RE: [digitalradio] Re: HF OFDM
Is not ISI only a concern when the symbol period is close to the multipath delay? Various sources I have read put typical maximum multipath at 10ms. If the symbol period is 25ms then there would not be a need for a guard interval since the critical part of the symbol is undistorted. So at symbol rates less than roughly 40 Hz there would be no need for a guard interval. Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of cesco12342000 Sent: Thursday, October 18, 2007 1:05 AM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: HF OFDM I would like verified: The raw bit rate is the same as the bandwidth and is independent of the symbol rate. Not in real word systems. Search for the guard interval used to cope with ISI. The bitrate will be higher with longer symbol rate, assuming the same guard interval (in ms) is used.
[digitalradio] Re: HF OFDM
Various sources I have read put typical maximum multipath at 10ms. If the symbol period is 25ms then there would not be a need for a guard interval since the critical part of the symbol is undistorted. Well, we want all of the symbol undistorted .. not only a part. So the 25ms symbol and the 10ms guard interval (your multipath time) adds up to a new 35ms symbol time. Note that carrier spacing is still based on 25ms to keep orthogonality intact.
RE: [digitalradio] Re: HF OFDM
Why does all the symbol need to be undistorted? Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of cesco12342000 Sent: Thursday, October 18, 2007 1:22 AM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: HF OFDM Various sources I have read put typical maximum multipath at 10ms. If the symbol period is 25ms then there would not be a need for a guard interval since the critical part of the symbol is undistorted. Well, we want all of the symbol undistorted .. not only a part. So the 25ms symbol and the 10ms guard interval (your multipath time) adds up to a new 35ms symbol time. Note that carrier spacing is still based on 25ms to keep orthogonality intact.
Re: [digitalradio] Re: HF OFDM
The FFT averages the signal over the entire sample period so any ISI during that interval will increase the error rate. I haven't implemented anything on HF yet, but there will be another affect that is important on ionospheric paths. Doppler spread is 1-10 Hz and can be up to 100 Hz on auroral paths. This will show up as phase errors when demodulating each subcarrier and limit the complexity of the signal constellation. The error rate should be decreased by shortening the symbol period (increasing subcarrier spacing) but this increases the percentage of time that must be devoted to the guard interval to eliminate ISI. Guard intervals are often 1/4 of the FFT sampling interval but it might be useful to make them 3/8 or 1/2 a symbol period to accomnodate poor propogation. 73, John KD6OZH - Original Message - From: Rud Merriam To: digitalradio@yahoogroups.com Sent: Thursday, October 18, 2007 06:25 UTC Subject: RE: [digitalradio] Re: HF OFDM Why does all the symbol need to be undistorted? Rud Merriam K5RUD ARES AEC Montgomery County, TX http://TheHamNetwork.net -Original Message- From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of cesco12342000 Sent: Thursday, October 18, 2007 1:22 AM To: digitalradio@yahoogroups.com Subject: [digitalradio] Re: HF OFDM Various sources I have read put typical maximum multipath at 10ms. If the symbol period is 25ms then there would not be a need for a guard interval since the critical part of the symbol is undistorted. Well, we want all of the symbol undistorted .. not only a part. So the 25ms symbol and the 10ms guard interval (your multipath time) adds up to a new 35ms symbol time. Note that carrier spacing is still based on 25ms to keep orthogonality intact.
