# qpsk viterbi decoding

Started by March 22, 2007
```hello everybody

what the customer have specified us for making viterbi decoding

includes some units that i want to know about

"SOFT SLICER"

what it is doing is converting 10 bits I and Q inputs to

two 3-bits output pair....any idea what it is....

and can someone provide some insight on "Node Synchronizer" used for
money.. ;) ) if possible...
```
```On Mar 22, 12:42 am, "khurram712" <khurram...@hotmail.com> wrote:
> hello everybody
>
> what the customer have specified us for making viterbi decoding
>
> includes some units that i want to know about
>
> "SOFT SLICER"
>
> what it is doing is converting 10 bits I and Q inputs to
>
> two 3-bits output pair....any idea what it is....
>
> and can someone provide some insight on "Node Synchronizer" used for
> money.. ;) ) if possible...

No idea what a "node synchronizer" is, but the
"soft slicer" is one that gives the probabilities
for one symbol (say -1) versus another (say +1).

Let's simplify by assuming BPSK (sending -1 or +1).
You see a noisy version y[n] = x[n] + z[n],
x[n] is the transmitted symbols, z[n] is white
Gaussian noise.  If you know the mean/variance
of z[n], then from the observed y[n] you can
compute (with some abuse of notation):

p(y[n] | x[n]=1) and p(y[n] | x[n]=-1).

Using Bayesian rule etc you can find

p( x[n]=1 | y[n]=Y),  p( x[n]=-1 | y[n]=Y).

In many cases you only care about the ratio of the
log of the two quantities above.  Look up "log
likelihood ratio".

This is probably what a "soft slicer" is.

The customer needs to tell you what format exactly
he/she wants.

Julius

```