Re: [Discuss-gnuradio] precise transmit scheduling

[Josh Blum]

> >
> I am also very interested in this. I have had some success by using
> gr.message_source, and having a python thread that periodically pushes new
> data into the system by putting it on the message queue. Transmitter auto_tr
> is enabled so that when the usrp runs out of data it turns off the transmit
> until the next message arrives. Scheduling at the python level seems very
> rough, with intervals varying ~+/- 1ms without tuning each time, and ~+/-
> 10ms when tuning before each burst. I was under the impression that calling
> set_rx_freq() to tune just the DUC rather than usrp.tune() should be
> faster/more responsive, but I haven't seen this to be true...

Im not sure how gr-usrp handles the tuning. If it helps, the UHD tuning 
interface will allow you to tune elements individually or as a complete 
chain. So you can tune the overall frequency, or just tweak the DSP 
frequency. Its all brought up into python FWIW.

http://www.ettus.com/uhd_docs/doxygen/html/structuhd_1_1tune__request__t.html
> Any scheduling / hopping tips would be greatly appreciated!

I am going to make a overload of the UHD USRP source that pushes packet 
decoration (such as time, freq, sample rate) onto a message queue. An 
upstream block will pop off the queue as it runs its work() function.

Now, this could be done in reverse with a message queue feeding into a 
UHD USRP sink with timestamp information. And there you have timed, 
scheduled transmission of packets.

Now, the problem being, this only is possible with USRP2 as USRP1 does 
not currently support timestamped transmission and reception.

-Josh