BER after Viterbi or RS decoding

Hi all,

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?

Hi,

I have once read a paper on how to estimate the BER without reference:
http://www.ebu.ch/en/technical/trev/trev_274-schramm.pdf

This is probably not exactly what you are looking for, as it estimates
the BER before viterbi decoding, but you might be able to work out a
solution from there.

Best wishes

Nic

Hi Nic,

thank you for the reference. To estimate the BER before Viterbi I just
re-encode the Viterbi output and compare the encoded bits with the Viterbi
input. Sometimes this estimate is called Viterbi error flag rate (VEFR).

Cheers, Lanbaba

Hi Lanbaba,

I think, this is exactly what the author in this paper also does, he
just calls it Pseudo-channel BER (pcBER), so not much new info for you,
I am sorry :(.

Anyhow, much luck with your search..

Best wishes

Nic

lanbaba 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?

You can get a BER estimate based on the average cost of the survived 
path for Viterbi. For the syndrome decoding of RS, the average number of 
corrected errors per block gives you the BER estimate.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

> You can get a BER estimate based on the average cost of the survived
> path for Viterbi.

That's a cool idea! So I guess you calibrate the system by injecting
known noise and measuring the path metric and the BER using a known
training sequence. Then the path metric in the case of unkown transmit
data will correspond to a particular BER (on average) using a lookup
table. How does this work with metric normalisation?

porterboy76@yahoo.com wrote:
> > You can get a BER estimate based on the average cost of the survived
> > path for Viterbi.
>
> That's a cool idea! So I guess you calibrate the system by injecting
> known noise and measuring the path metric and the BER using a known
> training sequence. Then the path metric in the case of unkown transmit
> data will correspond to a particular BER (on average) using a lookup
> table. How does this work with metric normalisation?

One idea is to compare the survivor path to the average of the other
paths. In pure noise, no particular path will dominate so the survivor
path will on average have the same cost as the others. As the signal
rises out of the noise, a lowest cost path should emerge and its cost
compared to the others should give an indication of the BER. Another
idea is to compare the best and worst paths. I don't know how this is
best done in practice but the information should be there to get a BER
estimate.

John

john wrote:


>>>You can get a BER estimate based on the average cost of the survived
>>>path for Viterbi.
>>
>>That's a cool idea! So I guess you calibrate the system by injecting
>>known noise and measuring the path metric and the BER using a known
>>training sequence.

That is brute force approach however yes you can do it this way.
> 
> One idea is to compare the survivor path to the average of the other
> paths. In pure noise, no particular path will dominate so the survivor
> path will on average have the same cost as the others. As the signal
> rises out of the noise, a lowest cost path should emerge and its cost
> compared to the others should give an indication of the BER. Another
> idea is to compare the best and worst paths.

You can do it this way also. However you should compare the best path 
and the next to the best path.


> I don't know how this is
> best done in practice but the information should be there to get a BER
> estimate.

The cost of the best path is already directly proportional to BER. We 
are takling about small BERs here.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

On Tue, 06 Dec 2005 02:35:54 -0600, "lanbaba" <lanbaba@gmx.ch> wrote:

>Hi all,
>
>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.   It's then a pretty simple mapping from input error rate to
output BER.   Many (if not most) systems that report their BER on
payload data do it this way.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

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'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.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com