Independent tests prove lack of frequency hopping with XPS

[XJet]

The big problem with channel-hopping at the level these guys are playing with is the potentially very long "holds" (periods of no data transfer) that it can introduce.

This leaves the programmer with a dillema...

Do you hop when interference levels are low (ie: *some* communications is still possible) and thus risk introducing unnecessary holds into the link, or do you wait until it's much higher and risk even longer (but less frequent) holds?

Because the switching overhead is so high, it's not a good idea to change frequencies unless it's absolutely necessary but how do you judge "absolutely necessary"?

Leave the hopping too late (once the 2-way link is lost) and the latency involved in finding a clear channel, hopping to it and resyncing with the other end can be so long as to be almost a waste of time.

When you're flying a fast model, or flying close to the ground, even a 1 second lockout is potentially disastrous.

Hop on demand sounds like a great idea but it is one fraught with compromises and potential problems.

That's probably why Spektrum uses redundancy and FASST uses constant hopping.

[sweetpea]

Actually if your read the whole thread they have gone to continuos hopping. Its slower than FASST though.

[XJet]

I notice that the DIY system in question has been built around Jenic modules and not XBeePro ones. I'm not familiar with the Jenic units so can't compare the intrinsic frequency agility they offer in terms of times to perform energy scans and actually change frequencies, however the following would apply to a XBeePro-based module configured in this way:

Continuous hopping implemented over a reliable-transport layer is prone to significant "holds" and can actually cause a whole lot more problems than it solves.

The problem is that the transmitter expects to get an ACK signal back from the transmitter and if it doesn't get one, it resends a minimum of three extra times. So if you blindly hop onto a channel with ahigh noise threshold, you're going to hit latency.

What's more if/when sync is lost between the transmitter and receiver, you've got to re-scan the available channels to find the new transmitting frequency before the link can be restored.

To make things worse -- the noise profile seen by the receiver can be radically different to that seen by the transmitter so establishing what the *new* frequency should be (for resync purposes) becomes very problematic. If the transmitter chooses a frequency that *it* thinks is quiet and starts transmitting there, the receiver may never hear it because "up there" that frequency might be obliterated by high noise levels not visible at ground level.

FASST doesn't have this problem because its receiver always knows where the next hop in the sequence is and the exact amount of time before the transmitter switches to that frequency. Even if sync is lost, all the receiver has to do is choose a nice quiet channel (at its end) and wait for the transmitter to appear on that channel. With a bodged XBeePro FH setup, this could be an unacceptable delay producing a nasty period of "no control" from the pilot's perspective.

And, with XPS/XBeePro you have a whole bunch of extra variables that can skew the effective switching time significantly which further adds to the resync/hop problems.

While it's easy to design and demonstrate a FH system with the XBeePro modules, such a system is far from robust and I certainly would never dare to fly with such a botch-job :-)

It'll work fine -- until you get enough noise or sufficient multi-pathing to provoke a resync which means it'll likely fail just when you need it most.

[zoomer260]

	Quote: Originally Posted by sweetpea
	Actually if your read the whole thread they have gone to continuos hopping. Its slower than FASST though.

I've read the whole thread and some of it several times and I missed anyone continually hopping except Futaba. Your talking about Spektrum ?

[sweetpea]

That is why I said read it and enjoy. Not ready for large scale.

Not everyone boosts the knowledge you do online and might find the article interesting.

[sweetpea]

	Quote: Originally Posted by zoomer260
	I've read the whole thread and some of it several times and I missed anyone continually hopping except Futaba. Your talking about Spektrum ?

These 2 should explain it.

	Quote: Originally Posted by Alan Hopper
	KreAture, these modules use a jennic chip that combines the radio with a 16Mhz 32bit processor (openrisc based). I have been thinking about making a smaller system using the jennic chip rather than the module, however the chip is bigger than the one you are using and requires external flash, but no seperate processor. I suspect your system is the way to go for small stuff. Do you know the legality of using just the chips rather than a pre approved module, on a diy basis? I currently use a hopping sequence that will always resync within two cycles if it goes out of sync. At start up the rx can either pick a clean channel and only start hopping after the first received packet, or just start hopping and wait for auto resysnc. The sequence index is passed in the packet. An example of a self resyncing sequence is 1 2 3 2 1 Alan

	Quote: Originally Posted by Alan Hopper
	KreAture , my hopping sequence is a psuedo random permutation of all the channels available. The random number generator is seeded by the mac address of the rx. This way the rx only needs to know the current sequence index to syncronise. The hopping is at a fixed time interval so it does not matter if a packet is missed, the rx uses its clock and the known sequence to know the channel to use. A sort of software pll is used to keep the clocks in sync. Alan

Def not for GS use "yet" but its interesting to see what is out there. No different than a guy building a Ham radio.

