2x1 Alamouti scheme using 3 USRP units

[Dor_Afriat]

Hello everyone.

I'm trying to implement 2x1 Alamouti scheme using 3 USRP 2920 radios, two as transmitters and one a a receiver.

I've been facing some questions I'll be glad if you could answer:

1. How can I keep all 3 devices synchronized? 2 of them are connected with a MIMO cable, but the third isn't. should I use the connected pair as my two transmitters?

2. is it possible to use a preamble for syncronization and chaneel estimation without a MIMO cable? by that I mean, is there a way to make a usrp detect a training sequence and start the receiving process only after the training sequence?

3. how can I reduce the impact of phase shift? from what I've been informed, if a pair is connected with a MIMO cable, there should not be a shift in the phase, but when i'm trying to transmit and receive a signal with 2 USRPs connected with a MIMO cable, I still get a shifted signal(I added an illustration).

thank you, much appreciation

[psisterhen]

1) Yes, connecting the two transmitters using the MIMO cable will be best.  These channels need to be tightly synchronized.  You could also share an external reference clock and PPS signal between all devices, if you have those available.  Otherwise, the receiver channel may have some frequency offset and won't sample at exactly the same time, but you can correct for that in your receiver code.

2) Yes, but you'll have to start streaming samples to the host and detect the preamble in your receiver code on the host computer.

3) You will have to detect the frequency offset and shift the frequency of your received signal in software.  There are many algorithms to do both.  You may want to see some of the community examples posted here:

https://decibel.ni.com/content/groups/ni-sdr-community?view=documents

[Dor_Afriat]

thank you.

can you please explain more about external reference clock and PPS signal? What hardware does it require?

[psisterhen]

The reference signal is usually 10 MHz and used as a reference clock for the sample clocks and the local oscillators on the devices.

The PPS signal (Pulse Per Second) is a synchronization signal that allows you to set the devices to the same time so that you can start generating/acquiring at the same time.

If you happen to have a PXI chassis as part of your system, that has a 10 MHz output you could route to the Tx and Rx devices.  Additionally, you could use a timing and sync module like this:

http://sine.ni.com/nips/cds/view/p/lang/en/nid/209100

If PXI is not an option in your system, consider the Octoclock:

http://www.ettus.com/product/details/OctoClock-G

[Dor_Afriat]

thanks, but I perfer to do a wireless sync. 

I've created a matlab code to prepare symbols to send as describe in Alamouti scheme. 

i've added a preamble and zero sequence. How can I now use those symbols and transmit them through 2 USRP transmitters connected with a MIMO cable?

it is imprortant to transmit in the same time slots from the two devices. Is there an example of using 2 USRPs connected with a MIMO cable as synchronized transmitters?

thank you very much.

[NasirHussain]

Hi, would you abe able to share some details to look into your problem?