--- In [email protected], Paul L Schmidt <[EMAIL PROTECTED]> wrote: > What has been the experience with frequency-selective > fading? I've noticed (watching waterfalls) that fading > appears frequently as a sliding notch through the > waterfall... might it be better to have the zero-time > signal in the middle, with the delayed data above and > below? Or is the probability of the fade syncing with > the retransmission not that high? > > 73, > > Paul / K9PS
Hi Paul, Yes, the "sliding notch" null seen in the waterfall, especially with wideband OFDM and similar modes is a very good visual indication of the frequency selective fading problem. It is rarely seen so clearly on narrowband digi modes, but it does show up as phase inversion or skew on the PSK31 polar display. It is easy to hear it on AM broadcast radio! :) And I agree with you, it would be better to have the capability of independently setting the frequency and time offset of each one of the carriers, to compensate for the different types of frequency selective fading and noises encountered. Some type of auto-feedback protocol from the receiving station might help determine the optimum setting on an as-needed basis. Each station could transmit a quick block of code that represents a received "spectrum map" of the passband. Then the other station could use the feedback to adjust the transmit carrier frequencies to the best S/N spots in the passband. Perhaps after some more experimentation on the air, a few standardised RMPSK combinations of frequency and time separation would be all that is needed to cover most situations.Then both could switch to single carrier PSK on that frequency. With standardised spacing between RMPSK carriers, two or more RMPSK QSOs and/or PSK QSOs could be more easily interleaved in the same passband. For narrower bandwidth utilization, another useful approach would be to call CQ or initiate the QSO with RMPSK... the responding station could reply with single carrier PSK on the best carrier frequency... and the originator could revert to single carrier PSK also. There are a lot of possible combinations :) 73---Bonnie KQ6A Need a Digital mode QSO? Connect to Telnet://cluster.dynalias.org Other areas of interest: The MixW Reflector : http://groups.yahoo.com/group/themixwgroup/ DigiPol: http://groups.yahoo.com/group/Digipol (band plan policy discussion) Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/digitalradio/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
