Conference cvg::hamradio_iii

494.0. "Spread Spectrum questions" by ROCK::SIEGRIST (Raise up a child in his own way...) Fri Mar 14 1997 08:37

SB QST ARL ARLB012
ARLB012 FCC proposes changes in spread spectrum regs

{stuff deleted} 
                        .....In spread spectrum the energy of
the transmitted signal is distributed among several synchronized
frequencies within a band and reassembled at the receiving end.
This reduces power density and duration of a transmission on a
particular frequency and lets spread spectrum almost invisibly share
the same spectrum with users of other, narrowband modes.  Spread
spectrum also provides for improved communication under poor
signal-to-noise conditions and in selective fading and multipath
environments, and the ability to accommodate more communication
channels operating simultaneously in the same spectrum.



 Questions:  
 * How is it that spread spectrum will 
   'accomodate more communication channels operating simultaneously
    in the same spectrum'?
 If the 'channels' were being used efficiently now then it does not seem
 possible that s.s. would accomodate more.  So is the problem inefficient
 channel use?

 * If we begin to use the 'channels' more efficiently using spread spectrum,
 won't that raise the noise floor, eventually cancelling out the s.s.
 advantage?  Is there not some limit to the number of s.s. transmissions
 that can be hopping around a particular area of spectrum?

 * In the amateur world, what are some of the proposed methods for
 syncing up remote stations?  I *assume* this is proposed for HF and it
 was not clear how the two stations communicating over their 'channel'
 knew where to start, and where to hop?  SSRY SSRY SSRY??

 Anyone know?
    dave  /nt1u

    I haven't thought about spread-spectrum for many, many years.. but
    here's my $0.02 anyway..
    
    Yes, SS generally raises the noise floor perceived by narrowband modes;
    this is conveniently ignored by commercial interests who want to
    "share" spectrum. 
    
    When narrowband modes have extra S/N available, they can indeed share
    space with some SS activity, even of those narrowband modes are
    maximally packed into their spectrum (which never happens on the hf
    bands; there always seem to be severl signals in too-little space, with
    no signals 'next door' .. but that's another soapbox..)
    
    multiple SS signals only interfere with each others in a statistical
    manner; the hopping sequences are usually chosen so that any pair isn't
    going to collide over a period of more than a few hops; this results in
    a small disruption, but the hop rate is generally so high that it is
    not noticeable for things like encoded voice (it causes a higher
    bit-error-rate)
    
    (memory is getting foggier by the minute here..) There is usually an
    initialization sequence where the two stations come to some agreement
    and march forward; in some (many? all?) cases there is also sufficient
    redundancy that the sequence can be recovered later. This all works
    MUCH better in a point-to-point situation, and gets a little messier in
    a multicast situation (similar to AMTOR, where you have to have idle
    time in FEC mode in order to resynchronize a listener)
    
    There was an article about some of this in QSY within the past couple
    of years, I think..
    
    ...tom W1PF