Re: [digitalradio] Re: OFDM Proposal
I don't know if this info is much use but Skysweep Technologies bundle an OFDM (TX RX) mode with their SkySweeper 5.12 package. http://www.skysweep.com/skysweep.html The mode is called SkyOFDM and from their help file: SkyOFDM is a state of art high speed modem based on the OFDM and turbo coding technologies. It offers several baud rates (300-9600 bps) and two different interleaving options (short and long). Also there are two bandwidth options: 2.0 and 2.6 kHz.The receiver should be set to the USB reception mode. The VHF/FM variant is not included in the SkySweeper Professional product. The high baud rates of SkyOFDM are very sensitive to signal clipping, so please set the transmitted signal level carefully. The peak to average ratio (crest factor) is up to 10 dB depending the baud rate. I haven't used it though. 73 Sholto KE7HPV. - Original Message - From: Rud Merriam [EMAIL PROTECTED] To: digitalradio@yahoogroups.com Sent: Sunday, October 21, 2007 10:13 PM Subject: RE: [digitalradio] Re: OFDM Proposal Ed, I am looking into the possibilities of OFDM. Right now I am stumbling over how to modulate the sub channels using the IFFT. g Actually, I think it is done by putting equal values in the real and imaginary parts of a bin and then doing the IFFT but need to try it out. I am a big proponent of FEC feeling it is woefully underused in ham digital modes. If you ain't FEC'n; you ain't tryin'. More pragmatically if you don't need FEC then you are using more bandwidth or power than necessary. One form of FEC I am curious about is putting FEC on each symbol. It has been an awfully long time that I looked at the error correcting codes (ECC) that are used in computer memory so I do not recall the details. A little research should remedy that memory failure. But putting 2 bit detection with 1 bit correction on a byte wide symbol might be worth the effort. In other words, stand by while I play with this. I will keep the group updated either by direct results or questions. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ed Hekman Sent: Sunday, October 21, 2007 11:55 PM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: OFDM Proposal 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.
[digitalradio] Re: OFDM Proposal
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? Try it. It's all there. FEC, data spreading and data rate can be selected in DRM, read the doc. Easypal has an additional FEC mode with much wider data spreading and another layer of high-efficiency FEC. By doubling the transmission time, you can recover up to 50% of the data ... 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? This is already done. Find out how the bsr-stuff works. The limitation is that it's not automatic. This was an intentional design decision. We dont want a bunch of drm-robots on the qrg.
Re: [digitalradio] Re: OFDM Proposal
Look at KA9Q's web site, especially http://www.ka9q.net/code/fec/, for FEC software. 73, John KD6OZH - Original Message - From: Rud Merriam To: digitalradio@yahoogroups.com Sent: Monday, October 22, 2007 05:13 UTC Subject: RE: [digitalradio] Re: OFDM Proposal Ed, I am looking into the possibilities of OFDM. Right now I am stumbling over how to modulate the sub channels using the IFFT. g Actually, I think it is done by putting equal values in the real and imaginary parts of a bin and then doing the IFFT but need to try it out. I am a big proponent of FEC feeling it is woefully underused in ham digital modes. If you ain't FEC'n; you ain't tryin'. More pragmatically if you don't need FEC then you are using more bandwidth or power than necessary. One form of FEC I am curious about is putting FEC on each symbol. It has been an awfully long time that I looked at the error correcting codes (ECC) that are used in computer memory so I do not recall the details. A little research should remedy that memory failure. But putting 2 bit detection with 1 bit correction on a byte wide symbol might be worth the effort. In other words, stand by while I play with this. I will keep the group updated either by direct results or questions. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ed Hekman Sent: Sunday, October 21, 2007 11:55 PM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: OFDM Proposal 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.
