Hello Rick, S/N in dB is measured versus a conventional noise bandwidth (3 KHz in general). This permits to compare modes against gaussia noise as you consider the signal power (indifferently of the way you modulate and the coding you use) and the noise power (the same for all modes).
Eb/N0 is related to the SNR at the output of the matched filter (it is the energy of the bit / the energy of the noise in the equivalent noise bandwith for the duration of the bit). It is interesting to compare modulations vis-a-vis of the Shannon limit (-1.6 dB) but it is not what we finally need (S/N as defined above). 73 Patrick ----- Original Message ----- From: Rick To: digitalradio@yahoogroups.com Sent: Friday, October 26, 2007 8:39 PM Subject: Re: [digitalradio] Re: QEX Article on HF Digital Propagation 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? 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? 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? 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 Vojtech Bubnik wrote: > > PSK31 bandwidth is much lower than of PSK100 that Pactor 2/3 utilizes. > PSK100 will lock to a signal 100/31 times far mistuned than PSK31. > > Symbol length of PSK31 is 32msec, symbol length of PSK100 is 10msec. I > would say that PSK31 will be oblivious to 2ms multipath, but I suppose > the phase difference of both reflections will not be stable, causing > phase modulation of the summed multipath signal, which PSK100 with > convolutional code will be able to handle. > > 73, Vojtech OK1IAK > >
