EU. LBT

[Mike Blandford]

Interesting! I don't have any official DSM transmitters, only 4-in-1 and Orange modules (flashed to the Multi protocol).

Checking on a 4-in-1, I see what appears to be two short transmissions, a bit over 1mS each from the power voltage dip, every 22 mS, so as long as the power isa bit under 100mW, then they are under 10% MU. However, in 11mS mode there are FOUR such transmissions in 22mS, so I reckon the power would need to be under 50mW to be under 10% MU.

Of course, a "real" DSM transmission may be less than 1mS in duration.

Mike

Edited By Mike Blandford on 11/08/2018 19:38:24

[Frank Skilbeck]

Posted by Peter Christy on 11/08/2018 18:34:41:

MattyB: Thanks for that! It confirms my guess!

Frank: The same issue would apply to any band where illegally high powers were in use. We've already had suspected instances in the UK, where people have been using high powered analogue TV transmitters on 2.4 GHz. In the US too, there are people using quite high power levels on 2.4 GHz, to the annoyance of others trying to use the band.

LBT is only a partial cure. There were instances in the early days of 2.4 GHz in the US - at their Nationals, I believe - where Spektrum transmitters were accused of "failing". DSM-2 tries to find two empty channels before powering up, a form of LBT. At that US Nats, there were so many flyers on 2.4 GHz, that all the channels were in use! The DSM-2 transmitters couldn't find an empty channel, and therefore, quite correctly, refused to transmit! Their owners thought they were faulty!

The same could happen with EU LBT, if a wide-band transmission gobbled up a significant amount of the band. But at least it would stop you taking off, I guess!

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Pete

As I understand it, it would prevent an LBT tx from transmitting on the bits of the band that were gobbled up, but just interfere with a non-LBT tx when it jumped into the affected area.

[Peter Christy]

Posted by Frank Skilbeck on 11/08/2018 21:05:13:

As I understand it, it would prevent an LBT tx from transmitting on the bits of the band that were gobbled up, but just interfere with a non-LBT tx when it jumped into the affected area.

That's partly true - the LBT system should avoid the part of the band already affected, but any interference between competing spread spectrum systems should be minimal.

Spread spectrum and frequency hopping are not the same thing. You can have spread spectrum without frequency hopping (Wifi works this way) and you can have frequency hopping without spread spectrum. If you think about it, when we used to change crystals on 35 MHz, it was a very slow form of frequency hopping. Ofcom were quite happy with ImmersionRC's 459MHz system because, although it was frequency hopping, it stayed within the narrow-band channel limits permitted on that band, rather than being a wide-band (spread spectrum) system.

Most RC systems combine spread spectrum with channel hopping. Spread spectrum on its own is pretty good at ignoring interfering signals on the same frequency. Wifi units will operate quite happily with another Wifi unit on the same channel - at least until the signal is completely overwhelmed. It may slow down a bit, but it will still work up to that point.

That being the case, if two signals hopped on to the same channel at the same time, any interference should be minimal (provided everyone is operating at similar power levels), and in any case, since the hopping sequence is "random", the next hop should be clean.

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Pete