Hi Daniel,
is a software solution impossible? You get the RX timestamps, and you could feed the TX
burst stream into both the USRP Sink and a block emitting messages describing but the
start timestamp TX burst. Messages can feed into a small queue that controls a state
machine e.g. in your carrier sense algorithm (and deafens it).
Best,
Marcus
On 9/29/25 12:15 PM, Daniel Estévez wrote:
Hello,
I am implementing a half-duplex modem with GNU Radio and a USRP B2xx that uses CSMA to
control medium access. I have a GNU Radio flowgraph with a USRP Source and USRP Sink
blocks both set to use the TX/RX antenna. The USRP Sink block uses burst transmission.
When a burst is sent, the B2xx automatically switches the receive to the RX2 port.
The issue is that there is enough leakage through the RX2 port that the system is able
to receive its own transmit packets with good SNR. This is a problem for the CSMA system
because it makes the system detect that the channel is busy while it is transmitting its
own packet. Due to the latency between the RX and TX streams in GNU Radio, this prevents
the modem from transmitting back-to-back packets (which is allowed and expected by this
CSMA scheme), because when the second packet is about to be sent, the modem receiver is
still seeing the end of the previous packet and determines that the channel is still busy.
To solve this issue, I would like the USRP Source to receive either zeros, no samples at
all, or something that at least does not have so much leakage of its own TX signal.
Is it possible to do this either by appropriate configuration in GNU Radio or
UHD?
I've been studying the sources of UHD and GNU Radio to see how this could be done. The
only thing I have found is that this would be possible with a small change to the FPGA
image. For instance, this line
https://github.com/EttusResearch/uhd/blob/master/fpga/usrp3/lib/radio_200/radio_legacy.v#L446
could be modified to replace the RX samples by zeros whenever run_rx is
asserted.
However I would prefer not to build a modified FPGA image for this system, so I was
wondering if other solutions exist.
Best,
Daniel.
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