Packet Loopback Example Using USRP

Hi again, I have a previous message thread regarding this problem, but I kept forgetting to 'reply all' to include the GNURadio email to keep it on the thread and my questions got kind of disorganized, so I wanted to start fresh with a new one and will make sure to reply all in the future.

So, I found a packet_loopback example (under the directory: examples/digital/packet) from this message thread and it says that I can replace the Channel Model block with the USRP source and sink blocks to try to send packets over the air (a screenshot of the original packet_loopback example and my modified packet_loopback example is attached as well). This is exactly what I want to do, but I'm running into some issues with the USRP sink.

I believe the problem has to do with the way that the USRP sink is receiving the packets. I believe it deals with 'bursty transmission', and so I specified the USRP sink's 'TSB tag name' parameter to 'packet_len', which is what the packet_tx block specifies as the length tag name (I've also attached a screenshot of the packet_tx flowgraph). I confirmed that the tag name is 'packet_len' by using the Tag Debug block.

Also, I've attached a video link here of what happens when I run the program. It seems like the USRP receives the packet here and there because of the short 'bursts' shown on the middle graph. Also, I noticed that there's a 'tG' printed in the terminal window (a screenshot of this is attached as well) when the program is first run. What does this mean? I also know that it's experiencing a lot of underruns, which is essentially the problem I'm trying to solve. It must do with the way the USRP sink is configured to receive the packets.

Are there any other parameters that need to be set?

Thanks so much in advance!! I appreciate any help :)

From:	Jada Mariano Berenguer
Subject:	Packet Loopback Example Using USRP
Date:	Fri, 22 Jan 2021 18:17:00 -0800