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: Sat, 12 Sep 92 13:44:13 GMT

In article <1992Sep12.041137.9338@rtsg.mot.com>, svoboda@rtsg.mot.com (David Svoboda) writes:
>In article <1992Sep9.190035.20060@usceast.cs.scarolina.edu> hutto@smtc.engr.scarolina.edu writes:
>|
>|    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.
>
>What you very nicely described is known as "frequency hopping" or "direct
>sequence" spread spectrum.  That method is fairly old technology, having 
>been used in some form or another since WWII.  (Quiz: who knows what famous 
>movie actress has the basic patents on frequency hopping?)
>
>A rather more elegant method of accomplishing spread spectrum is to take
>your binary signal sequence and xor it with a long binary key which has a 
>very high data rate compared with your signal.  The resultant signal is 
>modulated, but the high data rate of the key causes the power to "splatter" 
>about the spectrum.  Then, the receiver can cross correlate the key with
>the incoming wideband signal, and when a high correlation is found, the 
>signal can be recovered.  As long as the key of another "interfering"
>transmission is orthogonal to that of your receiver (by design), it
>will not correlate, and will simply look like low-power noise.  To a
>narrow band receiver, the total power of the spread-spectrum transmitter
>at a given frequency is down in the thermal noise.  And the spread
>spectrum receiver doesn't see the narrowband transmissions because
>their energy is concentrated in a narrow frequency region, and therefore
>will not greatly affect the overall correlation.
>
>If you make the key sufficiently long, say, 10e10 symbols, then repeat,
>you can have a staggering number of mutually orthogonal codes, and
>therefore non-interfering radios.  Synchronization between transmitter
>and receiver is critical (the longer you have to make your search 
>window for correlation, the more processing power it takes), but that
>might be as simple as plugging your transmitter and receiver together
>while charging.
>
>This spread spectrum method is what is planned for cellular radio,
>though rather more sophisticated in execution.
>
>		Dave Svoboda, Palatine, IL

    Actually, Dave, that's what I meant to say, I just didn't know it.   ;-)
    It's no fair posting if you actually know what you're talking about! ;-)

    I'd guess once it becomes the norm in celluar telephone, we have another
    five years or so to wait before it shows up in the first "high-end" R/C
    set.  Probably another five years after that before it reaches the
    "affordable" R/C sets.

-------------------------------
Brent Hutto
hutto@SMTC.engr.scarolina.edu