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.

Reply by PraZ●January 26, 20062006-01-26

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.

Reply by plusnet user●January 25, 20062006-01-25

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?

If you get any signal, is there a nice way of back propagating to get
optimum probabilities?

Reply by Tim Wescott●January 20, 20062006-01-20

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

Reply by Mike Yarwood●January 20, 20062006-01-20

"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

Reply by Davy●January 19, 20062006-01-19

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