CASTalk.com

Ant_Magma · Guest

I'm simulating an OFDM transceiver to be later converted and
implemented on a C6713 DSK.
I'm currently facing a problem with the simulation and i'm at my wits
end trying to figure
out what is wrong. Thus i'm really hoping you guys can lend me some
help otherwise i can't
proceed anymore...

Here are 2 pictures of my OFDM simlation in Simulink. Pic1 is the model
with the
convolutional encoder, viterbi decoder, BPSK mod & demod and the OFDM
transceiver. Pic2 is without the encoder/decoder

(both pics size <50k)
Pic1
http://geocities.com/antonio_magma/public/pic1.gif

Pic2
http://geocities.com/antonio_magma/public/pic2.gif

I'm generating 12 bits in a frame (Pic1) and encode with the 1/2
convolutional encoder with
its default setting "poly2trellis(7, [171 133])", the 24 complex
numbers generated by BPSK
are placed on to the carrier bins with symmetric conjugate and are
padded in this format:

(64 IFFT)
1(DC) 2~5(0) 6~29(data) 30~32(0) 33(Nyquist,0) 34~36(0) 37~60(conjugate
data) 61~64(0)

The "Input is conjugate symmetric" parameter in the IFFT block is
ticked so that it'll
produce at real output. Then a 16 sample cyclic prefix is added,
producing a 80 sample
output.

At the receiver, the inverse is performed. At the remove zero padding
block, only the
samples 6~29 are fed through since 37~60 is the complex conjugate and
are useless. Then it is demodulated and decoded with Viterbi decoder
having the same parameter "poly2trellis(7, [171 133])" with a traceback
depth of 35 (since based on Simulink's help, it says if coding is 1/2
then the traceback depth is about x5 of the constaint length which in
this case is 7).

The decision type is set to Hard Decision instead of Unquantized bcoz
if i use unquantized,
i would need an extra Unipolar to Bipolar converter which is not
necessary.

As you can see, i'm getting a BER of 0.5. However, when i tried
removing the convolutional
encoder and Viterbi decoder, set the Bernoulli to output 24 bits (so
that Tx & Rx will be
the same). I get zero errors! Which should mean that my OFDM
tranceivers are working
correctly. I have properly set the receiver delay parameter in the
Error rate calculation
block, 35 with the decoders and 0 without them.

I've checked the output of every single block and noticed that when i
place the
encoder/decoders, i get 35 zeros at the output of Viterbi (due to the
traceback path), then
the corrected bits. The first 12 bits are exactly the same as produced
by the Bernoulli
binary generator, however the rest after that 12 bits contains many
errors! That's why i'm
getting 0.5 BER.

But the question is why is my Viterbi producing such an output? Please
help, i can't
proceed anymore if i can't figure this out...i'm this close to killing
myself

PS: I've attached my OFDM model in Simulink for reference.
http://geocities.com/antonio_magma/public/ofdm.zip

Don Orofino · Guest

Hi Antonio,

Ant_Magma · Guest

Bless you Don :D

Thx 4 ur reply Martin.....
I've did noticed the different sample colors in my model but i didn't
thought much about it. You mentioned:

Ant_Magma · Guest

Bless you Don :D

Thx 4 ur reply Martin.....
I've did noticed the different sample colors in my model but i didn't
thought much about it. You mentioned:

Martin Clark · Guest

The problem is mixed sample times in the OFDM frame assembly (which causes
the Error Rate Calculation block to compare data signals with different
sample rates). In the DSP Constant blocks "DC" and "Nyquist", set the frame
period to inf. The model will then use a single frame rate.

--Martin Clark, Comms Development, The MathWorks

"Don Orofino" <dorofino@verizon.net> wrote in message
news:I5slf.3290$605.2831@trndny09...

Martin Clark · Guest

I may be using a different version of Simulink and so the prompts might
vary. In any case, the sample time of a constant block must be consistent
with the source (via sample time or frame period). You can set it
explicitly, but that can sometimes be an awkward calculation for complex
systems. An easier way with constants is to use inf. This will be
compatible with any other sample time.

