I checked few of the papers and I would like to make some correction what i
said earlier... 

>Hi vikram
>         i think there is serious misunderstanding on the OFDM itself on
>your side...   for diversity scheme you have to take one FFT only...

Not always true.....Not for all Dieversity schemes..... depends upon which
channel model is used... e.g Jakes,raliegh   etc... and how things are
implemented in specific....

>use linear combiner at the reciver... to recover individual signals...if
>you understand equalisation concept in OFDM it would be much easier for
>you to make out how things really fits well

That may be one way of implemeining things .... Aplying adavnced STBC
codes is another option... they provide better orthogonality but increase
computation complexity by a lot.. dont know much about MMSE or RLS
equalisation schems for STBC codes... 

>                      about preambles i can say that there are multiple
>ways of implementing detection schemes... i would rather prefer sending
>some pattern from one antenna while sending nothing from other and a
>distinct pattern from other antenna while  switching off the other
one...
> and dont confuse things with what some of the papers might be saying.. 
>
>you can refer almouti paper for details...


  certainly this is one of the options...

Again not very sure about all the the things..thats how i understand
things.... May be somebody else can answer in a better way  ...anybody 
???
sandeep
>
>                                  
>
>
>>Thanks for the replies.
>>
>>I cannot assume that the signals are arriving at equal time/phase.
>>I have to perform frequency and FFT symbol window synchronization for
>>each of the transmitted signals within a single receive signal.
>>Won't each signal act as an interferer for the other signal to make the
>>timing estimation more difficult...
>>
>>My guess is that the preambles should be made orthogonal/shift
>>orthogonal (as is mentioned in a paper by Stuber), but the spectrum of
>>the resultant signal is no longer band-limited, as we are sending
>>pseudorandom quasiorthogonal modulated sequences..
>>
>>Thanks
>>Vikram
>>
>>
>
>
>		
>This message was sent using the Comp.DSP web interface on
>www.DSPRelated.com
>


		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

>From a different perspective, what would be the benefit of applying
>STBC to the preamble?

Not more than the round-robbin approach, but it is a feasable way. In case
of 4 Tx antennas, round-robbin is even better in the sense that only 4
symbol periods are needed while STBC needs 8. But concerning the
nonlinearity of Tx power amplifier STBC allows a lower transmit power per
antenna compared to round-robbin. This reduces the nonlinear distortion.

>I think synchronization may be a bit more difficult if the correlation
>properties of the preamble are degraded by application of the STBC.
>In other words, if you're relying on nice correlator peaks to achieve
>synchronization if the code splatters the peaks it'll make
>synchronization harder.   I'm not sure what benefit would be gained by
>applying the STBC to the preamble, but I can see some definite
>complications by doing so.   Perhaps my "not possible" comment was too
>strong, but "more difficult" is certainly arguable.

I agree.



		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

On Wed, 19 Oct 2005 10:44:36 -0500, "lanbaba" <lanbaba@gmx.ch> wrote:

>> To your second question; if the channels aren't known and are to be
>> estimated using the preamble (which is often the case), then it isn't
>> possible to apply STBC to the preambles. Since decoding of the STBC
>> requires not only synchronization but knowledge of the channels, it's
>> tough to apply an STBC to the preamble.
>
>
>I don't catch you point. 
>
>In Alamouti's scheme (2x2 STBC) your transmitted signal is orthogonal. If
>such an orthogonal design is applied to the preamble for each subcarrier,
>then the channel estimation is straight forward given synchronization. Or
>do you mean the synchronization is too difficult?

From a different perspective, what would be the benefit of applying
STBC to the preamble?

I think synchronization may be a bit more difficult if the correlation
properties of the preamble are degraded by application of the STBC.
In other words, if you're relying on nice correlator peaks to achieve
synchronization if the code splatters the peaks it'll make
synchronization harder.   I'm not sure what benefit would be gained by
applying the STBC to the preamble, but I can see some definite
complications by doing so.   Perhaps my "not possible" comment was too
strong, but "more difficult" is certainly arguable.


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

> To your second question; if the channels aren't known and are to be
> estimated using the preamble (which is often the case), then it isn't
> possible to apply STBC to the preambles. Since decoding of the STBC
> requires not only synchronization but knowledge of the channels, it's
> tough to apply an STBC to the preamble.


I don't catch you point. 

In Alamouti's scheme (2x2 STBC) your transmitted signal is orthogonal. If
such an orthogonal design is applied to the preamble for each subcarrier,
then the channel estimation is straight forward given synchronization. Or
do you mean the synchronization is too difficult?

