Alamouti coding

Dear all,

I am loosing my mind trying to implement alamouti code between 2 tx and
1 rx.
I downloaded the trodeau repository but I cannot make the alamouti
works.

Any help will be appreciated, really, ANY!

Thank you

Vanessa

Thank you

Vanessa

Would you tell us what the problem is before we can help?
KZ

Sure,
I am using the code on the trodeau repository and the ofdm 1tx-1rx
works, however when I try to use the 2tx-1tx with the alamouti it
doesn’t.
I am not using USRPs now, but just 2 channel models without noise and
offset.
The tx codes 2 consecutive symbols as the alamouti says and add 1
preamble for each antenna:
first tx: |0…0preamble10…0| |0…0|
second tx: |0…0| |0…0preamble20…0|
length preamble1 = length preamble2 = 200 occupied tones
lenght |0…0preamble10…0| = 512 = fft legth
length |0…0| = 512 = fft legth
preamble1 is created as in the ofdm 1tx-1rx i.e. it contains 1s, -1s and
0s
preamble2 contains 1s and -1s

The rx performs synchronization and alamouti decoding taking 2
consecutive symbols each time.

Sometimes the rx is able to decode the header and the pkt length, and
the first 18 character of the payload. The length of the pkt is 400
bytes.

The first error that I can notice is that when it is decoding the
payload sometimes it finds the SYNC signal which means that it finds a
preamble at a certain point of the payload before the length of 400
bytes.

So I think that the problem is in the synchronisation process but it
can be also in the equalizer computation.
Moreover the course frequency is different from 0, but how can this be
possible if I do not introduce offset in the channel?

Can someone helps me in understanding at which stage the error is?

Thank you so much

Vanessa

On Fri, Nov 18, 2011 at 1:29 AM, Kyle Z. [email protected] wrote:

Thank you

Vanessa

Would you tell us what the problem is before we can help?
KZ


Vanessa GARDELLIN, Ph.D.
Researcher
Institute for Informatics and Telematics (IIT),
Italian National Research Council (CNR)
Via G. Moruzzi 1
56124 Pisa - ITALY
Phone: +390503153267
Room: B65/c
E-mail: [email protected]
WWW: http://www.iit.cnr.it/staff/vanessa.gardellin/
Skype: gardellin.vanessa

On Fri, Nov 18, 2011 at 3:57 AM, Vanessa Gardellin <
[email protected]> wrote:

length preamble1 = length preamble2 = 200 occupied tones
bytes.

Can someone helps me in understanding at which stage the error is?

Thank you so much

Vanessa

Vanessa,
I applaud your attempts at getting this working, but I hope you
understand
that this was an experimental development branch that is unfinished and
polished, which is why it never made it’s way into the mainline master.
Matt and I got parts of it working over a couple of days of hacking, but
it
was under a single experimental setup (the 2 USRPs were on opposite
sides
of a table), so we didn’t develop it at that point for robustness.

I just want to make sure that you know that you’ll be doing a lot of
hacking and experimenting here to get things to work. Mostly, I think we
provided a good framework for how to do a MIMO setup and structure of
the
flow graph and got a good start on the algorithms, but that’s where a
lot
of work is still required. The code is also probably a mess!

You say that you are having problems receiving packets. I’m assuming
that
you have tested out your setup with the standard OFDM benchmark code
that
is in GNU Radio right now. If you can get a solid setup using this code,
that’s where you want to start when working on the MIMO algorithms. Just
to
make sure that your gain settings and transmit powers are set correctly
for
your daughterboards and channel.

Next, it’s best to take these things one step at a time. The
gr_plot_ofdm.py and plot_ofdm.m files are tools we developed to look at
the
signals from different domains and different perspectives. They might
help
you understand the system and signal properties better to find where in
the
receive chain things are falling apart.

From there, the best way to get help is to use these tools to identify
where the problems are to be able to ask the list specific, pointed
questions about the algorithms and setup.

Hope that helps!
Tom

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