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Constellation symbol to bit's soft-probability?

D

Davy

Jan 1, 1970
0
Hi all,

I am new to demodulation and FEC.

How can I get each bit's soft-probability from the constellation?
For example, the modulation method is 16QAM, How can I get
P(bit1=0),P(bit2=0),P(bit3=0),P(bit4=0) from a symbol?

Is it belong to the subject of detection and need viterbi algorithm?

Or can you recommend some key words of this subject?

Best regards,
Davy
 
M

Mike Yarwood

Jan 1, 1970
0
Davy said:
Hi all,

I am new to demodulation and FEC.

How can I get each bit's soft-probability from the constellation?
For example, the modulation method is 16QAM, How can I get
P(bit1=0),P(bit2=0),P(bit3=0),P(bit4=0) from a symbol?

Is it belong to the subject of detection and need viterbi algorithm?

Or can you recommend some key words of this subject?
"bayes theorem"+"soft input" should work.

You could also look at page 72 to 74 of this for a very direct method;
http://4more.av.it.pt/docs/D4.4.pdf

Best of luck - Mike
 
T

Tim Wescott

Jan 1, 1970
0
Davy said:
Hi all,

I am new to demodulation and FEC.

How can I get each bit's soft-probability from the constellation?
For example, the modulation method is 16QAM, How can I get
P(bit1=0),P(bit2=0),P(bit3=0),P(bit4=0) from a symbol?

Is it belong to the subject of detection and need viterbi algorithm?

Or can you recommend some key words of this subject?

Best regards,
Davy
Viterbi decoding comes after soft decision making; the only real
connection is that Viterbi decoding is enhanced by soft decisions.

Mike's response is the correct one: you use Baysian estimation to figure
out what the probability that a certain bit was transmitted. You need
to know the probability density function of your noise and have a pretty
good grasp of the signal characteristics for it to work right.
 
P

PraZ

Jan 1, 1970
0
I don't completely comprehend what you mean by back propogation. But
yes, it is possible to get optimum probabilities for bits mapped to a
constellation symbol. You would find results on that if you search for
Bit Interleaved Coded Modulation, or BICM in short.
 
I

Ian Stirling

Jan 1, 1970
0
In sci.electronics.design PraZ said:
I don't completely comprehend what you mean by back propogation. But
yes, it is possible to get optimum probabilities for bits mapped to a
constellation symbol. You would find results on that if you search for
Bit Interleaved Coded Modulation, or BICM in short.

If you get a decoded signal of any form coming through the viterbi
decoder, a nice way of propagating back the actual symbol properties
into the decisions.
 
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