Re: Fair comparision: Reed Solomon code and convolutional code

On Wed, 16 Jul 2008 08:53:36 -0500, "cpshah99"
<cpshah99@xxxxxxxxxxxxxx> wrote:

On Jul 16, 2:10 am, "cpshah99" <cpsha...@xxxxxxxxxxxxxx> wrote:
On Jul 15, 3:03 pm, "cpshah99" <cpsha...@xxxxxxxxxxxxxx> wrote:

What should I do to make it fair? or is this comparision valid at
all?

Which is better for chiseling wood, a screwdriver
or a ball-peen hammer? Neither is very good for
the task, but you could do a little bit, maybe, with
the screwdriver and hardly anything with the
hammer. So, "clearly" the screwdriver is the
"better" tool. But, ah, when you have to drive
nails, the hammer is far far better than the
screwdriver (think of CDs and DVDs), isn't it?
And when you need to drive screws, the
screwdriver is the best tool, though you might
be able to do a little with a chisel if you have a
flat-head screw (forget Phillips heads!) and
nothing with the hammer.

%%%%%

Hi

Thanks for this carpentry stuff which I already know!!!!

chintan

Good. Now tell your boss that the convolutional code
is going to be better in almost any application and the
Reed-Solomon code is useless (unless he has a CD or
DVD (or a concatenated coding scheme) in mind.)
Reed-Solomon codes are not very good as channel
codes, but they make great straw men to be knocked
down while touting the merits of a coding scheme that
you really ought to be using in the first place.

%%%%

Hi

Thanks again. I had expected convolutional code to do better and it is
doing the same.

But another advantage of RS code is that it corrects burst error, which
is
problem in fading channel, and because of burst errors DFE (Decision
Feedback Equaliser) will not work properly. But if you use RS, this
problem
is solved.

Now to solve the same problem with convolution encoder, u will have to
use
'interleaver', otherwise forget that even BCJR will decode the burst
errors.

So my system with convolution encoder and interleaver outperforms RS
system.

Everything is same when I am comparing both the systems, channel,
doppler,
noise, adaptive algorithm for DFE.

But I was wondering that if I can claim that given the channel
perticular
channel conditions, convolution code will outperform RS.

BTW, nobody is boss.

Chintan

The bottom line is that care has to be taken to match codes to
channels. The assumptions about the error distributions in the
channel can make a big difference in how the code is applied or
architected into the system.

While on the surface the fact that RS codes can correct "burst errors"
is true, the nature of the "burst error" can make a big difference.
Often the length of the error burst can overwhelm the correcting
capability of the RS code. The performance curve (in an error
probability vs SNR sense) of a system with an RS code is pretty steep.
What often happens is that the region from performing well to
performing poorly is small, and RS codes exhibit a sometimes
undesirable characteristic of aliasing codewords when in the "poor"
region. i.e., many of the codewords coming out of the decoder will be
marked as corrected, but they'll be incorrect. At least with a
convolution code and a Viterbi decoder you can tell when it's unlocked
and spewing garbage.

So the nature of the error bursts matters, and the sensitivity of the
system to output errors matters. Interleavers are often used to
reduce the length of error bursts so that the channel code works
better. There are all kinds of considerations, and I think Dr.
Sarwate's initial response hints at the complexities of picking the
appropriate tool depending on the situation.


Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org

Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php


%%%%

Hi Eric

Thanks for your reply.

So it all depends on burst error patterns.

And as Dr. Sarwate said, convolution will do better than RS and I can
confirm (I am nobody to do this, but I am just saying) that from my
results.

Thanks a lot to everybody.

Chintan
.

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