[USRP-users] Full Duplex vs. Half Duplex operation?

Ian Buckley ianb at ionconcepts.com
Thu Oct 23 19:47:01 EDT 2014

Inside the FPGA there are two different state machines implemented in H/W logic, one each to control Tx and Rx…they each change state based on received packets, VITA times, buffer fullness, error conditions etc.

Each of these state machines drives a signal that shows when TX and RX are active (or not). (Active would be defined as samples passing through the DSP pipeline and data packets being packed/unpacked).

The TX and RX signals are routed to a variety of places including the so called ATR GPIO blocks. The ATR GPIO blocks drive signals that in the case of WBX directly controls the state of the 2 analog RF switches that route signals to and from the SMA's.

The ATR GPIO blocks work in the following way…..they are essentially a look up table that is programmed by the Host (UHD) with values that are to be driven out on the various GPIO lines (that are configured as outputs) in each of 4 states: IDLE, RX, TX, FULL-DUPLEX

Thus S/W pre-programs the ATR GPIO blocks during configuration before radio operation commences and then, once operation has commenced the various switch configurations are changed as a direct result of state changes within the FPGA.

The actual switches are on the Grand Daughter board that mounts to the WBX. I note BTW that there is a documentation error on the WBX schematics where its states that "io_rx_05" is the GPIO controlling which SMA is used by RX…it is infact "io_rx_15"

The gist of this is that in your case either: a) For some reason the ATR-GPIO has not been programmed by UHD to select the correct SMA or b) The USRP is not in the state you think it is (RX vs Full-Duplex)

Make sense?

On Oct 23, 2014, at 4:10 PM, Radio User via USRP-users <usrp-users at lists.ettus.com> wrote:

> Fellow USRPers.. 
> I recently saw this snippet in ...share/doc/uhd/manual/html/dboards.html
> -------------------------------------------------
> Receive Antennas: TX/RX or RX2
> Frontend 0: Complex baseband signal for selected antenna
> Note: The user may set the receive antenna to be TX/RX or RX2. However, when using a WBX board in full-duplex mode, the receive antenna will always be set to RX2, regardless of the settings.
> ------------------------------------------------
> When SoDaRadio sets the RX ant to "TX/RX" (via a call to set_rx_antenna) there is no effect
> (get_rx_antenna() returns "TX/RX" but the actual connection is still to RX2).   Apparently 
> the USRP FPGA and/or libuhd believes that the radio is in "full duplex" mode.  
> What condition of the radio determines that the radio is operating in full duplex mode?  
> Note that I'm not asking what "full duplex mode" means -- I'm asking how the USRP hardware/software
> determines that it is in full duplex mode.  
> rg
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> USRP-users at lists.ettus.com
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