Hi guys. I hope this message may be of interest to all of you, especially to Rein. The description of choosing number of non acknowledged frames is quite detailed IMHO. 73, Vojtech OK1IAK # ------------ Původní zpráva ------------ # Od: DuBose Walt Civ AETC CONS/LGCA <walt.dubose@xxxxxxxxxxxxxxx> # Předmět: RE: [digitalradio] 16QPSK Modulation and Baud # Datum: 20.9.2006 17:01:24 # ---------------------------------------- # 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@xxxxxxxxxxxxxxx [mailto:digitalradio@xxxxxxxxxxxxxxx] # Sent: Tuesday, September 19, 2006 3:04 PM # To: digitalradio@xxxxxxxxxxxxxxx # 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 # # # # # # # # # # # #