BER after Viterbi or RS decoding

On Thu, 08 Dec 2005 20:04:20 GMT, Vladimir Vassilevsky
<antispam_bogus@hotmail.com> wrote:

>Eric Jacobsen wrote:
>
>
>>>Does any one know a way to measure the BER after Viterbi or RS decoding at
>>>the Rx side if the uncoded data at the Tx side is unknown?
>> 
>> As mentioned already, re-encode and compare (with the uncorrected
>> stream) provides a good estimate of the Viterbi decoder's input error
>> rate.
>
>What if the input stream is in the soft decisions?

It usually is, but just taking the sign bit gives an easy comparison
for the re-encoder.   It's not a problem at all.  All you want is the
input BER, and the soft information isn't needed (or desired) for
that.

>> It's then a pretty simple mapping from input error rate to
>> output BER. 
>
>The cost of the best path is the distance between the input stream and 
>the data decoded. There is no need to re-encode the data since the 
>information is available already.

That sounds like a nice alternative, but re-encode and compare is
quite simple to implement and is a well-known and often used
technique.   The re-encoder for a convolution code is, as I'm sure you
know, quite simple.   The results that you can get with the re-encode
and compare technique are pretty accurate, usually well within
practical user requirements.

For people who implement their own Viterbi decoder I'm sure the
technique you mention has merit.  Often, though, the decoder is
purchased either as a discrete part or an IP block, and access to the
internal metrics may be difficult or not possible.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org