Rick wrote: > Something that has long been unclear to me is how can we have all these > modes that work far below zero db S/N and yet the Eb/No (energy per bit > relative to noise) can theoretically not go much lower than between 1 > and 2 dB below zero dB according to the Shannon Limit?
That's right... > Then you need to take the value of the baud rate and bandwidth of the > signal into consideration and that ratio is multiplied against the > Eb/No. Wouldn't that further raise the required S/N ratio? Actually, those "negative SNR's" are calculated on a 3 kHz (or similar voice channel) bandwidth. It does not tell the true story, but as a yardstick, it helps. > We often see measurements of modes that work -5, -10, even -15 dB S/N? > What are they measuring if not something related to the Eb/No? Yes, a 3 kHz voice channel...not the inmediate environment of the digital signal, but much, much farther away. And as noise floor is related to bandwidth... > Pactor has proven the worth (necessity?) of using full time FEC and a > moderate baud rate OFDM signal using PSK. Otherwise, you wouldn't you > need some kind of training pulse sequence as used on the 8PSK > MIL-STD/FED-STD/STANAG modems? > > 73, > > Rick, KV9U As I see it, Pactor does a whole lot more on the bandwidth it uses than the US_federal/military, non power limited standards. About the training sequence, the Viterbi demodulator ability to "guess out" the right bits out of the wrong received bits is another of the "hidden" Pactor II/III strenghts. Your mileage may vary... 73, Jose, CO2JA __________________________________________ Participe en Universidad 2008. 11 al 15 de febrero del 2008. Palacio de las Convenciones, Ciudad de la Habana, Cuba http://www.universidad2008.cu
