For one reason because AX25 is an abomination. It packs to many protocol layers into one format and does not provide any FEC.
Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -----Original Message----- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Graham Sent: Tuesday, August 05, 2008 7:30 PM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: Has anyone looked into FPGA-based digitalmodes? >>>>>>>>>> Packet, Pax or ARQ FAE, at least to be able to share the frequency (collisions must be managed), >>>>>>>>> Why not take the final step and code a narow band spread spectrum packet system ..using very narrow bandwith short packet bursts based on the ax25 system .enhanced by spread sprectrum . the system could fit inside the bandwith taken by one of the 'narrow' multi tone systems ? G .. --- In digitalradio@yahoogroups.com, "Patrick Lindecker" <[EMAIL PROTECTED]> wrote: > > Hello all, > > For my small experience about ARQ modes, it seems to me that: > > * a modern ARQ system does not really need a synchronous scheme as in Pactor > (with obligation to permanently exchange frames). It must be asynchronous as > Packet, Pax or ARQ FAE, at least to be able to share the frequency > (collisions must be managed), > > * I don't think a powerful coding is really necessary. I think a ratio of > 1/2 (one information byte received for two bytes transmitted) is sufficient > (as in ARQ FAE or ALE DBM). Big block codings as in JT65 or Olivia with > ratio of 1/5 or less would be exagerated and will decrease drastically the > characters throughput. I don't think convolutional codings are conveneint > for ARQ modes as you must introduce a relatively big delay before deciding > what was the received characters. These codings are more convenient for > continuous modes (as in PSK63F), > > * an "ARQ memory" is absolutly necessary. You can forget coding but you > can't forget this tool. It is equivalent to a repetition coding and it > permits to reduce drastically the number of retries, > > * if you have an "ARQ memory" the minimum S/N is not given by the message > itself but by the possibility to detect the frame. If you detect the frame, > you will be sure to decode it (directly or through one or two retries). > This means that you could do a system very quick and also sensitive in the > same time (if you accept the number of necessary retries). Practically, the > minimum S/N will be determined by the speed of transmission of the frame > prefix (in ARQ FAE , for 50 bauds the minimum S/N is about -13 dB. This > means that for 500 bauds it would be -3 dB and for 5 bauds it would be -23 > dB). The speed prefix transmission must be independant from the message > speed transmission, > > * if I would want to do a very quick ARQ mode (but I'm on very slow modes at > the moment), I will prefer a THROBX modulation (a choice of 2 carriers over > n) than an OFDM, this because the maximum power transmitted is very low if > you want to keep linear (1/sqrt(n) if n is very big). > A configuration with a mean power/peak power below 1/3 is not a good > configuration. > I would switch from a non coded transmission (good conditions) to a coded > transmission (bad conditions) according to ionospheric conditions (as > determined on frames reception). A predetermined (i.e known) sequence as in > 110A to determine the channel transfert function would be perhaps > interesting. > > * I think MFSK modulations are better than PSK modulations. > > * Doing a very quick ARQ mode is not very "fun"... Doing a system able to > permit exchange between several Hams would be much more fun. > > 73 > Patrick > ------------------------------------ Announce your digital presence via our Interactive Sked Page at http://www.obriensweb.com/sked Check our other Yahoo Groups.... http://groups.yahoo.com/group/dxlist/ http://groups.yahoo.com/group/contesting http://groups.yahoo.com/group/themixwgroup Yahoo! Groups Links
