Re: cyclic redundancy check 4-bit



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

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.

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.

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 PCs and sound cards and extremely low data rates, quite a few
operations can be performed on each bit.

What for?
Why can't you just make a telephone call to whatever place in the world?


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com


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