Re: RX/TX switching

[Johannes Demel]

Hi Fabien,

Personally, I use PDUs to control when TX starts. GR 3.10 integrated a 
lot more of these.
https://github.com/gnuradio/gnuradio/tree/main/gr-pdu
Besides, the gr-pdu_utils OOT might be a good reference.
https://github.com/sandialabs/gr-pdu_utils
It mentions SOB/EOB in their docs.
EOB is required to signal that a burst ends. I'm not sure if a SOB is 
really required or if a USRP would just "wake up" and start transmitting.

I suggested the SOB/EOB mechanism because it sounds like you transmit 
long bursts. Short bursts might be handled via tagged streams. The UHD 
sink would handle these cases. By short bursts I mean <=4k complex samples.

Generally, the idea is: trigger transmission with a PDU block, use a PDU 
to tagged stream block to fead you data into the GR streaming interface. 
Then make sure to signal to the UHD sink when a burst ends.

Cheers
Johannes

On 04.03.22 12:50, Fabien PELLET wrote:
> Hi Johannes,
>
> Yes this is exactly my problem : after unlocking the flowgraph, I get 
> some underruns (only, no overrun).
>
> Do you have an example of code using EOB/SOB ? Ok for sending EOB before 
> locking but is it necessary to send a SOB also and when ?
>
> Your suggestion of idling the TX flowgraph is something I looked for 
> during several days but I never found how. Indeed, if I do not connect a 
> source and a sink at each ends, gnuradio will fail to run. How to stop 
> feeding one branch of a flowgraph ? Do you have examples of codes for 
> doing that ?
>
> Thanks for your help,
>
> Fabien, F4CTZ.
>
> Le 04/03/2022 à 12:03, Johannes Demel a écrit :
> > Hi Fabien,
> >
> > do those underruns occur after you lock/unlock and switch from TX to 
> > RX or vice versa? Do you see overruns as well?
> >
> > I'd assume the USRP expects a constant sample flow and even a short 
> > interuption, like your lock/unlock task interrupts that flow.
> > Still assuming this is the root cause, you might fix this by sending 
> > an EOB signal to the USRP before you lock the flowgraph.
> >
> > Besides, the whole SOB/EOB mechanism might help you with your issue. 
> > You can just stop the TX flowgraph portion while you don't need it. By 
> > stop I mean, the flowgraph is not fed any new samples and just idles.
> > As an added bonus, you minimize LO leakage which might be an issue for 
> > your RX flowgraph. I know this is a more intrusive change.
> >
> > Cheers
> > Johannes
> >
> > On 04.03.22 10:45, Fabien PELLET wrote:
> > > Hello again,
> > >
> > > I build a flowgraph in C++ with an RX chain and a TX chain. I'm using 
> > > a USRP N210. When I'm in RX, the TX chain is connected to a constant 
> > > zero source and feed a null sink and the RX chain is connected to 
> > > USRP source and sink. When I'm in TX, it is the opposite.
> > >
> > > I have to keep the two chains running at the same time for switching 
> > > speed purpose and do not have to stop the flowgraph and restart it. 
> > > So for my switching, I just lock the top block and unlock it after.
> > >
> > > If I always stay in RX, I have no underrun. Same thing in TX. My 
> > > problem is that when I lock and unlock the flowgraph, I get a lot of 
> > > underruns.
> > >
> > > As a test, I remove the TX part of my flowgraph and only keep the RX 
> > > part and I try do some lock/unlock sometimes : same problem with 
> > > underruns.
> > >
> > > How to do lock/unlock safely in C++ to avoid underruns ?
> > >
> > > Thanks for the help,
> > >
> > > Best regards,
> > >
> > > Fabien, F4CTZ.