> Why do the reports about Pactor indicate it is more robust than the QEX > article would indicate?
I did not read the QEX article, but I hope I learned something about PSK modulation with regard to ionospheric flutter over the years I am developing PocketDigi. 1) PSK is very efficient in white noise. 2) Ionospheric flutter modulates reflected signal. If the digital modulation is slower or comparable to the modulation caused by ionospheric flutter, the signal will not be intelligible. 3) Coding gain is your friend. Trade bandwidth for improved coding gain. It works, if the raw channel S/N is higher than some threshold, see the graph on the following page and read the whole article. http://www.qsl.net/n9zia/a105/index.html So if a slow PSK signal is reflected by fluttering ionosphere, it will be distorted. If a fast PSK signal is reflected by the same ionosphere, the distortion will be less severe, but one would need to increase power to keep the wider signal readable. The solution to beat ionospheric flutter is to combine higher modulation speed and coding. Pactor 3 raw modulation speed is 100Bd. In worst conditions, effective data rate will be reduced by a convolutional encoder to 50Bd. Pactor improves the reliability further by memory ARQ. Pactor will really work even in a bad ionospheric flutter. There are less elaborate PSK modes than Pactor 2/3 used by HAMs designed to beat ionospheric flutter. PSK63F is a mode derived from PSK63 and MFSK16. It uses binary PSK modulation of raw 63 bits per second, but it is coded by MFSK16 varicode and MFSK16 convolutional coder, decreasing effective data rate to 31 bits per second. If comparing PSK31 against PSK63F in white Gaussian noise, there will be a S/N threshold, under which PSK31 will produce less errors than PSK63F. Above this threshold, PSK63F will produce less errors than PSK31 for the price of doubled bandwidth and turnaround delay caused by convolutional encoder/decoder. If comparing the two modes under ionospheric flutter, PSK63F will work under much more severe flutter than PSK31 independent of S/N. I think Patrick has a similar mode in MultiPSK, which limits further the character table, making the mode work with even lower S/N. 73, Vojtech OK1IAK
