>>>>>>>>>> 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 >
