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## RE: [Discuss-gnuradio] Re: Simultaneously transmitting on two transmitte

 From: Sanmi Subject: RE: [Discuss-gnuradio] Re: Simultaneously transmitting on two transmitters Date: Fri, 7 Mar 2008 14:37:52 -0600

Hi Wee Shinhan,

Short answer: The idea is feasible.

Using two USRP's is a little more complicated because of the same issue of
synchronization.

The first step is synchronization of the on-board clocks. The current
solution requires some hands-on work using directions here
process disables one of the USRP board clocks and so the "slave" USRP can no
longer be used as a stand-alone board.

The next issue is synchronization of the actual waveforms in TX and RX. This
can be accomplished in two ways (that I know of):

1) Using the m-block/inband timestamps: Using M-blocks, you can define
timestamps that allow you to control when data is transmitted and
appropriately synchronize RX. The m-block/inband code is still in
development but is sufficiently stable to start some experiments. The
m-block code is all in c++ and does not use gr-flowgraphs so you might have
to build your application around it, or find some way to integrate m-blocks
into your flowgraphs (hint: SWIG). The m-block code can also be used for
many other functions (search the list for more details). Most of the code is
available in the trunk.

2) Custom FPGA code: the current gr-flowgraphs can already handle multiple
description). There is no implemented TX parallel, but it is possible to
build a multi-TX FPGA and gr code using the same ideas.

So the multi board radios are possible, but it is not necessarily
straightforward to implement right now.

-Sanmi

-----Original Message-----
Shinhan
Sent: Friday, March 07, 2008 1:27 AM
To: jiqun qi
Subject: [Discuss-gnuradio] Re: Simultaneously transmitting on
twotransmitters

Hi Jiqun,

I would try your suggestion, another idea is what if i
used 2 USRP? is the idea feasible? Thanks again for
the help.

> >
> In principle, in USRP, if these two transmit
> daughter boards share "one
> common clock", it could not achieve the
> "strict-sense simultaneously
> transmitting" because your program is executed in
> "sequence", not
> "parallel". However, you can "first" transmit the
> symbol s1 on antenna 1,
> "then" transmit s2 on antenna 2 "almost
> simultaneously" from the view point
> of well programming.
>
> For example,  you can send symbols s1 and s2 to the
> transmit buffer first,
> then enable the two antennas for transmitting, so
> the transmit time cycle
> difference \delta_t will be smaller than the way
> that you send symbol s1 to
> the antenna 1, and enable it for transmitting, then
> do the symbol s2,
> because there is the time cycle consumption that you
> send symbol s2 to the
> transmit buffer.
>
> I am not sure how actually it would be in practice.
> And I will try my best
> to do it in the following days.
>

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