Re: cyclic redundancy check 4-bit

On Fri, 11 May 2007 09:45:12 -0500, Vladimir Vassilevsky
<antispam_bogus@xxxxxxxxxxx> wrote:



Paul Keinanen wrote:

Sounds like some amateur radio LF experiments on 135 kHz, in which the

transmission time of a single bit can be minutes. Distances of several
thousand kilometers with less than 1 W radiated power can be achieved
despite the strong atmospheric background noise. Due the extremely low
antenna efficiency with practical antenna towers (10-30 m), you still
have to feed up to 1000 W into the antenna to get the 1 W radiated
power.


For 135kHz and 30m, the radiating resistance would be at the order of 5
milliohms at the best. To have the 1Wt radiated, the current of 14A has
to be pumped into the antenna.

The usual practice is to put a large capacitance hat on top of the
tower, consisting of horizontal wires spreading out radially from the
top.

To create such current in the antenna,
the input voltage has to be about 15kV.

There have been cases when the loading coil catch fire :-).

The loss resistance would be at
the order of Ohms, so the antenna efficiency is ~0.1% like you
mentioned. I can see some technical problems there, and the practical
part is rather questionable :)

The real issue is the grounding resistance, so unless you live on the
coast of a salty ocean, much of the power is dissipated in the ground.

To automatically detect such slow signals, one could for instance
multiply the incoming signal (with an I/Q-multiplier, to avoid the
unknown phase problem) with a locally generated carrier at the same
frequency and do some integration to measure the amount of power
received.

The obvious limitation is the stability of the frequency, so the
integration time can't be longer then tens of seconds for the reasonable
free running oscillators. However, it can be better if locking on the
external frq reference like GPS.

Or previously Loran-C, which was a nuisance anyway, with much
sidebands above 135 kHz, even if the Loran-C nominal frequency was 100
kHz.

For 2FSK, generate the carriers for the two possible tones and
multiply by two I/Q multipliers and compare which frequency had the
larger output.

Due to the high Q, the antenna bandwidth is ~hundreds of Hz. Not much
space for FSK, no space at all for the spead spectrum.

If the symbol rate is well below 1 Baud, a 1 Hz FSK separation is more
than sufficient.

A 100 Hz antenna bandwidth would be more than enough if for instance a
SS system with 10 Hz chip rate and 102.3 s bit period could be used.

This compensates for signal strength variations due to
propagation changes and receiver gain drift as long as the bits are
shorter than the fade cycle.

Other possibilities might be to use direct sequence spread spectrum
with some moderate chip rate and sending two alternate sequences and
then try to detect, which gives the best correlation.

Synchronization is the key.

With very low data rates and hundreds of megabytes or PC RAM and lots
of processing power, storing a sequence of signals into RAM and
matching all possible delayed versions of the PRN key is sooner or
later going to bring up a good correlation.

Paul

.

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