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SYMBOLS SUPERIMPOSED.  IN EFFECT, THE SIGNAL INTERFERES WITH ITSELF.  ONE
SOLUTION IS TO USE AN EQUALIZER.  THIS IS A SPECIAL TYPE OF DIGITAL FILTER
THAT FILTERS THE DEMODULATED SIGNAL TO REMOVE THE DELAY DIFFERENCES SO THAT
MULTIPLE PATHS BECOME TIME ALIGNED.  SINCE THE PATH CHARACTERISTICS CHANGE
AS THE MOBILE STATION MOVES AROUND, A SPECIAL TRAINING SEQUENCE IS SENT
PERIODICALLY SO THAT THE RECEIVER CAN RE OPTIMIZE THE FILTER COEFFICIENTS
BASED ON THE KNOWN SYMBOLS IN THE TRAINING SEQUENCE.  ISI IS AN EVEN BIGGER
ƒ  QST DE W1AW  ‰
PROBLEM AT HF THAN AT VHF/UHF BECAUSE THE PATH LENGTHS ARE MUCH GREATER.
IT IS NOT UNUSUAL TO HAVE PATH LENGTH DIFFERENCES UP TO 3000 KM 1800 MILES,
WHICH CORRESPOND TO PROPAGATION DELAY DIFFERENCES OF UP TO 10 MS.  THAT IS
WHY DIGITAL MODES ON HF WITH SYMBOL PERIODS OF LESS THAN ABOUT 10 TO 20 MS
‰  FROM PAGE 98  ‰
331 G3AAE/381 G3KMQ/353 G3LZQ/342 G3TJW/353 HL5NBM/331 I0SSW/349 I1YRL/339
I2RFJ/341 I8WY/339 IK2UKW/333 IK4SDY/333 IK6GZM/336 IK8AUC/336 IT9QDS/338
‰  END OF 13 WPM TEXT  ‰  QST DE W1AW  ƒ