OFDM MIMO 2x2 with NI USRP 2920

[pixus]

Hi,

I am trying to set up MIMO 2x2 system with 4 NI USRPs, 2 MIMO cables and one switch like in example https://decibel.ni.com/content/docs/DOC-23285, but without external PPS REFin, and i have some problems.

Simulation doesn't work because this error: "Packet had timestamp that was late (or to early)".

Does anyone know how to solve this problem?

Thanks

[sshearman]

Hi Pixus,

The example really requires the external PPS in right now in order to avoid the error you are seeing. It's used to synchronize the transmit and receive operations. The problem is that without the PPS in signal, the timing of the operations isn't deterministic and you'll likely have cases where the transmit preceeds the receive operation, which can lead to that error.

--Sam

[pixus]

Hi Sshearman,

Example doesn't requires the external PPS, I have try it without any external PPS,and it works. The problem was in the shame, in example transmitters TX2,TX3 and TX5 are connected directly with Gb Ethernet cable with switch and PC, and for their channels (1,2 and 4), trigger parameters must be set. In the schema, there is some text string (1,2,4), who defines channels in witch trigger must be set.

I was connected two NI USRPs 2920 on channel 0 and 1, but i didn't change txt string. After changing txt string to (0,1), everything works correctly.

I have another question. 

I tryed to set mimo 2x2 with alamauthi codeing like in example http://www.ni.com/white-paper/13878/en. Everythime this error pops up:

"A stream command was ishued in the past". What can i do to solve this problem?

Thanks

Pixus

[horvlint]

Hi Pixus,

I have found a forum post discussing the same error you that you have. There are possinle solutions to your problem:

http://forums.ni.com/t5/LabVIEW/NIUSRP-error-in-receiving-data-with-synchronized-5-USRP2-boards/m-p/...

Please let me know if any of these advices was helpful or if there's any progression regarding your issue.

[0111]

Hi Pixus,

I have tried 2x2 MIMO OFDM System and received the error "A stream command was ishued in the past", too. Do you solve this problem? If you do, could you explain how did you fix it?

Thanks

Arda

[ErikL]

You need to time sync over the MIMO cable. See figure 6 step 4 and figure 7 step 5.

http://www.ni.com/white-paper/14311/en

[ErikL]

Note that using a MIMO cable and no internal reference the firs channel in step 4 should be set to "internal" and the timing event in step 5 should be set to "now".

[pixus]

Hi ErikL,

What is the full allocated bandwidth that is used in your example from https://decibel.ni.com/content/docs/DOC-23285,

if we transmit signal on 915Mhz, with IQ rate of 1 MS/s?

Or what is the frequency space between subcarries?

Thanks

[madong]

Hi Pixus,

         This is the first time that I use NI USRP 2920,and I want to build a MIMO-OFDM system for a platform to achieve signal detectation.But I just have three NI USRP 2920 and six antennas.Therfore,I want to ask you whether I can use two antennas on a USRP? Thank you very much!

[ErikL]

Each USRP has a TX and RX chain that is independent although you will see a strong singal on the reciever from the transmitter due to leakage inside the device.