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


Graham
 

Hi Kristoff 

Thread is  a  little  wide to , pick up all the  points 

amazed ,  no live  psk on the  bands ,  must admit ,  it was  never too  robust , worked nicely  on the  higher  bands , where not too  much  'turbulence  , especially   10 meters  

/OFDM systems are good at dealing with where where interference at one very precise specific frequency./

Possibly not so, this is a ofdm signal ,[was a image ?] showing selective fade , a high % of the signal escapes the attenuation , the out come being, the system continues to function , ie its very capable of dealing with noise distributed in the channel , in terms of time and spectrum , only needs to capture ' bits / patches of the transmission , to recover the pay-load ..

But for that, it must have a rough idea when the bits start.
That's why systems like very narrow-band modes (WSPR and similar) use an external time-reference (i.e. the clock of the PC) to determine when it must start receiving.

Not so , opera is a free running system, with no time locking required , once the data bits are recovered , the sync is then extracted ..noting opera is packet like system where the data frame is fixed length and the payload is also fixed length ,
the coding encapsulates the payload , and not having to provide a open ended , non time limited decode , there is no start or finish ,

only the need to capture 40 + % this requires a high redundancy level , noting the deep search [ correlation] second chance detector provided for 477&136 KHz , retains the 40+% min capture criteria , as well as the AFC , making it very robust and independent of equipment stability


Other  low s/n  modes , require to  establish   ''lock'  before the  decoding can  commence , the  two  tone start to olivia , or  the  ROS  pre-amble , the  delay to  print is  the  FEC time , The ROS-MF  modes  have a  much longer  FEC time to  over come the  QSB found  on  MF , ROS-HF much  shorter , as Opera   op8   bridges the  slow  qsb  MF , Op32  30 mins  , works  well  on 136  where  fade  times  are  very long ,  each  retains the  40 + %  requirement 

Opera  could be  categorised  as  a  data  burst  system  , perhaps  not  so [much ?]  packet , as that  requires the  conventional  start - data- stop  format , but  still  a single  burst ,  could be  received .. 

wspr uses time frame , as the coding is very low level , make use of correlation , to provide low s/n , correlation is normally used to recover the data bits from the RX stream , going for large blocks , requires accurate time matching ,
this  couple  with  multiple  passes , where the  signal is  repeated , verbatim, slowly  recovers the  signal [ may be 2 dB per pass] , not having  handled the  detection  in DSP , this  limited  in terms of  AFC -  has none ,  It responds to tones  +/- of the  selected  qrg,  to  provide  drift accommodation , the  down side being   strong  carriers , desensitise  the  system  , also   requires  high % recovery to  function .. 

Its a  detection  system ,  it detects  signals ,  that  was its  original  function, the  issue's  occur  when , comparisons  are made with  free  running  data  modes , one  repeating  issue  is the  50% duty cycle  of  opera ,  to deliver the  same  energy  as  wspr-2 ,, opera needs to  run for  x2  the  time , however  as the  coding  efficiency is  very high   op2 and wspr2  have  similar s/n ratings , with  only  half the  supplied  energy and  single  pass , 

APSK and QAM  , I don't have the  proof ,  only  point to  the  concept  being  used  ,  I assume  provides the  maximum  pay load for  the  minimum  bandwidth  , the orthogonal aspect ,  is linked to  carrier  stacking in the  B/W , as not to  interfere with  each  other   .. being FM , the  side bands  are  exact replicas  of the  main and extend  -/+ , where  in mfsk , the  tones  are  placed  by the modem pattern  generator ,  each  only  occupying  a  single  frequency , being  left to the  dsp to  determine where  and what  fills the  pass  band 

73-Graham 








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