Doubt

>Chintan, let me give you a homework ;-).
>
>Suppose that you have a simple QPSK modem at carrier freq fc,
>sampled at sampling rate fs > 2*(fc+R), symbol rate R, and doppler
>is \delta.
>
>Suppose that you sample directly at passband.
>
>Now look at your current receiver and answer the following:
>1.  At the output of the ADC, what is the dopper rate normalized
>    per sample?  What is the normalized frequency offset?
>2.  Same thing, but now after your mixer.
>3.  Same thing, but now after you decimate to close to R.
>
>I hope that you can at least convince yourself if working in baseband
>is a good idea or not.
>
>Good luck!
>Julius

%%%

Thanks a lot Julius.

Now I am not using any ADC or mixer in my simulations, but my TX, channel
and RX looks like this:

Tx:
info->enc->mapping->pulse shaping->carrier modulation

Channel:
delay->doppler->noise

RX:

Down Conversion->matched filter->doppler correction
DFE->demapper->decoder

Now, let say fc=12KHz, R=4KHz, Fs=48KHz

So Ns=number of samples per symbol=12

If I define doppler delta=V/c;

so my received signal will appear as r(nTs)=s(n(1+delta)Ts)

so according to this my sampling frequency at RX should be 

fs_rx=(1+delta)*fs;

I dont know if I am answering your ques but, it seems that if I knew this
'delta' at receiver, it is good to sample at passband.

Sorry if I am wrong. 

Thanks again.

Chintan

On Aug 5, 4:47&#4294967295;pm, "cpshah99" <cpsha...@rediffmail.com> wrote:

>
> Thanks a lot Julius.
>
> Now I am not using any ADC or mixer in my simulations, but my TX, channel
> and RX looks like this:
>
> Tx:
> info->enc->mapping->pulse shaping->carrier modulation
>
> Channel:
> delay->doppler->noise
>
> RX:
>
> Down Conversion->matched filter->doppler correction
> DFE->demapper->decoder
>
> Now, let say fc=12KHz, R=4KHz, Fs=48KHz
>
> So Ns=number of samples per symbol=12
>
> If I define doppler delta=V/c;
>
> so my received signal will appear as r(nTs)=s(n(1+delta)Ts)
>
> so according to this my sampling frequency at RX should be
>
> fs_rx=(1+delta)*fs;
>
> I dont know if I am answering your ques but, it seems that if I knew this
> 'delta' at receiver, it is good to sample at passband.
>
> Sorry if I am wrong.
>
> Thanks again.
>
> Chintan

Chintan, please put in the actual values, and normalize the drifts,
the frequency offsets, etc. to the sampling rate, meaning that the
expressions should be in radians instead of in absolute values.

Is your goal only to simulate or to eventually build a system?

What stops you from knowing \delta at passband or at any point
in your receiver stages?

Personally, I like to split up my receiver designs into "inner"
and "outer" receivers.  That way I can assume that one is
perfect and optimize the other, and reverse the role.  I have
a feeling that somehow you've convinced yourself to be limited
to a particular architecture or algorithm, and I think that this is
why you are having so much trouble, or seem to think that you
do.  To your credit, you know the causes the grief.

Now, are you sure that your doppler emulator in your simulation
is good enough for what you want to do?  Have you verified?
Julius

>On Aug 5, 4:47=A0pm, "cpshah99" <cpsha...@rediffmail.com> wrote:
>
>>
>> Thanks a lot Julius.
>>
>> Now I am not using any ADC or mixer in my simulations, but my TX,
channel
>> and RX looks like this:
>>
>> Tx:
>> info->enc->mapping->pulse shaping->carrier modulation
>>
>> Channel:
>> delay->doppler->noise
>>
>> RX:
>>
>> Down Conversion->matched filter->doppler correction
>> DFE->demapper->decoder
>>
>> Now, let say fc=3D12KHz, R=3D4KHz, Fs=3D48KHz
>>
>> So Ns=3Dnumber of samples per symbol=3D12
>>
>> If I define doppler delta=3DV/c;
>>
>> so my received signal will appear as r(nTs)=3Ds(n(1+delta)Ts)
>>
>> so according to this my sampling frequency at RX should be
>>
>> fs_rx=3D(1+delta)*fs;
>>
>> I dont know if I am answering your ques but, it seems that if I knew
this
>> 'delta' at receiver, it is good to sample at passband.
>>
>> Sorry if I am wrong.
>>
>> Thanks again.
>>
>> Chintan
>
>Chintan, please put in the actual values, and normalize the drifts,
>the frequency offsets, etc. to the sampling rate, meaning that the
>expressions should be in radians instead of in absolute values.
>
>Is your goal only to simulate or to eventually build a system?
>
>What stops you from knowing \delta at passband or at any point
>in your receiver stages?
>
>Personally, I like to split up my receiver designs into "inner"
>and "outer" receivers.  That way I can assume that one is
>perfect and optimize the other, and reverse the role.  I have
>a feeling that somehow you've convinced yourself to be limited
>to a particular architecture or algorithm, and I think that this is
>why you are having so much trouble, or seem to think that you
>do.  To your credit, you know the causes the grief.
>
>Now, are you sure that your doppler emulator in your simulation
>is good enough for what you want to do?  Have you verified?
>Julius
>

%%%%%

Hi Julius,

My ultimate goal is just to simuate the system.

