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Constellation symbol to bit's soft-probability?

Started by Davy January 19, 2006
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

"Davy" <zhushenli@gmail.com> wrote in message 
news:1137726871.700967.266740@o13g2000cwo.googlegroups.com...
> 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
Davy wrote:

> 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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
In sci.electronics.design Mike Yarwood <mpyarwood@btopenworld.com> wrote:
> > "Davy" <zhushenli@gmail.com> wrote in message > news:1137726871.700967.266740@o13g2000cwo.googlegroups.com... >> 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?
<snip> "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
If you get any signal, is there a nice way of back propagating to get optimum probabilities?
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.

In sci.electronics.design PraZ <prasanna.sethuraman@patni.com> wrote:
> 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.