To be explicit: UHD has no built-in feature for this problem. I think one of the solutions suggested here (incl. yours) is the best solution.
--M On Tue, Sep 30, 2025 at 4:12 PM Marcus Müller <[email protected]> wrote: > 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. > > > > > > _______________________________________________ > > USRP-users mailing list -- [email protected] > > To unsubscribe send an email to [email protected] > _______________________________________________ > USRP-users mailing list -- [email protected] > To unsubscribe send an email to [email protected] >
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