Lanbaba

		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

>My guess is that the preambles should be made orthogonal/shift
>orthogonal (as is mentioned in a paper by Stuber), but the spectrum of
>the resultant signal is no longer band-limited, as we are sending
>pseudorandom quasiorthogonal modulated sequences..

What do you mean with "pseudorandom quasiorthogonal"? 

In the sense of a minimum estimation variance with a least squares
estimator 
the optimal time domain preamble sequences that you send from different TX
antennas are only defined within one OFDM symbol duration. As you read in
those papers, they should have two properties
1. cyclic cross-correlation vanishes within a window whose length is the
guard period.
2. cyclic auto-correlation is perfect within a window whose length is the
guard period.

Taking DFT of those time domain sequences you get pilot sequences defined
in the frequency domain. Each pilot symbol modulates a subcarrier in one
of the Tx antennas and the orthogonality is not destoried at all.

Lanbaba


		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

Hi vikram
         i think there is serious misunderstanding on the OFDM itself on
your side...   for diversity scheme you have to take one FFT only... and
use linear combiner at the reciver... to recover individual signals...if
you understand equalisation concept in OFDM it would be much easier for
you to make out how things really fits well

                      about preambles i can say that there are multiple
ways of implementing detection schemes... i would rather prefer sending
some pattern from one antenna while sending nothing from other and a
distinct pattern from other antenna while  switching off the other one...
 and dont confuse things with what some of the papers might be saying.. 

you can refer almouti paper for details...

i hope this will help you a lot

sandeep

                                  


>Thanks for the replies.
>
>I cannot assume that the signals are arriving at equal time/phase.
>I have to perform frequency and FFT symbol window synchronization for
>each of the transmitted signals within a single receive signal.
>Won't each signal act as an interferer for the other signal to make the
>timing estimation more difficult...
>
>My guess is that the preambles should be made orthogonal/shift
>orthogonal (as is mentioned in a paper by Stuber), but the spectrum of
>the resultant signal is no longer band-limited, as we are sending
>pseudorandom quasiorthogonal modulated sequences..
>
>Thanks
>Vikram
>
>


		
This message was sent using the Comp.DSP web interface on
www.DSPRelated.com

Hi
   I am implementing COFDM. Can any body tell me how actually The Jakes
wireless channel works...Because the parameters and everything are
deterministic.... Plz let me know why.....

Wow, thats pretty interesting, i.e the fact that the timing
misregistration between the different transmit antennas is ignorable..
actually, It seems intuitive. If that is indeed the case, (i.e
identical symbol timing), then the papers are justified in giving a
single symbol timing instant for the received signal, otherwise, they
are building castles in the air.

My project will have 2.4 Ghz antennas separated by less than a 1m
apart. They will transmit using MIMO-OFDM scheme.  I will have a
receiver which will acquire 1 ms of data, demodulated the MISO OFDM
signal and display a clean constellation.

Once the project is over, I plan to put up the source code freely on a
web-site and cover it with a GNU license...

On 16 Oct 2005 06:57:38 -0700, cvikram@mac.com wrote:

>Thanks for the replies.
>
>I cannot assume that the signals are arriving at equal time/phase.
>I have to perform frequency and FFT symbol window synchronization for
>each of the transmitted signals within a single receive signal.
>Won't each signal act as an interferer for the other signal to make the
>timing estimation more difficult...

Unless the distance between the transmit antennas is huge, the timing
misregistration between the incident signals at the receive antenna
will be much less than the channel delay spread.   In this sense
symbol timing recover shouldn't be much of an issue, I'd think, but I
don't know what application you're addressing.

>My guess is that the preambles should be made orthogonal/shift
>orthogonal (as is mentioned in a paper by Stuber), but the spectrum of
>the resultant signal is no longer band-limited, as we are sending
>pseudorandom quasiorthogonal modulated sequences..

Another method is to make sections of the preamble time orthogonal,
i.e., coordinate the transmitters to each transmit a short section of
preamble in a time-multiplexed fashion.   This provides an easy way to
separate the channel estimates at the receiver.   It might make the
preamble a little bit longer, but that's a system tradeoff.


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

Thanks for the replies.

I cannot assume that the signals are arriving at equal time/phase.
I have to perform frequency and FFT symbol window synchronization for
each of the transmitted signals within a single receive signal.
Won't each signal act as an interferer for the other signal to make the
timing estimation more difficult...

My guess is that the preambles should be made orthogonal/shift
orthogonal (as is mentioned in a paper by Stuber), but the spectrum of
the resultant signal is no longer band-limited, as we are sending
pseudorandom quasiorthogonal modulated sequences..

Thanks
Vikram