Message stuck in the OFDM TX chain

[Anthony B.]

Hi all,

I am currently experimenting an OFDM transmission/reception using message passing and two USRP B200.
The TX/RX flowgraphs (each USRP handling one)  are based on the examples provided in GNU Radio :

https://github.com/gnuradio/gnuradio/tree/master/gr-digital/examples/ofdm

I added at the beginning of the TX chain and at the end of the RX chains custom blocks to handle messages, based on the Chat Application example here :

https://wiki.gnuradio.org/index.php/Guided_Tutorial_Programming_Topics#5.3.3_Example:_Chat_Application

So far I have been able to transmit and receive messages correctly.
My main issue is that while sending messages through the OFDM Tx chain, I am only able to actually send every second message.
I am checking that with the TX LED, which corresponds with the good reception on the RX side. So there is one message that stays stuck in the OFDM TX chain.
While figuring out this issue, I changed the device args according to transport notes for the USB link :

https://files.ettus.com/manual/page_transport.html

I observed that if I set send_frame_size to a value less than 152 bytes, I am able to send messages continuously (not one every second message anymore).
If I set a value above the latter one, I only receive the first message sent on the RX side, and then nothing else.
At a certain value (I guess the default value for the setting), I start having again my initial issue: one message stays in the TX chain every two messages.
My goal is to make it work for a bidirectional flowgraph (having both USRP Source/Sink), but I did not find a correct setting to have a continuous message transmission.

Does someone know what would cause the messages to be blocked in the OFDM TX chain ? Is there a relationship between the size of the message sent and the USB transport parameters I should work on ?

I am using GNU Radio 3.7.14.0 and UHD 3.14.0.

Thanks for any suggestion!

Anthony