Independent tests prove lack of frequency hopping with XPS

[XJet]

	Quote: Originally Posted by Leseirf
	It occurs to me that there could be a simple software solution. The way I understand it (and I've followed pretty much every thread for the beginning) when the TX doesn't get acks, it retransmits. No acks for x amount of time it alternates looking on other frequency's with retransmitting on the original freq. Why couldn't the rx after x time with no signal just hop to clean air. The TX would then find it on its own. With the transmit-ack sequence being (IIRC) in the 2ms range and only 12 frequencies it shouldn't take long to re-establish connection.

The problem with a "hop on demand" system is two-fold:

First you have to determine *when* to hop, then you have to determine *where* to hop.

If you suddenly find that the link with your receiver is lost -- do you wait and hope the interfering signal goes away (as it often does on 2.4GHz) or do you instigate a hop?

Just how long do you wait before deciding that it's better to hop?

If you hop too soon then you may be start introducing unnecessary latency and the risk of lost synchronization between transmitter and receiver.

If you wait too long you run the risk of the resulting "lockout" causing the model to crash.

A second's wait-and-see may be a reasonable for a sports-type model but totally unacceptable for someone flying a jet or doing low-level 3D flying -- yet a quarter-second may introduce a greater overall latency by allowing relatively low levels of noise to repeatedly trigger the hopping sequence.

Then there's working out where to hop. You can't assume that *any* other channel will be noise-free enough to allow continued control and you can't simply pre-program in a default -- or a single burst of noise could send all the XPS systems operating on a number of channels straight to the default channel where they'd effectively knock each other out.

The best way might be to maintain a constant spectrum analysis at each end of the link and negotiate an alternative channel *before* any interference even appears. Both ends of the link *must* be analyzed because the spectral profiles of each can differ significantly. A row of trees or buildings can completely shield the transmitter from detecting a noise source that effectively knocks out one or more channels at flying altitude and the close proximity of another weaker 2.4GHz signal at the transmitter end could also render different channels marginal at that end.

Unfortunately the fastest that the XBeePro modules can perform a spectral analysis is about 16mS and that's a very limited scan that spends just a few mS on each channel so may miss some very strong but bursty (typical of 2.4GHz) noise. All you need are a few retries thrown in there and you can rapidly run out of time -- since each frame of data is just 20mS apart. So the very act of scanning could introduce random levels of latency to the link.

If I were doing this, I'd regularly (but not constantly) scan between frames and build up a rapidly moving weighted average of the noise levels on the unused channels (at both Tx and Rx) then use that weighted average to negotiate a "jump-to" channel and pass that info back and forth once per frame. This would allow the Tx and Rx to agree on where to go if the current channel got clobbered.

Unfortunately this isn't bullet-proof either -- since there's no guarantee that the agreed alternative channel would not also be hit by the same (or other noise) at the same time.

Fortunately, the risk of this happening could be minimzed by using some simple algorithms (ie: give channels further from the current channel a higher weight when calculating the best alternative).

However, no matter which way you look at it -- any solution is going to be a kludge at best. The XbeePro simply isn't designed to be frequency agile.

This botch-job "hop on demand" is still not as good as FHSS or automatic redundancy though IMHO.

I can see why JD went with the XBeePro modules but it has locked him into some less than optimal technology (for RC purposes) on the 2.4GHz band.

The XBeePro modules are *easy* to use but that doesn't make them the *best* solution to the 2.4GHz RC problem. Various people (including myself) have knocked up XBeePro-based 2.4GHz RC links in just a few hours and they work well -- but just don't have the reserve of safety that other solutions offer in a busy 2.4GHz spectrum.

If the XBeePro solution was so good you could bet that lots more of different brands would already be using them due to the low barrier to entry and the incredible simplicity of building transmitters and receivers around them. The fact that nobody else has, speaks volumes me thinks.

[ChrisHays]

	Quote: Originally Posted by ChrisHays
	hows that new china system you are testing xjet?

?

[XJet]

	Quote: Originally Posted by ChrisHays
	?

Well one of the systems I'm testing seems to work okay but it's not suitable for giant scale -- no failsafes -- however it is dirt cheap (under $50 per 8-channel receiver *with* satellite).

There's video here:

iMax 9X

Full review will be posted shortly.

[exeter_acres]

	Quote: Originally Posted by XJet
	Full review will be posted shortly.

teeeheeee!

