A point that should be kept firmly in mind.
A spread spectrum signal, applied to the basic underlying modulation
technique, does not make the communications channel more efficient. That is to
say, for a particular modulation and FEC system, the same transmit power , the
same receive Eb/No, will produce the same or better bit error rate before the
application of spreading/dispreading to the channel. In most cases the
dispreading implementation is not perfect, so spreading will degrade the
channel by some finite amount.
Having said this, I must qualify the above, as except when the bandwidth of the
despread or unspread modulation/FEC transmission has an interference with a
bandwidth significantly less than the bandwidth of the carrier. In other
words, the only time that spreading allows weak signal performance to be
improved is when the modulated bandwidth is significantly wider than the QRM.
In most amateur situations, the QRM environment is WIDER than the signal of
interest.
So having said why spread spectrum will usually buy us nothing ( I assume that
we are not trying to hide our signal under the noise floor, a typical reason
for its commercial or military use), what are the real methods by which things
can be improved.
(1) Frequency diversity, in the form of encoding the source to allow it to
be transmitted (as adjacent multiple carriers) on multiple frequencies
simultaneously, is needed to combat the frequency selective fading present on
HF paths. This also can be used to lower the baud rate of the individual
carrier.
(2) FEC coding layers, to combat, with one type of FEC, the low signal to
noise ratio (QRN) inherent in weak signal work, and additional layers of FEC,
of a type appropriate to combat the time carrying interference environment
typical of QRM and atmospheric QRN.
(3) Time diversity coding, to combat the channels dispersive distortion in
time over HF (short baud bad, long baud good), and frequency selective, but
short duration, fading. Incidentally the “short baud bad” is one reason why
spreading tends to underperform on real HF circuits compared to a flat white
noise channel in a laboratory environment.
If you have a transmission system that is crafted to meet all three of the
above, then adding spreading will improve it in our HF environment.(For example
MT63). So if the “ROS” seems to be an efficient method of transmitting low
rate bits over a poor HF channel, removing the spreading, and keeping the
underlying modulation and coding, should make it even better.
As an aside, frequency hopping does not by its self imply spread spectrum.
There are a number of M-ary FSK systems, including of historical note, the four
frequency duplex teletype (FRA-86 hint hint) and PICCOLO.
73
Les
Lester B Veenstra MØYCM K1YCM
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From: [email protected] [mailto:[email protected]] On
Behalf Of Dave Sparks
Sent: Wednesday, June 02, 2010 10:50 PM
To: [email protected]
Subject: Re: [digitalradio] What is here Spread Spectrum and why and what is
not?
More importantly (to me, at least) is Spread Spectrum the most effective or
efficient way of using a given amount of bandwidth to deliver a given data
rate, from a weak signal point of view? IOW, would ROS work better than,
let's say, MT-63, WINMOR, or Olivia if those three modes were adjusted to
use the same bandwidth and data rate as ROS? If it were open source, I
would have included Pactor-3 in that list, too.
If not, then using SS is counter-productive as well as legally problematic.
(I'm not implying that ROS is SS, BTW.)
--
Dave Sparks
AF6AS