Re: [digitalradio] Re: OFDM Proposal
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
Re: [digitalradio] Re: OFDM Proposal
The inputs to the FFT for each subcarrier are the sine and cosine of the phase of the signal. This is usually done in a lookup table. However, you're right that a FFT is inefficient for 9 subcarriers. Each could be generated by an accumulator and a sine/cosine lookup table. The input to the accumulator is the frequency of the subcarrier and the phase can be altered by adding to the output of the accumulator (modulo table size). Receiving would be done by multiplying the incoming signal by the sine and cosine of each subcarrier center frequency. The in-phase and quadrature outputs are then accumulated over the sample period. The phase change has to be done at the beginning of each sample period. This actually generates multiple sidebands as the modulating signal is a square wave. However, all sidebands except the first sideband cancel out if the subcarriers are evenly spaced. 73, John KD6OZH - Original Message - From: Rud Merriam To: digitalradio@yahoogroups.com Sent: Monday, October 22, 2007 15:40 UTC Subject: RE: [digitalradio] Re: OFDM Proposal Thanks, John, for this reference and the other responses. I do keep an eye on Phil's work and comments. Time for a direct question: How do you modulate an OFDM sub channel using an IFFT? Or is that not the way to do it in practice? I have experimented with Excel. Looking at the FFT of a cosine wave I get a nice solid single frequency bin. Doing a modulation of it with a phase change in the middle I get a number of bins which looks impractical to implement. A value in a single bin then running an IFFT generates a nice cosine. I can kind of make a PSK modulate signal by copying 3 values from the FFT experiment above but other attempts generate a mess. g Again, that does not seem like a feasible approach. Musing about it while going to sleep I got thinking about another approach based on the observation that the sign of a value in the complex number controls the phase of the start of the curve. The process is: Generate a symbol If a phase change is needed change the sign in the bin Generate the next symbol The trick is that the symbols are offset by 1/2 the timing period, i.e. the start of the 2nd symbol is actually the midpoint of the 2nd symbol. This works because in OFDM the symbol period contains complete cycles of the waves. Possibly using the IFFT for an HF OFDM signal is inefficient, especially when working with a 500 Hz bandwidth signal. The 62.5 baud suggestion you made only using 9 tones so generating them directly would not be a CPU intensive process, especially using table lookups. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of John B. Stephensen Sent: Monday, October 22, 2007 7:40 AM To: digitalradio@yahoogroups.com Subject: Re: [digitalradio] Re: OFDM Proposal Look at KA9Q's web site, especially http://www.ka9q.net/code/fec/, for FEC software. 73, John KD6OZH
[digitalradio] Re: OFDM Proposal
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. What other uses can others suggest that would have distinctly different requirements? Ed --- In digitalradio@yahoogroups.com, Rud Merriam [EMAIL PROTECTED] wrote: Here is a proposal for an OFDM protocol with a bandwidth of 493 Hz. The symbol rate is 29 Hz allowing 17 subcarriers. That provides 16 subsymbols and 1 pilot carrier. With PSK modulation and a 5 ms guard interval the effective symbol rate is 25.3 Hz which provides 405 bps. Any guesses on how well that would work? g Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net
RE: [digitalradio] Re: OFDM Proposal
Ed, I am looking into the possibilities of OFDM. Right now I am stumbling over how to modulate the sub channels using the IFFT. g Actually, I think it is done by putting equal values in the real and imaginary parts of a bin and then doing the IFFT but need to try it out. I am a big proponent of FEC feeling it is woefully underused in ham digital modes. If you ain't FEC'n; you ain't tryin'. More pragmatically if you don't need FEC then you are using more bandwidth or power than necessary. One form of FEC I am curious about is putting FEC on each symbol. It has been an awfully long time that I looked at the error correcting codes (ECC) that are used in computer memory so I do not recall the details. A little research should remedy that memory failure. But putting 2 bit detection with 1 bit correction on a byte wide symbol might be worth the effort. In other words, stand by while I play with this. I will keep the group updated either by direct results or questions. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ed Hekman Sent: Sunday, October 21, 2007 11:55 PM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: OFDM Proposal 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.