And the doppler correction that I am using, it actually converges to the
delta.

You are right in the sense that I had convinced myself abt this perticular
algorithm.

But again as u said, I will consider the points that you have made and
work on it.

Thanks a lot.

Chintan

On Aug 6, 2:11 am, "cpshah99" <cpsha...@rediffmail.com> wrote:
>
> Hi Julius,
>
> My ultimate goal is just to simuate the system.
>
> And the doppler correction that I am using, it actually converges to the
> delta.
>
> You are right in the sense that I had convinced myself abt this perticular
> algorithm.
>
> But again as u said, I will consider the points that you have made and
> work on it.
>
> Thanks a lot.
>
> Chintan

Chintan, can you tell me where you found the thesis of
T H Eggen, please?

For any communication system, I recommend as follows:

 * Assume perfect synch, go end-to-end including channel,
   front-end, and try to see if it's implementable in the platform
   of your choice (fixed-point, etc.)

 * Now add synch mismatch, but assume that the receiver
   knows the nuisance parameters exactly.  Consider a few
   architectures and algorithms, test the performance.  This
   is useful for telling you the complexity of the "inner receiver".
   It sounds trivial to most people who are not very concerned
   with implementation, but this stage of the design should
   tell you the order of the filters, interpolators, oversampling
   rates.

 * Now try to come up with an architecture and algorithm to
   estimate the nuisance parameters.  Can you do a one-shot
   estimate based on a preamble?  Or do you have to use a
   smoothed / tracked estimate via a loop filter?

 * Then try to put all of the above together.

In principle, you can do everything in baseband, of course, but
the devil is in the details.  If your Doppler is so big that your
baseband correction's loop filter bandwidth has to be very big,
then you are going to suffer in terms of performance.

Good luck on your project.  I assure you that you'll learn a lot
from using a more structured approach to solving your problem.

Julius

On Aug 5, 3:13 pm, "cpshah99" <cpsha...@rediffmail.com> wrote:
>
> As u have said, I am going to read Mengali again.
>
> Also there is a good book:
>
> Simulation of Communication Systems, 2nd ed, by  Michel C Jeruchim et al.
>

So I think that the Mengali book is not as useful for you at this
stage.
If you already have a choice of architecture or algorithm in mind,
it's
a great reference, but this book does not tell you which architecture
to
use.

I know it seems really frustrating, but a non-trivial project is what
makes
a person stronger :-P.  In fact, the non-trivial-ness makes it more
worthwhile, otherwise any dude will have been able to get the answer
off
of a textbook!

Julius

>On Aug 6, 2:11 am, "cpshah99" <cpsha...@rediffmail.com> wrote:
>>
>> Hi Julius,
>>
>> My ultimate goal is just to simuate the system.
>>
>> And the doppler correction that I am using, it actually converges to
the
>> delta.
>>
>> You are right in the sense that I had convinced myself abt this
perticular
>> algorithm.
>>
>> But again as u said, I will consider the points that you have made and
>> work on it.
>>
>> Thanks a lot.
>>
>> Chintan
>
>Chintan, can you tell me where you found the thesis of
>T H Eggen, please?
>
>For any communication system, I recommend as follows:
>
> * Assume perfect synch, go end-to-end including channel,
>   front-end, and try to see if it's implementable in the platform
>   of your choice (fixed-point, etc.)
>
> * Now add synch mismatch, but assume that the receiver
>   knows the nuisance parameters exactly.  Consider a few
>   architectures and algorithms, test the performance.  This
>   is useful for telling you the complexity of the "inner receiver".
>   It sounds trivial to most people who are not very concerned
>   with implementation, but this stage of the design should
>   tell you the order of the filters, interpolators, oversampling
>   rates.
>
> * Now try to come up with an architecture and algorithm to
>   estimate the nuisance parameters.  Can you do a one-shot
>   estimate based on a preamble?  Or do you have to use a
>   smoothed / tracked estimate via a loop filter?
>
> * Then try to put all of the above together.
>
>In principle, you can do everything in baseband, of course, but
>the devil is in the details.  If your Doppler is so big that your
>baseband correction's loop filter bandwidth has to be very big,
>then you are going to suffer in terms of performance.
>
>Good luck on your project.  I assure you that you'll learn a lot
>from using a more structured approach to solving your problem.
>
>Julius
>
%%%%%

Hi Julius,

Thanks a lot for the encouraging words. This is what a frustrated person
like me needs!!!

Here is the link for the thesis:

http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA343081

The points u have mentioned above are very useful.

Thanks a lot.

Chintan