Model Forum / Radio Controlled / Air Models / January 2009

all depends on the TX maker,
http://www.rcmodelreviews.com/spreadspectrum01.shtml

enjoy the read

well nlya few are hoppers as such.

OK, this is as far as I have been able to ascertain the facts.

The 2.4Ghz band is split into channels. These are pretty broad.. IIRC
5MHz or so.

Spektrum picks two of them, and uses them.
Simpler systems like Assan use just one.

Futaba FASST uses all of them in sequence.

Thats teh first bit.

The next bit is the modulation scheme. This is spread spectrum. There
are a number of digital numbers which, when convoluted with the incoming
digital signal to be sent, spread it out as a pseudo random spread
spectrum across the entire channel. To decode you deconvolute with the
same number.  I am not sure, but I suspect all systems from a given
manufacturer use the same spreading code. There are not enough spreading
codes to really use them to distinguish between transmitters at the field.

So, at this point we have the possibility of one, two or hopping
channel, and spread spectrum across the channel in use with a
deconvolution code being needed to pick the (digital) signal up.

The next stage is that the transmitters don't transmit all the time. Its
a bit like Ethernet. They transmit a burst of frames, all with error
correction and unique ID's associated with the transmitter. Binding a
receiver consists in it picking the largest local signal that fits its
manufacturers modulation scheme, and reading the transmitted code and
storing it. It then will, when switched on subsequently, scan all the
channels to find a transmitter it recognizes sending frames with that
number.

If frames get lost, or corrupted they are corrected, or discarded: Thus
'busy' airspace merely reduces the number of 'good' frames a receiver
gets. Response gets sluggish, but does not stop. Failure to receive ANY
vaid frames puts the receiver into failsafe.

In addition two systems - XPS and JETI, transmit back from the model to
the transmitter, and request re-transmits if frame are lost. I think.

I've been thinking about buying a large-ish RC

You will. Basically a typical 2.4Ghz video signal will hog a complete
channel and saturate it. It is MUCH higher data rate that a RC TX. If
you are lucky and you have a spektrum or Futaba FASST system they will
select channels that are different (Spektrum) or hop past it (FASST).
However our experience with simple ASSAN single channel sets was
complete and utter loss of control. To the point where it never even got
launched :-)

To put it simply, the video transmitter will not allow any other
transmitter to use that *channel*. If your set is good, or detects that
and uses another channel (switch video on before transmitter) or uses
two channels or hops you *may* get away with it. BUT having 100mW of
2.4Ghz power transmitting next to even a next door channel at a few
picowatts sensitivity, is not ideal. Its highly likely range will be
seriously reduced.

We didnt have huge resoures to ursue: we equipped the plane with an old
35Mhz set, and flew it..er..out of range! (headset stuff) fortunately
the camera showed where it had landed more or less...about 3 miles away :)

NOTE that this sort of irresponsible behaviour..flying a RC model out of
sight, probably violates your insurance. It's a lot of fun though.