"Ant_Magma" <vcteo1@gmail.com> wrote in message
news:1133936245.849953.268760@g47g2000cwa.googlegroups.com...

Ant_Magma · Guest

It's solved!! Thx martin for the tip, the model is now working.

I have another series of questions, hope you don't mind....

I've now included the Puncture and Insert Zero block to my model to
obtain a better code rate of 3/4. However, this has lead into another
problem.

As i have mentioned before, my Viterbi decoder is set to Hard decision.
Hard decision works based on 0s & 1s. When i included the puncture
pair, again i get 0.5 BER. However, if i change the decision type in my
Viterbi to Unquantized and add a Unipolar to Biploar Converter (since
unquantized viterbi works on -1 n 1) before the Insert Zero block, it
works perfectly.

I was wondering is there any way to work around the problem? meaning
without using the bipolar converter?

2nd Q:

I obtained somewhere from the help in Simulink that for a 3/4 code rate
the optimum puncture vector is [110110].' correct?
Based on your experience, is there any optimum parameter for
interleaving? rows & columns (i plan to add interleaving after i've
figure out the puncturing)

I've noticed in the Simulink demos (both HiperLAN2 & WLAN802.11a) which
involves OFDM, interleaving is done twice using the Matrix interleaver
and the General block interleaver. Is it just a choice or preference?
Or is there a specific reason to it? And if so, how do i choose the
Elements for the General block interleaver?

Sorry for the barrage of questions....really appreaciate u guy's help...

Ant_Magma · Guest

It's solved!! Thx martin for the tip, the model is now working.

I have another series of questions, hope you don't mind....

I've now included the Puncture and Insert Zero block to my model to
obtain a better code rate of 3/4. However, this has lead into another
problem.

As i have mentioned before, my Viterbi decoder is set to Hard decision.
Hard decision works based on 0s & 1s. When i included the puncture
pair, again i get 0.5 BER. However, if i change the decision type in my
Viterbi to Unquantized and add a Unipolar to Biploar Converter (since
unquantized viterbi works on -1 n 1) before the Insert Zero block, it
works perfectly.

I was wondering is there any way to work around the problem? meaning
without using the bipolar converter?

2nd Q:

I obtained somewhere from the help in Simulink that for a 3/4 code rate
the optimum puncture vector is [110110].' correct?
Based on your experience, is there any optimum parameter for
interleaving? rows & columns (i plan to add interleaving after i've
figure out the puncturing)

I've noticed in the Simulink demos (both HiperLAN2 & WLAN802.11a) which
involves OFDM, interleaving is done twice using the Matrix interleaver
and the General block interleaver. Is it just a choice or preference?
Or is there a specific reason to it? And if so, how do i choose the
Elements for the General block interleaver?

Sorry for the barrage of questions....really appreaciate u guy's help...

Ant_Magma · Guest

It's solved!! Thx martin for the tip, the model is now working.

I have another series of questions, hope you don't mind....

I've now included the Puncture and Insert Zero block to my model to
obtain a better code rate of 3/4. However, this has lead into another
problem.

As i have mentioned before, my Viterbi decoder is set to Hard decision.
Hard decision works based on 0s & 1s. When i included the puncture
pair, again i get 0.5 BER. However, if i change the decision type in my
Viterbi to Unquantized and add a Unipolar to Biploar Converter (since
unquantized viterbi works on -1 n 1) before the Insert Zero block, it
works perfectly.

I was wondering is there any way to work around the problem? meaning
without using the bipolar converter?

2nd Q:

I obtained somewhere from the help in Simulink that for a 3/4 code rate
the optimum puncture vector is [110110].' correct?
Based on your experience, is there any optimum parameter for
interleaving? rows & columns (i plan to add interleaving after i've
figure out the puncturing)

I've noticed in the Simulink demos (both HiperLAN2 & WLAN802.11a) which
involves OFDM, interleaving is done twice using the Matrix interleaver
and the General block interleaver. Is it just a choice or preference?
Or is there a specific reason to it? And if so, how do i choose the
Elements for the General block interleaver?

Sorry for the barrage of questions....really appreaciate u guy's help...

Ant_Magma · Guest

It's solved!! Thx martin for the tip, the model is now working.

