Lester,
Months of testing of all available modes on a 200 mile, weak signal,
path on 432 MHz support what you say. Contestia (or Olivia, but slower)
has surfaced as the most reliable mode we have found in the difficult
environment of signals marginally above the noise, fading (QSB) as deep
at 5 s-units, Doppler shift, and Doppler spreading. ROS's spread
spectrum simply fails completely, as do any of the PSK modes. Contestia
surpasses Olivia simply because it takes only half the time that Olivia
takes to pass information, and for our purposes of ragchewing, the
constraints of all upper case are not a problem. If you do not like all
upper case, in fldigi we have added an option to use all lower case...
73, Skip KH6TY
On 7/14/2010 3:51 AM, Lester Veenstra wrote:
Now let's cut to the chase:
* *
*THE USE OF SPREADSPECTRUM, THAT IS, THE USE OF BANDWIDTH EXPANSION
TECHNIQUES BY ADDING PSEUDORANDOM DATA, NOT CREATED FROM THE USER
INPUT INFORMATION DATA, IS OF NO ADVANTAGE IN IMPROVING THE END TO END
PERFORMANCE OF A LINK, WHEN COMPARED WITH PROPERLY SELECTED MODERN
ENCODING AND MODULATION TECHNIQUES.*
* *
What I am proposing for consideration is the point that for a given
transmission bandwidth, and a given end to end data transmission rate
(user information), the bits added should actually perform an error
reduction function and interference mitigation function. This can be
performed using with tradition FEC codes and in modulation selection
and encoding (PSK, MFSK,Multicarrier PSK, M-ARY FSK, multicarrier
M-ARY FSK, etc.).
My point is, why add bits to the transmission that at the receive end,
do not improve the performance.
For your consideration of the above, I repeat some previously stated
basics:
(1) Any proper transmission encoding coding scheme, will, as one of
its first steps, scramble or randomize the incoming data, in order to
provide a uniformly random data stream to the subsequent steps in the
process. These randomizers come in a few well defined, published,
forms, so it is not that hard to derandomize the result , once you
have demodulated, and stripped off the FEC layers. This is typically
the first and last step in an end to end process. This process does
not produce any encoding or bandwidth expansion. It is a bit in, a bit
out process.
(2) FEC coding layers, to combat, frequently with one type of FEC,
for low signal to noise ratio (QRN)(white noise), inherent in weak
signal work to correct random errors, and then outside (around) of the
previous FEC, additional layers of FEC, usually a type appropriate to
combat bursty errors of the type caused by 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.
(4) Finally, mapping the encoded transmit data into unique modulation
states. This is most commonly done as frequency and phase conditions.
For example, frequency diversity, in the form of encoding the source
to allow it to be transmitted as adjacent multiple carriers or are
single carriers on multiple frequencies, is needed to combat the
frequency selective fading present on HF paths and to make use of
frequencies that at any given instant (in this case, instant = the
symbol time) have less noise (QRM) present.
There is a practical limit to what can be done in a single carrier
system with encoding on HF circuits in particular, because the
dispersive (multipath) nature of the HF path is hash on short baud
transmissions (high symbol rate). There are a number of ways to
reduce the symbol rate of the actual encoded transmitted bits.
Changing from BPSK to QPSK actually creates two orthogonal synchronous
BPSK transmissions at half (longer) the symbol rate. (FYI: Changing to
OFFSET QPSK results in no symbol rate reduction)
Using M-Ary FSK where the number of frequencies in the set and the
symbol rate are inversely related. For example. Assume a conventional
50 baud(synchronous) FSK transmission. Each transmit symbol is 20ms
long. Changing this directly to 8-ary FSK creates eight distinct
frequencies, the particular frequency in this case determined by the
value of three bits of transmit data used to encode a single transmit
baud, that at are used one at a time, with a symbol that is now 160 ms
long or 6.25 baud.
The result is the same (longer symbol times, easier HF transmissions)
with changing from a single psk carrier to multiple adjacent,
simultaneous psk carriers, each carrying part of the FEC encoding data
stream.
In addition to transmit baud rate reduction (symbol time duration
increase), multi frequency systems, both single carrier or multiple
carrier, can be used as a time diversity encoding to combat dynamic
frequency selective degradations such as QRM from other users, QRN
from atmospherics, and fading.
One final point that should be obvious by now; SS is not necessary to
whiten the noise in the transmission bandwidth. In fact, there are
more efficient techniques, described above, to do the same thing,
described above. In fact, if your transmission bandwidth has a
uniform rate of interference, either QRM or QRN, SS is if no help at
all. The only way to improve the channel performance is FEC and other
forms of mapping the input user data in a deterministic manner, to
best match (compensate for) the impairments of the channel.
* *
/Lester B Veenstra MØYCM K1YCM///
les...@veenstras.com <mailto:les...@veenstras.com>
m0...@veenstras.com <mailto:m0...@veenstras.com>
k1...@veenstras.com <mailto:k1...@veenstras.com>
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