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PCMA value Calculation

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Posts: 15
Feb 4th, 2013 at 9:59am  
Last day Idirect people work On our system we increase 2 mhz bw. then he change PCMA parameter


in this picture i mention our previous and present value.

but  i want to know how does idirect ppl calculate

Near symbol rate? which formula he use?

can anyone one tell me?
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« Last Edit: Feb 4th, 2013 at 1:26pm by Admin1 »  
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - Feb 5th, 2013 at 10:32am  
Why don't you ask the iDirect person who made the changes ?
Ask particularly what the "F" means in "Fsym" and what "Near" refers to. I have no idea.  Incidentally the iDirect software is wrong in its display of Centre Frequency, Cancel BW and MaxFreqOfset; the units should be written as "kHz".  Note the use of lower case letter "k" and "z".

If you want to investigate yourself, have a look at the modem configuration and find the information rate, FEC ratio and modulation mode.

Symbol rate = information rate / (FEC ratio * bits/symbol).

Information rate = 3 Mbit/s
FEC ratio = 0.793  (other possibilities include fractions like 0.431, 0.533, 0.495, 0.793, 0.978, also 1/2, 2/3, 3/4, 7/8 etc)
Modulation mode = QPSK  (2 bits per symbol).  BPSK = 1 bit per symbol.   8PSK = 3 bits per symbol

Symbol rate = information rate / (FEC ratio * bits/symbol).
Symbol rate = 3000000 / (0.793 * 2) = 1891551 symbols /s = 1891 ksymbols/s

A further thing you should do is observe the satellite spectrum and check that your new, wider, carrier is within the bandwidth agreed by the satellite operator. If you increase the symbol rate your carrier gets wider and you must first agree this with the satellite operator. Provided you don't change the carrier power or symbol rate you can mutually rearrange bit rate, FEC and modulation mode yourself; it won't make any difference on the satellite but first tell the NOC so they know, and can see what you are doing.

Best regards, Eric.
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