Rick, The measurement of SNR and Eb/No are two different measurements. The confusion comes because they are both cited in dB. It took me quite a lot of rereading material to clearly understand them. I dumped my understanding of it onto my web site at http://thehamnetwork.net/wiki/#Shannon-Hartley%20%5B%5BShannon%20Limit%5D%5D . To see the math and graphs clearly you need to have some support software installed. See http://thehamnetwork.net/wiki/#Graphics%20%5B%5BMath%20Expressions%5D%5D for details.
The actual Shannon Limit is -1.6 dB for Eb/No. The limit for SNR is not expressible, that I have seen, as a single number. Instead it is determined by the power, noise, and bandwidth. More simply, by the SNR and bandwidth. One of the datum I found interesting is that below 0 dB SNR the channel capacity drops precipitously. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Rick Sent: Friday, October 26, 2007 1:39 PM To: digitalradio@yahoogroups.com 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
