[USRP-users] Phase alignment

Jawad Seddar jawad.seddar at gmail.com
Mon Dec 8 03:34:03 EST 2014


Hello all,

I am not sure if the following belongs on this list or on the
discuss-gnuradio list but here goes :

I have 2 USRP N210 (let's call them A and B) equipped with SBX
daughterboards and a GPSDO. Even though they both have a GPSDO, B gets its
references (clock and time) from A through a MIMO cable.

I use A and B to send the same signal and a third USRP (C) as a receiver to
observe what's happening. The signal sent by both A and B consists of GMSK
modulated packets (whith a fixed arrival period) sent on a carrier
frequency of around 1GHz. The samples forwarded to A and B are tagged for
each packet with a "tx_sob" and a "tx_time" tag at the beginning of the
packet and a "tx_eob" at the end of each packet.

This setup works quite well and I can see both signals from A and B added
together with a phase offset that is different for each execution. I can
compensate for this offset by rotating B's signal and thus I observe fully
constructive signals. Great results so far !!!

I do get some problems when I have some extra packets to send from A and
not from B. Then the packets sent from A and B do not add up constructively
anymore, in fact I observe a 180° phase shift after each non shared packet
(see attached images for illustration).

The attached images contain the following :
common.jpg : signal sent by both A and B
constructive.jpg : signal received by C when there is constructive
interference

A.jpg : signal sent by A including small packets every 4 common packets
phase_shit.jpg : signal received by C when A sends small packets and B does
not.
phase_shit2.jpg : Clearly shows the phase shift after each small packet

All signals shown are measured from C. You may notice some burst with large
amplitude (goes outside of the window) before each interesting burst, these
are not of any interest here. The tags displayed here are tags added by a
receiving block, not tags set by the transmitters.

I am not sure where these phase shifts come from. I have read things about
the CORDICs but it seemed to me that they should be aligned after each
start of burst, which are present for all packets along with a tx_time tag (
http://files.ettus.com/manual/page_sync.html).

Is there something I am missing here?

I am using UHD 3.8 and GNURadio 3.7.5

Thanks,
Jawad
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