[zoomer260]

OK.

[sweetpea]

Here's an independant test proving XPS did hop. Of course no data to support why, but heck.....we have no data to support why it hasn't

I think this just raises even more questions.

So what does everyone have to say about this?

	Quote: Originally Posted by bruff
	Question when one flies on XPS and takes off, he is on one channel per a XPD and when he lands (never turning his radio off) he is on another channel. Again per the XPD doesn't that mean he has hopped? Bob

	Quote: Originally Posted by Daemon
	Yes if that scenario can be reproduced, then it would indicate that it had hopped. Have you witnessed this? ian

	Quote: Originally Posted by bruff
	It has happen to me at the flying field. Only once so far. Been playing with my XPD and a Laptop at the field when I fly. Use it to see what is going on. Never noticed anything with the plane. Bob

[Kiwi]

Data,???

Is Bruff scanning with his laptop or does he have something to read the Rx before and after flight.

What is the ground noise like and can he maybe post some screen shots of what he is seeing. So far I have done everything except put the XPS Rx in a bucket of water and I'm buggered if it moves. But I have not exhausted my attempts just yet.

The bucket of water is coming up (RX in a sealed container of course.

Hopefully he can drop in and share what happened and how it may have occurred.

[XJet]

	Quote: Originally Posted by sweetpea
	Here's an independant test proving XPS did hop. Of course no data to support why, but heck.....we have no data to support why it hasn't I think this just raises even more questions. So what does everyone have to say about this?

The real issue is not whether XPS *can* hop but whether it hops in a way that is useful/meaningful.

Perhaps the guy who posted his experience had encountered "potential interference" but what we want to know is "does XPS hop when it encounters *actual* interference?"

Based on what's been disclosed so far, the parameters required to make XPS hop are so narrow and precise that even if it can be provoked into hopping, it's unlikely that the feeble algorithm it uses will be enough to save your model in the event of "real world" interference.

Something as simple as a single carrier transmission will seemingly knock XPS out (because it won't hop) yet it will leave Spektrum/JR and FAAST users unaffected.

We have yet to see any proof that XPS has a *useful* degree of frequency agility.

As I've said -- all JD has to do is submit his system for independent testing -- but he refuses to do so. Nor will he provide the exact parameters (so we can reproduce it) that he uses to get hopping to occur.

I think that speaks volumes as to the veracity of his claims.

[sweetpea]

Hey, don't shoot the messengar......from what I gathered..they guy went to the field one day with a laptop and XDP.

The XDP can be used as a SA device. He checked his system before and after flight and it changed. Meaning it changed freqs sometime from startup to land. He noticed no problems in flight.

I never said that the system was robust or anything like that. Only that the system appears to hop. Bruff is a RCgroups user so I doubt he will post here.

[ricoalonso]

One question to ask is: Did he notice if other 2.4GHz flyers were around or near the flying area, ie: FASST? Or, any other 2.4GHz source?

[sweetpea]

t

	Quote: Originally Posted by bruff
	Never noticed any unusual 2.4 activity. We do have other members who fly XPS, Spectrum and FASST. Not sure who/what was up when I was. There were a number of 2.4 pilots at the field that day. Bob

[ricoalonso]

Yup, he just answered my post/question directed to him at RCG and that exactly what he said. Although he is not sure who/what was up, I'd say maybe (it's possible) that one of the FASST was turned on and caused it to hop which will confirm the other finding of another poster who mentioned that he was able to make XPS hop by introducing FASST in the environment.

If his observation is to confirm that XPS will hop when FASST is around (based on my assumptions above), then question is: is this a good or bad sign? What if there's a few more FASST in the field (on the air and/or turned on), won't it make XPS get crazy hopping around?

[xed]

	Quote: Originally Posted by ricoalonso
	If his observation is to confirm that XPS will hop when FASST is around (based on my assumptions above), then question is: is this a good or bad sign? What if there's a few more FASST in the field (on the air and/or turned on), won't it make XPS get crazy hopping around?

It would be extremely BAD and pointless to Hop with FASST active. Each Hop with XPS is roughly 400ms. Do you want to be locked out for that period of time at unpredictable intervals of time which may get worse with more FASST systems online?

I'm inclined to believe that the receiver rebooted due to low voltage and happened upon a new channel when it came back on. The pilot stated he didn't feel any latency or lockout during the flight, which he may have not honestly noticed in level flight.