[ChrisHays]

xjet.. please include a pic of the tx!..thanks

[zoomer260]

	Quote: Originally Posted by XJet
	Well one of the systems I'm testing seems to work okay but it's not suitable for giant scale -- no failsafes -- however it is dirt cheap (under $50 per 8-channel receiver *with* satellite). There's video here: iMax 9X Full review will be posted shortly.

Xjet, I'm NOT a basher on this thread I am following with accute interest. But I am wondering why you said a particular system needs failsafe for one airborne craft but not another ? They both can be lethal. Is it because meet rules require it on one but not the other or is it a personal requirement ?

[XJet]

	Quote: Originally Posted by zoomer260
	Xjet, I'm NOT a basher on this thread I am following with accute interest. But I am wondering why you said a particular system needs failsafe for one airborne craft but not another ? They both can be lethal. Is it because meet rules require it on one but not the other or is it a personal requirement ?

Well it's just a personal preference but I would not feel at all comfortable using any RC system that didn't have failsafe on a model larger than .40-size.

Imagine the mayhem that could ensue if you're taxiing even a 50cc model and got hit by interference -- even a 22x8 spinning at 7K RPMs will make mincemeat of anyone who gets in its way.

All my larger planes have PCM receivers, I keep FM/PPM sets for the smaller "somwhat less" dangerous ones ;-)

Does anyone fly 50cc+ without failsafe these days? Why would you?

[XJet]

	Quote: Originally Posted by ChrisHays
	xjet.. please include a pic of the tx!..thanks

Here's a picture of the transmitter, 2.4GHz receiver (with satellite) and 2.4GHz module.

Looks suspiciously like a JR module but it won't work without a small mod in my 9X II (9303).

[zoomer260]

	Quote: Originally Posted by XJet
	Well it's just a personal preference but I would not feel at all comfortable using any RC system that didn't have failsafe on a model larger than .40-size. Imagine the mayhem that could ensue if you're taxiing even a 50cc model and got hit by interference -- even a 22x8 spinning at 7K RPMs will make mincemeat of anyone who gets in its way. All my larger planes have PCM receivers, I keep FM/PPM sets for the smaller "somwhat less" dangerous ones ;-) Does anyone fly 50cc+ without failsafe these days? Why would you?

No particular reason to not use it I've just seen severe rush to the hospital damage from a .40 size airplane. My current radio is both but all I have is PPM receivers. The one PCM receiver I've used (I got with a used radio system) cost me a plane to lockout even though I had it checked out before and after the incident. As a result I'm not sure about it's benifit. I've had "hits" and with PPM you know it, with PCM you can get hit and maybe not ?
But to answer your question is I have no PCM receivers and have plenty PPM.

[zoomer260]

And to show that I'm not trying to start anything, I've never used two receivers either. But just for the heck of it I'm gonna do it in this plane I'm building now just because I have them and thought why not ? No reason to do it other than it's something I haven't tried and I'm thinking maybe two antennas are better than one ?

[DesertHucker]

	Quote: Originally Posted by zoomer260
	No particular reason to not use it I've just seen severe rush to the hospital damage from a .40 size airplane. My current radio is both but all I have is PPM receivers. The one PCM receiver I've used (I got with a used radio system) cost me a plane to lockout even though I had it checked out before and after the incident. As a result I'm not sure about it's benifit. I've had "hits" and with PPM you know it, with PCM you can get hit and maybe not ? But to answer your question is I have no PCM receivers and have plenty PPM.

In Australia you can not fly IMAC without fail-safe at least on the throttle.

So a system without fail-safe could not be used for IMAC. I'm not sure about other disciplines.

[sweetpea]

There are cheap after market failsafe device that can be put inline with the throttle servo.

So this system could still be used, not that I would use it.

[DesertHucker]

	Quote: Originally Posted by sweetpea
	There are cheap after market failsafe device that can be put inline with the throttle servo. So this system could still be used, not that I would use it.

Did not think of that

[XJet]

These devices are ony useful if the receiver stops outputting pulses when the signal is lost or interference is received -- a lot of the DSP-based PPM receivers just go into a default "last position hold" for quite a while before they stop outputting pulses.

I'll check this cheap Chinese 2.4GHz set and see what it does.

[KrisW]

	Quote: Originally Posted by XJet
	These devices are ony useful if the receiver stops outputting pulses when the signal is lost or interference is received -- a lot of the DSP-based PPM receivers just go into a default "last position hold" for quite a while before they stop outputting pulses. I'll check this cheap Chinese 2.4GHz set and see what it does.

Any idea how many frequencies the Chinese items can use? It would be nice to know if they can switch at all.