Re: [Discuss-gnuradio] set_auto_tr(), gr_stall, and PTT

[Michael Dickens]

Robert - To answer a few questions (and some background to answer them):

0) I did get the PTT working, and can see the TX on the RX antenna -  
no matter which antenna I select (on an RFX board).  If I select the  
TX/RX antenna, then the RX is attenuated ~25 dB or so.  If I select  
the RX antenna, then the RX is not significantly attenuated.

1) I also got simultaneous TX/RX working, using different antennas  
("full duplex").  It was -quite- handy that the "select_rx_antenna()"  
method was added just as I was searching for it ;)  If only this had  
been there a week earlier (9/1; it was added in r6377 on 9/10), my in- 
house demo would have been well-received!  At least I got those bugs  
worked out in time for "the big demo".

2) I also got the PTT working on a single TX/RX antenna ("half  
duplex"), by turning on the USRP_TX object ("enable(true)" I think)  
when the PTT is "pushed", then turning it off when the PTT is  
"released".  This works quite well, and is modeled after the USRP  
NBFM PTT example.

3) I never got to the "gr_stall()" block, because I got (1) and (2)  
working.  It would still be interesting to see if "set_auto_tr()" and  
the scheduler would work with a "stall" block on the TX side.  I  
don't have time to look into this right now.

4) What Matt says is true, roughly, regarding simultaneous TX/RX on a  
single antenna.  I haven't tried this yet, it's purely observational  
at this time (as per 0 above) but: You actually can do simultaneous  
TX/RX on that antenna of an RFX board, but there's a penalty on the  
RX side in terms of attenuation ... something like 25 dB.  I believe  
that simultaneous TX/RX could be made to work if the penalty is OK  
for your needs.

5) You can do time multiplexed switching of the TX/RX antenna (as per  
2 above).  You do this by disabling the USRP_TX object, to allow for  
RX.  Then you enable the USRP_TX to do TX.  There have been  
discussions on this email list regarding how quickly you can enable /  
disable TX .. something like a few ms IIRC.  Keep in mind that the  
flow-graph side of GNU Radio has no concept of "time" and isn't  
designed to do TDMA; you can probably implement "coarse" TDMA via OS- 
specific timers.  I think for anything reasonably fine-grained or  
time-precise, you'll need to use the USRP-server and mblock modules  
of GNU Radio.

Hope this helps! - MLD