Syncing 2 USRP 2921's TX without an external reference or GPS?

[filipfilip]

Dear all,

I have NI USRP 2921's (so, no full duplex, no GPS) and I want to send packets from 2 TX exactly at the same time. As far as I understood from reading older posts, one needs to either have an external source + a MIMO cable or GPS-capable USRPs.

The precision I need is on the level of microseconds, I can tolerate desync of up to 500 microseconds.

These are my dilemmas:

1. Is there a simpler, programmatic alternative that would enable 2 devices to send at the same time, e.g.

*connect a PC with 2 USRPs using an ethernet and a MIMO cable

*put 2 IP addresses in the device name field

Or perhaps

*use a switch and ethernet cables to connect PC with USRPs and send commands with a broadcast IP address to both USRPs at the same time?

Is teh   'Configure Trigger' block helpful in any way?

2. Any other ideas?

3. Any examples in LabVIEW (not requiring full-duplex USRPs) that could help?

4. What exactly is an external reference, hardware-wise? is it a signal generator set to produce square wave?

Thanks in advance!

Filip

[ErikL]

Synchronization can mean many different things.  More accurate sync requires more robust synchronziation methods.

This is not unique to USRP.

[filipfilip]

I was unclear, thanks for pointing that out. I need sync in the sense that 2 devices transmit almost at the same time, where I can tolerate deesync of up to 500 microseconds.

[filipfilip]

Thanks for the info! I guess I do need an external source in order to achieve the desired sync level. Could you please also clarify a few more things:

1. can an external reference be a signal generator?

2. if I connect signal generator output to a splitter, I have two outputs. So, each USRP should be connected to 1 signal genrator output, right?

3. USRPs have 2 inputs for external reference, REF IN and PPS IN. Which input should I use? For time sync, do I need both? I mean, if I have 2 USRPs, each with 2 inputs, and only 2 outputs from the signal generator, something doesn't fit.

Thanks!

Filip