OFDM is used by a number of programs as Ed mentions, but isn't this also the prime basis for many of our sound card modes? And Pactor 3?
Most of the items mentioned below have been done or are currently used technology. One glaring exception is the lack of a narrow ARQ mode for keyboard that runs on the major platform. There is PSKmail on Linux but nothing for MS Windows, so this technology has not advanced very much. We do have wide ARQ mode now with FAE which uses the 8FSK signaling waveform from the older ALE designs. It is very wide and has mediocre performance by today's standards, but at least it is available now. Narrower and more robust waveforms would be welcome as a replacement. The timing issue with ARQ modems has not been a problem with moderately long transmissions of several seconds or more. Perhaps the main consideration is not the length of time for the transmission as much as the length of time of the window for the ACK/NAK. OFDM has trade-off issues since it spreads many tones simultaneously over a wide area. This leads to a crest factor that reduces power for any given tone compared to putting all the energy into one tone at a time. Because of the interpretation of the Part 97 rules, here in the U.S., we can use very wide band OFDM modes that have multiple carriers in the text digital areas of the bands, as long as none are faster than 300 baud. At the same time, we can not use high speed single tone modems when they exceed 300 baud and yet from what I have been able to determine, they are similar in bandwidth to OFDM. But we can use the high speed modems on the voice/image portions of the bands with what appears to be unlimited baud rates, and yet the proponents of these modems, never seem to be tested and reporting back to us how well they work. Instead they complain how we can not use them in the text digital portions of the bands. Can anyone explain this? 73, Rick, KV9U Ed Hekman wrote: > Rud, > > I would really enjoy hearing some discussion on the use of OFDM on > HF. I work on an OFDM project and have an understanding of how it > works although I am not a system designer. I have also tried OFDM > with HamPal (digital SSTV) and WinDRM (digital voice) and it seems to > me that it offers a lot of potential for improving performance with > HF multipath fading. To stimulate some discussion I would like to > ask some questions for the experts in the field. > > The limitation that is often experienced with HamPal is that the > probability of getting a complete file through without errors is > small so it usually requires a manual request to fill in the holes. > Looking at the waterfall display it is apparent that frequently there > are transient holes in the spectrum due to multipath fading. Could a > higher higher success rate without the fill requests be achieved with > some combination of better FEC, slower data rate, better spreading of > the data over frequency and/or time? > > There have been many discussions of the timing problems associated > with using Pactor with computer sound cards. Would it be feasible to > use OFDM (or any other mode) with much longer ARQ cycle times > (several seconds) to accomodate the computer timing limitations? > > BPSK31 has become very popular due to its success with relatively low > signal to noise ratios. I have noticed, though, that it does not > perform well with the rapid flutter experienced with propagation over > the pole even with good signal levels. Could this be be overcome > with a little lower data rate (longer bit periods), better FEC, wider > bandwidth, OFDM, etc? > > I have noticed that with signals coming over the North Pole, RTTY > often works better than PSK31. Since RTTY (and MFSK, Olivia, etc.) > only uses one tone at a time, it seems to me that OFDM which uses the > entire bandwidth all of the time would be a more efficient use of the > bandwidth. Is this reasonable? How do the various modes compare for > efficiency of the bandwidth usage? I am familiar with Shannon's > theorem but would like to know how OFDM compares to other modulation > modes. > > There is quite a range of applications or uses of digital modes > within the ham radio community. Each application has very different > requirements. The applications range from: > > 1) Weak signal communications that require minimal information > exchange and can take extended time periods (JT65, etc.). > 2) Real time keyboard to keyboard QSOs with speeds ranging from a few > words/minute to 50+ wpm. These uses can usually tolerate some > errors. Narrow bandwidth also seems to be an advantage for these > uses for a few different reasons. > 3) Net operations. ARQ cannot be used since this is one to many > communications. I have not participated in this type of operation so > I am not familiar with how it is used or what is required. > 4) File transfer or email - The highest possible data rate is > preferred but the mode must be adaptable for varying propagation > conditions. This requires zero error so usually uses some form of > ARQ. > >
