the impact of an overcrowded spectrum is interesting there are several problems; for instance it would only work if *all* access points in an area had the system implemented which is impractical.

 

The work is still at the research stage - the fact it made it into a conference means it's new, but there are people begining to look into the issue of different protocols competing for the same bandwidth, and so there might be some interesting reseach output in the next few years.

[dbuckley]

At work we're having a lot of WiFi problems in public spaces, and it seems that mobile phones in hotspot mode is one of the prime offenders. Wasn't a problem last year, seems to be the new generation of phones and data-generous plans.

[bruce]

 

I started listing potential uses of 2.4Ghz and I'm sure there are more;

 

(snip)

 

... and microwave ovens trump them all...

[Stuart91]

At work we're having a lot of WiFi problems in public spaces, and it seems that mobile phones in hotspot mode is one of the prime offenders. Wasn't a problem last year, seems to be the new generation of phones and data-generous plans.

 

I'm curious - I have my iPhone set up to act as a hotspot. (Indeed, this is how I'm connecting at the moment). If I don't actually turn the Personal Hotspot off when I'm not using it, is the phone still transmitting in the 2.4GHz band even though there's nothing connected to it?

 

(As a side note, I don't imagine this'll be particularly great for battery life either)

 

 

When I was at University I remember hearing about some Trolls (for want of a better word) from one of the engineering faculties who would turn up at a spot popular with model aircraft enthusiasts, and fire up some jamming gear. Much laughter as the planes flew off into the distance with the owners powerless to do anything about it.

 

I'm surprised in a sense that people of a similar mindset don't mess up radio mics at concerts this way. Has anyone ever come across any instances of deliberate jamming of radio mics?

[bruce]

 

I'm surprised in a sense that people of a similar mindset don't mess up radio mics at concerts this way. Has anyone ever come across any instances of deliberate jamming of radio mics?

 

"technical terrorism" - here's what we were saying about a similar subject 7 years ago...

[smalljoshua]

It's hassle trying to take a bottle of water into a concert these days, imagine the response if I took in a bag of unknown electrical devices with aerials attached?

 

Combine with steel walled buildings making it harder to interfere from the outside and it really reduces the risk!

 

Josh

[Stuart91]

It's hassle trying to take a bottle of water into a concert these days, imagine the response if I took in a bag of unknown electrical devices with aerials attached?

 

Combine with steel walled buildings making it harder to interfere from the outside and it really reduces the risk!

 

One of the early shows on the U2 360 tour had a bootleg movie made of it and released online. The crew shot it using handheld cameras in the audience. They recorded the audio from as many radio signals as they could find, and built the mix using a combination of radio mics/packs and IEM mixes.

 

I guess the gear in question is less cumbersome, but there'll probably be a way around it. Build the jammer into a powered wheelchair - you've got a power source there, and it would be relatively easy to conceal stuff within it.

[timtheenchanteruk]

 

 

I'm curious - I have my iPhone set up to act as a hotspot. (Indeed, this is how I'm connecting at the moment). If I don't actually turn the Personal Hotspot off when I'm not using it, is the phone still transmitting in the 2.4GHz band even though there's nothing connected to it?

 

 

yes, in the same way any other wifi hotspot transmits, otherwise you wouldn't be able to connect to it, its not like phone wifi that can turn itself on/off on demand, a portable hotspot is on continuously.

[david.elsbury]

... except for when you turn it off... I have a widget for my Android phone that enables or disables the hotspot, mainly because it chews the battery.

 

David

[cedd]

... and microwave ovens trump them all...

Not to mention Civilian Radar. I'm pumping out 1MW EIRP at the day job! Thankfully the power rolls off very quickly - our local venue is only a mile away, line of sight.

[timtheenchanteruk]

... except for when you turn it off... I have a widget for my Android phone that enables or disables the hotspot, mainly because it chews the battery.

 

David

 

 

same here, I only use it when I need it, but the quote specifically says he doesn't turn it off :P

[Stuart91]

same here, I only use it when I need it, but the quote specifically says he doesn't turn it off :P

 

To be honest, I hadn't really given it much thought until now. I don't think many other people will have either.

 

The one thing I wasn't sure about was whether the hotspot would produce less interference when there isn't a connection in place, as opposed to when it is actually pushing data.

[timtheenchanteruk]

possibly, because theres wouldnt be a datastream, but tis still broadcasting its SSID etc.

[bruce]

It'll be beaconing - typically 10 frames per second - even when not transmitting data.