How to synchronize two transmitters

I was wondering if there is a way to syncrhonize two transmitters
running
on two machines.
Both transmitter use OFDM. I could use a shared reference clock, but the
problem is that
I don’t know how exactly I can send packets SIMULTANEOUSLY from both
transmitters.
I want to measure co-channel interference and study multiuser detection
alogorithms.

Regards!

What I think that might be a solution is using only one computer with
two
USRPs. In the flowgraph, you will have two sinks. If your network card
are
both identical and the distance between the USRPs are not large enough
to
be considered, I don’t see why the signals would not be syncrhonized
(or,
at least, very similar to each other).

Att,

2012/1/22 yend B. [email protected]

On 27/01/12 01:16 PM, André Selva wrote:

I was wondering if there is a way to syncrhonize two transmitters
running on two machines.
Both transmitter use OFDM. I could use a shared reference clock,
but the problem is that
I don't know how exactly I can send packets SIMULTANEOUSLY from
both transmitters.
I want to measure co-channel interference and study multiuser
detection alogorithms.

Regards!

If you have a USRP that can take external synchronization signals (1PPS
and 10Mhz clock), then
you can use the raw UHD primitives to have transmit streams start at a
particular time. You’d
first do a “setup” where you’d use set_time_at_next_pps(), and have
the two hosts agree on what
time to set on the two USRP units. Then both hosts would start a TX
stream at a particular time
in the future–some point past where the set_time_at_next_pps() is
set.

Or if you run on a single host, you can use a uhd_usrp_sink block with
multiple channels–one for
each USRP, and set it up for PPS synch, etc. I believe UHD will
arrange to time-align all the channels
when they’re inside a single block. UHD source/sink block can be used
to talk to multiple USRP
units, and conceptually form a “synchronization domain”, when the
appropriate synch hardware is
in use.

Thanks for the reply!
If I use one computer, I would even combine and stream them to the same
USRP antenna. What I wanted to have is two independent
transmitters ( primary and secondary) transmitting over the same
channel.
The secondary (cognitive) transmitter will have to
synchronize to the primary transmission so that it can encode its
message
(eg. dirty-paper coding) to mitigate the interference.

Regards!