I have talked to some scientist at a large research independent facility who are doing HF modem research for the government. Here is some of what they believe...
For a broadcast mode, use heavy FEC. If the receiving stations have transmit capability, let them NAK missed data periodically. For individual or group connections, use a small to moderate amount of FEC with CRC and ARQ based on NAKs rather than ACKs. Start off with moderate FEC and send 3-7 frames depending on the length/size of your frame. Short frames send 7, long frames send 3. If no station sends a NAK, send 6-14 frames. If no NAKS, send 12-28 frames, etc. If at any time only one NAK from one station, resend the frame and continue on. If you are for example sending 6-14 frames and receive two NAKs, back off to 3-7 frames. If sending 12-28 frames and receive two or more NAKs, back off to 6-12 frames OR just drop back to 3-7 frames. They also recommend manually setting a real-time propagation index for the frequency used and base your baud rate on that or use a fixed baud rate for various MUFs or bands. There was much discussion among the group concerning using varying baud rates or a single baud rate. About half felt that a 45.5 baud rate (or perhaps 31 baud rate) should be used on HF. The other half thought that 31, 45, 90 and 180 baud rates could be used. For their testing using a channel simulator close to a Watson channel simulator (they tested to a poor CCIR channel with varying fading, noise, etc. with a goal of 0 to -10 dB SNR). Their modem manually switched baud rates depending on the frequency (band) used and of course the band chosen was based on the projected path distance and MUF. Their transmission length were from 10-30 seconds. I don't know how many frames they sent but I do know that a 10 second transmission took 15 seconds to decode with moderate to heavy FEC in the broadcast mode. Their 30 second transmission produced a little over two pages (72-76 characters per line and 60 lines per page) of ASCII characters. They were getting 3 bits of information per tone and were using multi-tone. They said that their "mode" use much like OFDM and I am almost sure they were using somewhere between 50-80 tones. Their ultimate goal was a full page of ASCII characters being "decoded" in less than 15 seconds in a broadcast mode. The did mention the error rate but don't know what it was...but I am almost sure that had to be less than 1 character per page considering the type of information the system was to send. I wish I could tell you more, but the entire project is considered intellectual property by the research organization. Walt/K5YFW -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] Sent: Tuesday, September 19, 2006 3:04 PM To: digitalradio@yahoogroups.com Subject: Re: [digitalradio] 16QPSK Modulation and Baud Jose Amador wrote: > Taking adventage of SCS experience, they chose PSK > (cannot tell by heart if differential or not, a peek > to the manual is needed) as a modem, and depending on > the retry rate (closely related to BER) it tries more > complex constellations and more carriers. One of the > "secrets" is the switchover criteria...when retries > rise, then jump to the next "lower speed", whatever it > means. I think that FEC could be used wisely... For instance: Initially, use ARQ, with the modulation "A", working at "A" bauds. When retries rise, enable FEC dinamically. If it fails again, jump to the lower speed, or even to another stronger modulation (versus noise, I mean). When the retries diminish, it may try with more carriers, or more complex constellations, or more speed. The key is to do it automatically, or adaptatively. The switchover criteria is the most complex problem... but it could be reached even with the trial and error mode. 73 de Nestor, CM3NA __________________________________________ XIII Convención Científica de Ingeniería y Arquitectura 28/noviembre al 1/diciembre de 2006 Cujae, Ciudad de la Habana, Cuba http://www.cujae.edu.cu/eventos/convencion Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/digitalradio/join (Yahoo! ID required) <*> To change settings via email: mailto:[EMAIL PROTECTED] mailto:[EMAIL PROTECTED] <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
