Re: Understanding APSK and QAM Rohde and Schwartz video #VARA


Graham
 

On Mon, Aug 9, 2021 at 08:13 PM, Kristoff Bonne wrote:
use a combination of QAM and APSK constelation
Kristoff

Its never immediately  obvious , from the  video,  the  issues round  the  pk-pk  levels of the  vara  modem are  perhaps , a little more  translucent  [as opposed to  transparent ] as are the  various  displays .. 

The hybrid serves to  maximise the  pay load of a  single  carrier ,  perhaps more  fragile, but ,  that's  linked to  the rate ,  the  faster , with the  same  energy , then the  higher  becomes the  errors , either  reduce the  rate ,  more  energy/time ..or  increase the  energy , again  more  energy , just the  rate of supply  increases ..

So, is the  issue with  single  high  speed  carriers ,  quite  incompatible with  turbulent  conditions  encountered  on HF
one  single  jamming  signal in the  pass band and everything  stop's  ,higher rates fall  over due to  delays and  multi path  etc ..

then comes the  maximum  rate  compatible with HF ,

Its reasonable , to  take a  guide from the  vara  modem , the maximum rate for  HF is nearly  x10 slower than the  300 baud  limit , quoted as  a limiting  factor  , 300 providing a b/w limitation  , for  single  carrier  systems ,  under ideal  conditions ,  anything and  everything  works ,  DRM broadcast requires , direct or  single  hop  , anything  after ,  then  it fails to  'lock'  
[ as the  sun spot cycle  progresses , the  opportunity for  multiple hop connections will  increase]

B/W utilisation , Much the  same as  say a   Tank with  tracks , and multiple wheeled  vehicle , one track  off ,  stops,  one  tyre  off  , keeps  going ..

Taking the vara   modem has the  optimum   single  carrier  rate  for  HF ,

Then ,  as can be seen,  in the  OFDM  levels,  multiple  low rate  carriers   , are  combined , to  provide ,  maximum  rate,  maximum  ''in channel ''  jamming /  qrm rejection , where as the  on-air  carrier spectrum's  are   provided with  16 ? fixed  patterns ,   not  seen, in the  coding  of the  modem is the  mathematical  'engine' that seeks to  distribute the  payload  , making  best  use the  the  available  channel b/w  and recover it post demodulation . 

.selective  qsb  can  remove  sections the  b/w, but  as long as the  minimum  criteria is met , for the  recovered  data , the  pay-load can be  extracted   if not ,  a more  robust  level  can be  selected , or vice versa 

NB: Reaching the  limitation of psk,  the  lower  levels  revert to   mfsk , simply  as during  trials ,  high  latitude users
encountered link  survival issues  coupled to  psk modulation ,  mfsk provides ''the'' most   robust and  energy efficient 
modem , 

Hope that  fills  in some of the  gaps , 

73-Graham
g0nbd 

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