Problem with two USRP

[A87]

Hello,
When i use same USRP 2952R for transmitting and receiving, The BER results are
satisfactory for different SNR. However, when i use two separate USRP for
transmitting and receiving, Then BER is .5 for all SNR.

Why it is happening? 

[rnv]

Hi A87,

Can you shed some more light on the problem you are facing ? Can you please provide the details of the configuration of the USRP used for TX & RX ?

Plus if you are working with digital modulation schemes, have you ensured that you are providing the synchronization parameters for demodulation ?

If the reference pattern and the demodulated bits are even offset by a few bits, then you could observe this problem.

Some of the fundamentals of the synchronization while working with digital modulation schemes is explained in the forum post mentioned below.

http://forums.ni.com/t5/LabVIEW/How-to-use-Generate-Synchronization-Parameters-for-PSK/td-p/1763962

http://forums.ni.com/t5/LabVIEW/Generate-Synchronization-Parameters/td-p/2744418

Hope some of the above information is helpful.

Thanks & Regards,

Raghunandan N V

Application Engineer

NI - India

[A87]

Hi mv,

I connect the two usrp by SMA to SMA cable for PPS Trig and REF.

And for synchronization, i am using preamble which is PN sequence.

When the received signal is correlate with this preamble, i get peak so that i confirm that actual data is received.

[brooksprumo]

Unfortunately daisy chaining 294x/295x USRPs is not supported. I feel this might be the root cause of your problem. Are you doing anything else to ensure your USRPs are synchronized (besdies the cable wiring)?

[A87]

No, I did only cable wiring.

is there any other option?

[muffin.vi]

A87,

The best way to synchronize multiple USRP RIOs is to use an CDA-2990. This will let you distribute one external clock from a timing card to multiple USRP RIOs, or to generate a clock in the CDA-2990 and feed that clock to the USRP RIOs. 

[A87]

Can i do carrier phase synchronization?

[muffin.vi]

What level of synchronization are you talking about? If two USRP RIOs share the same 10 MHz and PPS reference signals, they will have phase coherence,  but not phase alignment. 

[A87]

Hi, When i use Labview example of a QPSK i get proper constellatation without any synchronization.

For more clarity, i attached note and some gvi file 

Thanks

[A87]

Hi