Alamouti 2x2 using USRP- PPL did not lock within the allotted time

Question

Hello.
I've been trying to use the following example VI:
http://www.ni.com/white-paper/13878/en/
I can't understand how to connect the two USRPs 
What clock source should I use? How should I connect it?
right now I have my USRPs connected with a MIMO cable, and both are connected to my laptop with a gigabit switch.
currently, when I'm tring to run the VI without the clock source, there is this error:
niUSRP initiate.vi<ERR>The reference clock PPL did not lock within the alloted time.
Thank you in advance.

Answer 1

Sorry for the late reply, I did not have access to the lab over the weekend. 
Yes I am uing HW configuration 1.  i set my IPs for the Txs and Rxs (4 NI 2921 boards), set the symbol rate, and appropriate IQ sampling rate. I then set the Tx element 1 Ref Frequency Source and timbase clock to internal. and for element 2 to be MIMO. I do the same for the Rx element 1 and element 2 as mentioned in the white paper. I set an appropriate carrier freq according to my machine (shown in snapshot below)
I then run the code, and the error shown in the picture shows up. Apparently, the error code is not recognized. I also show a picture of the error recorded on the front panel (which is the actual error, that needs to looked at). I then re-wire the error out in the block diagram to be connected to the last block in the chain 'Calculate BER VI' (shown in the 3rd picture below) which was previously not the case. Now i rerun the VI, and the error showing up in the block diagram is gone but the error mentioned on the front panel is still there (which is normal becuase i have not changed anything important in the block diagram). 
I look up this error 'Error -107411864 - A stream command was issued in the past' and find this NI-known issues document: http://www.ni.com/product-documentation/52445/en/ 
And I see this error there. ID number 467209 (corresponding to the above error) . In this it states:
This error occurs when you set Timebase Clock Source to PpsIn and do not provide a timebase clock source to the PPS IN port.
If you set Timebase Clock Source to PpsIn and do not provide a timebase clock source to PPS IN, the USRP device will revert back to its internal timebase clock source. This may result in error -1074118649 "A stream command was issued in the past."
Since I have not even setup the configuration on the front panel using the Ref_in and PPS_in (i have actually set them to internal) why am i still getting this error. 
I also looked at the ID number '466351' in the same list (right below the prev one) and find out that also seems to be fine since not all of the follwing conditions are being met:
The slave device has a GPSDO.
The niUSRP Set Time VI is called only on the master.
The timestamp is applied immediately.
as mentioned in the description. 
And since, there is a clear conflict between the white paper configuration designed for this 2x2 scheme (HW configuration 1) and the actual HW configuration corresponding to this same white paper (HW configuration 2), how can i expect the code to run without the Refi_in and PPS_in for all 4 devices (for sync between Transmitters and Recievers) which seems to important for this code to run properly. 
I do not understand this error either 'A stream command was issued in the past' since i have not found a soultion to it online. 
Now can you elaborate on what seems to be a conflict and elaborate on this error, which has been reported on other threads as well but the NI people do not seem to bother to respond as to why this is so.