From: svoboda@rtsg.mot.com (David Svoboda)
Subject: Re: Radio dreams
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Fri, 11 Sep 1992 00:14:26 GMT

In article <1992Sep10.103654.756@csc.canterbury.ac.nz> kaiser@elec.canterbury.ac.nz (Chris Kaiser) writes:
|In article <ERICJ.92Sep9124702@calcutta.cfsat.Honeywell.COM>, ericj@hwcae.Honeywell.COM (Eric Jacobsen) writes:
|
|> Another method, though is to use a spread spectrum technique where you don't
|> really care (as much, anyway) about how much radios are interfering with
|> each other. This also complicates ($$$$ again) the transmitter and receiver.
|
|Add to that that eash flier would have to have their own unique code, and there are
|only a finite number of codes. Therefore you'd still have to have a control pegboard,
|although it'd be a code control board rather than a frequency control board. Also, its
|much easier (more idiot-proof?) to change a crystal than a spreading code.

Well, this is really not a problem.  If you make your code long enough, you can
acheive pseudo-orthogonality between literally millions of units.  For the 
upcoming CDMA digital cellular telephones (any Qualcommers out there?), there 
will be a unique code for each and every unit manufactured.  That well out-
weighs the likely numbers of RC transmitters in the world.

I think spread-spectrum techniques would be really nifty-neato to use for RC.
Imagine, if you will, an exceedingly low-power-output transmitter, on the order
of a milliwatt, which uses the entire RC spectrum for communication, yet 
is of such a low power, with it's power so spread around the spectrum, 
that it looks like low amplitude thermal noise to everyone else's receiver 
(and even that diminishes as their airplane gets further from the flightline).  
To the spread-spectrum receiver, everyone else looks like narrowband noise, 
including 3IM, pagers, and everybody, and is ignored in favor of your specific 
signal.  

But gee, do you need to put up a frequency pin someplace, just to make all your 
narrow-band pals feel more comfortable?  :-)

|These three techniques (frequency division multiplexing, time division mux'ing, and code
|division mux'ing) are the major methods of allocating an available frequency spectrum
|between multiple users. Each has advantages and disadvantages for different applications.
|For RC applications, frequency division mux'ing (our current system) is currently the
|best system in terms of cost, time delay, ease of use, control of clashs, etc.

Not all of those apply to code-division systems.  Cost, yes, definitely, spread
spectrum is an expensive technology.  Time delay?  No reason for extra time 
delay there; it's quite deterministic.  Ease of use?  Control of clashes?  If 
you have a unique code, these are not concerns.  What could be easier than a 
radio that nobody could shoot down?

Hopefully, as the cost of processing power plummets (little would I have 
imagined in 1973 that I would someday own a radio with more processing power 
than my TI calculator! :-)  we will see this CDMA technology come a bit more
"down to earth".

(PS, I am a DSP engineer with Motorola Cellular, and have some familiarity
with CDMA technology, although it is not my focus here.)

		Dave Svoboda, Palatine, IL
Newsgroups: rec.models.rc