> You can use a MAP decoder to
> non-iteratively decode a stream encoded with a convolutional encoder.
> Performance can be slightly better than that achieved with a Viterbi
> decoder under certain conditions.

Sorry for bringing up an old topic. But, if there is no a-priori
information (or for uniform priors), MAP decoding reduces to ML
decoding (which is what the Viterbi algorithm does). So unless there is
an iterative procedure to estimate the a-priori probabilities, the MAP
decoder doesn't do any better than the Viterbi algorithm. So although
you CAN perform MAP decoding non-iteratively, it wouldn't be any better
than the Viterbi algorithm.

- Ravi Srikantiah

On Fri, 13 Jan 2006 02:57:53 -0600, "lanbaba" <lanbaba@gmx.ch> wrote:

>>I am not looking at a Iterative scheme. My interest is to know the
>>performance difference between Viterbi and MAP (BCJR) when used to
>>decode a sequence which is encoded using a binary convolution encoder
>>(say of constraint length=6).
>
>MAP decoder itself is an iterative scheme. You cannot get the estimate of
>apriori probabilities of all bits untill you do the first decoding cycle
>with the assumption of equal distribution.

That's not quite correct, if I understand what you're saying.

The MAP or APP decoding algorithm, (two names for the same thing),
isn't iterative by itself.   You can use a MAP decoder to
non-iteratively decode a stream encoded with a convolutional encoder.
Performance can be slightly better than that achieved with a Viterbi
decoder under certain conditions.

MAP decoders are typically used iteratively, though, in things like
Turbo Codes where the system iterates between two separate
convolutional encodings of the same data (with the order of one being
scrambled by an interleaver).

The MAP itself is not iterative.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

>I am not looking at a Iterative scheme. My interest is to know the
>performance difference between Viterbi and MAP (BCJR) when used to
>decode a sequence which is encoded using a binary convolution encoder
>(say of constraint length=6).
>
>

MAP decoder itself is an iterative scheme. You cannot get the estimate of
apriori probabilities of all bits untill you do the first decoding cycle
with the assumption of equal distribution.

Viterbi decoding is Maximum-Likelihood, so it gets lower W( word )ER.
But in very low SNR, MAP gets lower BER, because it is optimal for each
bit.

I am not looking at a Iterative scheme. My interest is to know the
performance difference between Viterbi and MAP (BCJR) when used to
decode a sequence which is encoded using a binary convolution encoder
(say of constraint length=6).

Assuming the convolution coder as specified by the IEEE802.11 or DVB
standard (G1=133 and G2 =171) which has a constraint length of 6. The
channel is AWGN.
I am interested in knowing the performance gain if I replace the
Viterbi decoder in the receiver with the MAP decoder which by design is
to give me lowest BER.

The thought process started when I was looking at the DVB standard. It
has a inner convolution code and a outer Reed-Solomon code. If I use a
MAP decoder in place of a standard (not SOVA) Viterbi decoder,will the
performance of the concatenated system improve. I am not currently
thinking of a iterative scheme. I am only going to use the hard outputs
from the  MAP decoder which i will stream into the RS decoder.

So coming back to question what i asked before, what is the performance
improvement of MAP over Viterbi when decoding a stream coded using a
convolution encoder as specified above assuming the noise is AWGN.

thanks
Sudhi




john wrote:
> sudhindra.bellary@gmail.com wrote:
> > Hello,
> >
> > Can any one point me to paper/book which tabulates the performance of
> > MAP decoder when compared to Viterbi. I see most sites only mentioning
> > that the MAP algorithm outperforms Viterbi at low SNR and high BER
> > condition. Can any one tell me how much is the performance gain by
> > using MAP at low SNR conditions.
> >
> > Thanks
> > Sudhi
>
> Are you talking about decoding a convolutional code in AWGN channel?
> 
> John

On 12 Nov 2005 01:53:27 -0800, sudhindra.bellary@gmail.com wrote:

>Hello,
>
>Can any one point me to paper/book which tabulates the performance of
>MAP decoder when compared to Viterbi. I see most sites only mentioning
>that the MAP algorithm outperforms Viterbi at low SNR and high BER
>condition. Can any one tell me how much is the performance gain by
>using MAP at low SNR conditions.

Do you mean to compare Viterbi and MAP decoders for a single
convolutional code used by itself?   The difference isn't going to be
large, and will probably depend on the constraint length.

Or do you mean to compare the two when used in an iterative code like
a Turbo Code?   There the difference is more significant.


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

sudhindra.bellary@gmail.com wrote:
> Hello,
>
> Can any one point me to paper/book which tabulates the performance of
> MAP decoder when compared to Viterbi. I see most sites only mentioning
> that the MAP algorithm outperforms Viterbi at low SNR and high BER
> condition. Can any one tell me how much is the performance gain by
> using MAP at low SNR conditions.
>
> Thanks
> Sudhi

Are you talking about decoding a convolutional code in AWGN channel?

John

sudhindra.bellary@gmail.com wrote:
> Hello,
>
> Can any one point me to paper/book which tabulates the performance of
> MAP decoder when compared to Viterbi. I see most sites only mentioning
> that the MAP algorithm outperforms Viterbi at low SNR and high BER
> condition. Can any one tell me how much is the performance gain by
> using MAP at low SNR conditions.
>
> Thanks
> Sudhi

Are you talking about decoding a convolutional code in AWGN channel?

John

Hello,

Can any one point me to paper/book which tabulates the performance of
MAP decoder when compared to Viterbi. I see most sites only mentioning
that the MAP algorithm outperforms Viterbi at low SNR and high BER
condition. Can any one tell me how much is the performance gain by
using MAP at low SNR conditions.

Thanks
Sudhi