# What happens to 5.8 GHz signals when too many transmitters are in a confined space? [duplicate]

This question already has an answer here:

I'm helping set up an Airsoft arena in the UK and I plan to have RC-FPV cameras mounted on the weaponry and masks which the players use/wear (similar to wireless drone cameras). As it's an open space we'll be testing the 5.8 GHz bandwidth first using 40 channel transmitters/receivers.

A few people have suggested that using anything over 8 channels in this bandwidth concurrently would cause problems but what would those problems be?

For example:

• Would the video become merged or blended with neighboring channels?

• Would the receiver swap from transmitter to transmitter depending on the signal strength?

• Would it just not work?

• Or something else?

It just not working and signal merging would represent a problem for me but just channel hopping is no major issue.

These are the channels available in one of the systems we are considering.

In case it's relevant here are the available specs.

5.8G 600mW 40 Channel Wireless AV Transmitter:
Transmitter Frequency /Channel Amount: 5 bands / 8 channel each = 40 frequencies
Output Power:600mW
Frequency Control: Built-in frequency & phase lock loop
AV input: Analog AV signal input
ANT connector: SMA (needle inside)
Power Supply: DC 7-12 volts
Current Consumption: 220mA
Size: 54*32*10mm
Neat weight: 22g


Some additional specifications as a clarification to answer questions:

I'll be streaming low quality AV (320tvl at 24fps and 22 kHz audio) which should come in at under 18mhz bandwidth per device. The modulation is most likely to be Airwave on one side and Boscam on the other.

To clarify, this is not a duplicate question to How to avoid interference in wireless communication. While everyone would like to avoid interference my question relates to what would happen when interference is encountered.

## marked as duplicate by Dmitry Grigoryev, PeterJ, Daniel Grillo, brhans, laptop2dSep 20 '16 at 18:46

• How is the signal modulated? How much bandwidth does each channel need? How will you deal with the duplicate frequencies listed in the table? – Ignacio Vazquez-Abrams Sep 20 '16 at 7:57
• At any point in space, the received signal is the sum of the (inverse square distance) received signals. For any wanted signal, the others add to become 'noise' which increases the bit error rate. – Sean Houlihane Sep 20 '16 at 8:06
• I've added the details of the modulation and intended bandwidth per stream under the specification in the original question. The maximum distance would be 150m with direct line of sight up to the receivers in the 8m ceiling. I don't see any duplicate frequencies. – Andy Gee Sep 20 '16 at 20:27

Your system doesn't offer 40 non-overlapping channels.

The bandwidth of an analog video signal is around 10 MHz (I couldn't find the bandwidth for your type of transmitter, could be 8 MHz, could be 15 MHz), but as you can see ch 1 band FR1 and ch 8 FR2 have a 1 MHz spacing between carriers: the channels are significantly overlapping. If two emitters use two overlapping channels, a receiver that receive them with the same power won't be able to demodulate anything correctly. If a receiver is much closer from an emitter than from the other one, the image quality could still be significantly degraded.

And even if the channels are not strictly overlapping (spacing between carriers > signal bandwidth):

1. a radio transmitter always send some energy above and bellow its set channel, more so if it's a cheap device (crappy radios) and/or a very compact device (less room for filters)
2. a radio receiver always received some energy from above and bellow its set channel (it's the receiver selectivity, rarely specified for consumer products), more so if it's cheap and/or compact

That's why you have to space the channels that are used simultaneously if you want to receive each of them with an acceptable quality.

Looking at your table, each "band" is in fact a "frequency plan", a way to arrange 8 channels in a non overlapping way (19 MHz minimum spacing) in a set frequency band of ~5.6 to ~5.9 GHz. I assume the different frequency plans were designed for interoperability (eg. maybe FR4 match the way a popular device manufacturer) and for legal compatibility (eg. FR3 has some channels around 5.9 GHz, and might be forbidden in some countries; FR1 doesn't).

Assuming you're the only one using videos transmitters in a set location (if not you'll have to coordinate with the other users), you should pick the same "band" setting for all your hardware, and select a different channel for each transmitter/receiver pair for a total of 8 usable channels, like it was suggested to you.

If 8 is definitely not enough, by carefully picking sufficiently spaced channels in several "bands" you might get a dozen of usable channels, but not 40.

• Thank you for your detailed and helpful response. If you could expand slightly on "won't be able to demodulate anything correctly" I'll accept your answer. My question related to what would the symptoms be of overlapping signals. This may be manufacturer specific, it may be total failure or merging of signals (like a cross fader). It's this information I'm primarily interested in. even if it's a gut feeling. I understand the bandwidth required for the cheap compact cameras is around 18MHz – Andy Gee Sep 20 '16 at 20:37
• Assuming the transmission is analog, and not digital, I don't know what the exact symptoms will be. It depends on hardware, how much the signals are overlapping (eg. chroma might be more affected than luma, as they use different sub-carriers) and relative power of the two signals received. I am pretty certain though that it won't simply be a superimposition/mixing of the two pictures as both source won't be in sync. – Sylvain Sep 21 '16 at 16:03
• Thanks, that's I can handle anything but superimposition as I'll be selecting the best x feeds based on image quality, static and motion detection for display online and in-house. Crisp merged signals would be near impossible to reject as candidates. – Andy Gee Sep 21 '16 at 16:29