From: hutto@SMTC.engr.scarolina.edu (Brent Hutto)
Subject: Re: Radio dreams
Reply-To: hutto@smtc.engr.scarolina.edu
Organization: NIST Southeast Manufacturing Technology Center
Date: Wed, 9 Sep 92 19:00:35 GMT

In article <1992Sep9.150729.4381@informix.com>, robertw@informix.com (Robert Weinberg) writes:
>OK, set loose your electronic imagination.
>
>SUPPOSE all transmitters used the SAME frequency, sending out packets of digitally  
>encoded signals. The receiver would look for a packet addressed to it with a  
>digital code, and only pay attention to that packet.
>
>Everyone would have to time-share the same airwaves, so the packets would have to  
>be brief, and there would have to be assurances that if there were, say, 25 planes  
>up, nobody got bumped.
>
>One way would be to have a transmitter in the plane as well, sending back a  
>"received" signal, so that the pilot's transmitter would re-send until it got a  
>"received" back. But that's expensive and adds weight, etc.
>
>Instead, would it be possible to make the transmitted packets so brief that they  
>could be broadcast repeatedly? Say, a .1 ms packet, sent every 5 ms, so that the  
>chances of 4 or 5 packets in a row getting bumped by somebody elses radio would be  
>small (true statistically?). That would assure that within 5x5 ms, or .025 seconds,  
>a signal got received.
>
>Another possibility: a crude receiver inside the transmitter box. The transmitter  
>would wait until it could detect no transmission from other pilots, then send out  
>its .1 ms burst. This way there would be a lot fewer signals in the air during any  
>given second. If 20 pilots each needed to send out 20 signals per second, that  
>would only occupy .1X20X20ms or 40ms of each second.
>
>I am suggesting something like this instead of trying to eliminate interference  
>between multiple frequencies. 

    Actually, what you are asking for is best done using a set of techniques
    collectively known as "spread spectrum".  Each transmitter/receiver pair
    uses a unique pseudo-random sequence of numbers to "hop" frequency every,
    let's say, 10ms.  As long as they stay in sync, the receiver "hears" the
    transmitter pretty much continuously (except for a tiny interval where
    the frequency is settling on its new value).  However, as long as all
    transmitter/receiver pairs us sequences that are sufficiently random
    (actual they just have to be sufficiently independant of each other),
    the worst thing likely to happen is a very occasional missed 10ms frame
    when two of the pseudo-random sequences "cross" on the same frequency
    for one frame.

    If you make the range of frequencies sufficiently wide compared to the
    bandwidth of a single (and the number of signals in use at any one
    time), each additional transmitter/receiver pair just costs a little
    bit of signal/noise ratio within that band and there is no "hard" limit
    on how many signals can use the band at once.  More users just means a
    little less range and reliability for everyone, no catastophic failure
    for one of two users whose channels conflict.

    The military uses this technology for reasons of security (they just
    keep their pseudo-random sequence a secret) so one day in a few years
    it will trickle down.  First commercial use may be in celluar phones.
    Once a large volume of encoding chips are being produces, it'll just
    be a matter of making the frequency-agile analog components cheap[*],
    miniature and reliable enough and we'll see it in R/C.  Of course, it
    will also require regulatory changes to allow it on the currently used
    bands, but one advantage is that a good spread-spectrum system can
    coexists (for a small number of users) with existing fixed-frequency
    users on the same band with little or no damage to either type of
    user.

    I expect to see spread-spectrum in high-volume commercial application
    in 8-10 years, maybe sooner if the government is cooperative in allowing
    the needed regulatory changes.  Maybe we'll see it in R/C before we're
    too old to fly any more ;-)

    * NOTE: by the time these systems become popular, direct digital
            synthesis of RF waveforms may be practical in which case
            no fancy analog electronics will be required.

-------------------------------
Brent Hutto
hutto@SMTC.engr.scarolina.edu
Newsgroups: rec.models.rc