I have another series of questions, hope you don't mind....

I've now included the Puncture and Insert Zero block to my model to
obtain a better code rate of 3/4. However, this has lead into another
problem.

As i have mentioned before, my Viterbi decoder is set to Hard decision.
Hard decision works based on 0s & 1s. When i included the puncture
pair, again i get 0.5 BER. However, if i change the decision type in my
Viterbi to Unquantized and add a Unipolar to Biploar Converter (since
unquantized viterbi works on -1 n 1) before the Insert Zero block, it
works perfectly.

I was wondering is there any way to work around the problem? meaning
without using the bipolar converter?

2nd Q:

I obtained somewhere from the help in Simulink that for a 3/4 code rate
the optimum puncture vector is [110110].' correct?
Based on your experience, is there any optimum parameter for
interleaving? rows & columns (i plan to add interleaving after i've
figure out the puncturing)

I've noticed in the Simulink demos (both HiperLAN2 & WLAN802.11a) which
involves OFDM, interleaving is done twice using the Matrix interleaver
and the General block interleaver. Is it just a choice or preference?
Or is there a specific reason to it? And if so, how do i choose the
Elements for the General block interleaver?

Sorry for the barrage of questions....really appreaciate u guy's help...

Ant_Magma · Guest

It's solved!! Thx martin for the tip, the model is now working.

I have another series of questions, hope you don't mind....

I've now included the Puncture and Insert Zero block to my model to
obtain a better code rate of 3/4. However, this has lead into another
problem.

As i have mentioned before, my Viterbi decoder is set to Hard decision.
Hard decision works based on 0s & 1s. When i included the puncture
pair, again i get 0.5 BER. However, if i change the decision type in my
Viterbi to Unquantized and add a Unipolar to Biploar Converter (since
unquantized viterbi works on -1 n 1) before the Insert Zero block, it
works perfectly.

I was wondering is there any way to work around the problem? meaning
without using the bipolar converter?

2nd Q:

I obtained somewhere from the help in Simulink that for a 3/4 code rate
the optimum puncture vector is [110110].' correct?
Based on your experience, is there any optimum parameter for
interleaving? rows & columns (i plan to add interleaving after i've
figure out the puncturing)

I've noticed in the Simulink demos (both HiperLAN2 & WLAN802.11a) which
involves OFDM, interleaving is done twice using the Matrix interleaver
and the General block interleaver. Is it just a choice or preference?
Or is there a specific reason to it? And if so, how do i choose the
Elements for the General block interleaver?

Sorry for the barrage of questions....really appreaciate u guy's help...

Guest · Posted: Fri Dec 09, 2005 1:16 am Post subject: Re: OFDM Simulink problem, SOS!

See Below.

Ant_Magma wrote:

Ant_Magma · Guest

The first method you mentioned is soft-decision viterbi correct?

Guest · Posted: Fri Dec 09, 2005 5:16 pm Post subject: Re: OFDM Simulink problem, SOS!

The first method was really just an example of what is required to
provide the decoder with the correct information for the missing bits.
If you provide the decoder with a value of 0.5, from the input range
{0,1} then you are effectively providing 'soft' bit decisions aren't
you?

What I'm really trying to say is that you need to decide what your
maximum positive value (equiv binary one) and your maximum negative
values (equiv binary zero) are, and then for the missing bits, due to
puncturing, you insert a value that is smack bang in the middle of
these two extreme values (assuming, of course, that you don't have any
prior information pertaining to the missing bits!). Why, do you choose
a value smack bang in the middle? Well, because I'm assuming that the
transmitted bits are equiprobable and therefore, you won't have any
knowledge of the values of the missing punctured bits in the receiver.

For example, if you have only hard bit decisions at the input, then you
either convert the binary stream of ones and zeros into a bipolar
signal and then perform zero insertion. Or, you leave the input data
as a string of ones and zeros and you perform 0.5 insertion. Both
methods work equally well, it's just that you have to work out whether
the decoder you're implementing can handle one input or the other....no
more, no less!

cheers.

nt_Magma wrote:
> The first method you mentioned is soft-decision viterbi correct?