# Alamouti coding **URL:** \ **Category:** GNU Radio\ **Created:** [November 17, 2011, 5:47pm UTC](https://www.ruby-forum.com/t/alamouti-coding/213969 "2011-11-17T17:47:25Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![luislavena](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/l/0ea827/32.png) [@luislavena](https://www.ruby-forum.com/u/luislavena)\ **Post date:** [November 17, 2011, 5:47pm UTC](https://www.ruby-forum.com/t/alamouti-coding/213969/1 "2011-11-17T17:47:25Z")
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 ---
**Author:** ![Vanessa\_Gardellin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/ebca7d/32.png) [@Vanessa\_Gardellin](https://www.ruby-forum.com/u/Vanessa_Gardellin)\ **Post date:** [November 18, 2011, 1:26am UTC](https://www.ruby-forum.com/t/alamouti-coding/213969/2 "2011-11-18T01:26:45Z")
> Thank you > > Vanessa Would you tell us what the problem is before we can help? KZ ---
**Author:** ![Vanessa\_Gardellin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/ebca7d/32.png) [@Vanessa\_Gardellin](https://www.ruby-forum.com/u/Vanessa_Gardellin)\ **Post date:** [November 18, 2011, 9:59am UTC](https://www.ruby-forum.com/t/alamouti-coding/213969/3 "2011-11-18T09:59:08Z")
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. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) 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: [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) WWW: [http://www.iit.cnr.it/staff/vanessa.gardellin/](http://www.iit.cnr.it/staff/vanessa.gardellin/) Skype: gardellin.vanessa ---
**Author:** ![Vanessa\_Gardellin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/ebca7d/32.png) [@Vanessa\_Gardellin](https://www.ruby-forum.com/u/Vanessa_Gardellin)\ **Post date:** [November 18, 2011, 4:03pm UTC](https://www.ruby-forum.com/t/alamouti-coding/213969/4 "2011-11-18T16:03:53Z")
On Fri, Nov 18, 2011 at 3:57 AM, Vanessa Gardellin \< [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)\> 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