Re: [Discuss-gnuradio] TDD mode with USRPs in GNU Radio

[Andy Walls]

Hi Johannes:

On Tue, 2019-02-26 at 11:05 +0000, Johannes Demel wrote:
> Hi Andy,
> 
> thanks for your answer and help. That's the pointer I was looking
> for.
> 
> Just for clarification, I' refering to this schematic:
> https://files.ettus.com/schematics/ubx/ubx.pdf
> 
> The 'SKyWorks SKY13350-385LF' switch is this SPDT component in the
> TXPA 
> block?

Yes, on Sheet 1.

It is also depicted on Sheet 11, Grid C-2, designated U32.


> I expect a configuration with separate antennas for TX and RX. But
> since 
> isolation between TX and RX chains is limited by the switch, this is
> the 
> component I need to worry about?

In my opinion, yes.

It seems kind of funny that Ettus would make a board that can blast out
20 dBm on Tx, can only tolerate -15 dBm max on Rx, and then have only a
single device that provides only 17-20 dB isolation between the Tx and
Rx.

If all the specifications from the UBX-160 page are to be believed, 
you always have to have the UBX Tx gain turned down by at least 15 dB
to not damage your UBX board.




> Otherwise I'd just separate my TX and RX antennas spatially. But
> that 
> doesn't make sense in my case since the critical component is this 
> switch in my case.

Right.

Although antenna separation needs to happen as well, so you have at
least 35 dB loss over the air.  Keep in mind that equations derived
from far field approximations don't necessarily hold in the near field.


> I aim at working in the 3.7GHz band. Thus, I assume that my receive 
> signal goes through the VMMK-3603 LNA in the RXLNA block in the 
> schematic. I wonder how that component right after the RX2 SMA works?
> I 
> assume this is a Skyworks AS236-321LF as shown on page 11 of the
> schematic.
> 
https://store.skyworksinc.com/Products/Detail/AS236321LF-Skyworks-Solutions-Inc/88944/

Take a look at the SPDT switch in Figure 1 here:
http://www.skyworksinc.com/uploads/documents/PB_RFSwitches_PB121_15B.pdf

It's not too hard to imagine what a DPDT switch looks like from there.


> Are there in fact 2 switches concatenated? Would it be appropriate
> to 
> just add up their isolation values?

Ah, yes.  As long as the USRP switches this one over to RX2, (and that
sounds like your configuration,) it looks like you get an additional
13-15 dB of isolation.  Still not enough to cover the whole 35 dB, but
you're up to a worst case minimum 30 dB isolation.

If all of the specifications are to be believed, and you know you'll
receive negligible energy on the RX2 port antenna, you only need set Tx
output gain to 5 dB of attenuation, and make sure you are using RX2 for
Rx.


> Another thought, would it be possible to configure a USRP in GR such 
> that it does continuously receive on TX/RX and then switch for the 
> duration of a transmit burst and switch back afterwards? Of course, 
> preferably this happens automagically.

I'm pretty sure this already is handled automatically by the
USRP/libuhd, but it would take a bit of searching through code to
verify.

In the case of telling the USRP you're using the TX/RX port for both Tx
and Rx, you may not get that Skyworks AS236-321LF switch switched into
the position to get the additional isolation.  You'll have to check the
code or check with Ettus.


> This thread [0] suggests that there is some kind of control. But so
> far, 
> I didn't find a definite way to tell if this covers my case with GNU 
> Radio. Also, in [1] it sounds like switching is done automagically. 

In GNURadio you should be able to set a USRP Sink to "TX/RX" and a USRP
Source to "TX/RX", and libuhd should take care of the switching
automatically, if I recall correctly.  Again, you might not get the
isolation of the second Rx switch using this configuration.


> Though, I wonder if I can just use 'UHD: USRP Sink' and 'Source' or
> if I 
> need to use the Async Sink?

I have no idea.  Never used it.

Regards,
Andy

> Cheers
> Johannes
> 
> [0] 
> 
http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2016-October/050112.html
> [1] 
> 
https://stackoverflow.com/questions/39321262/switching-usrp-from-rx-to-tx-using-gnuradio
> 
> Am 25.02.19 um 20:30 schrieb Andy Walls:
> > 
> > > Date: Mon, 25 Feb 2019 10:29:56 +0000
> > > From: Johannes Demel
> > > To: "address@hidden" <address@hidden>
> > > Subject: [Discuss-gnuradio] TDD mode with USRPs in GNU Radio
> > > 
> > > Hi all,
> > > 
> > > I plan to implement a TDD system with GNU Radio and X310s w
> > > UBX160s.
> > > My
> > > lab setup is as follows:
> > > - multiple USRPs (start with 2, extend to more)
> > > - each USRP shall use 1 TX and one RX port. Preferably on one
> > > daughterboard in order to extend it to MIMO later on.
> > > 
> > > At the moment the receiver runs continuously. While TX bursts
> > > happen
> > > occasionally. I'd like to turn up TX power as much as possible.
> > > But
> > > I'm
> > > concerned that this will damage the RX frontend. Especially if
> > > the
> > > RX
> > > gain is set to some high value too. Should I be concerned about
> > > that?
> > 
> > Yes.
> > 
> > https://kb.ettus.com/UBX
> > 
> > "TX Power (Max)
> > 
> >   10 MHz - 3 GHz: 20 dBm
> >   3 - 6 GHz: 8 - 20 dBm"
> > 
> > "Input Power Levels
> > 
> > The maximum input power for the UBX is -15 dBm."
> > 
> > 
> > The TX chain is isolated from the Rx chain by a SKyWorks SKY13350-
> > 385LF
> > switch.
> > https://files.ettus.com/schematics/ubx/ubx.pdf
> > http://www.skyworksinc.com/Product/712/SKY13350-385LF
> > 
> > which claims typical isolation of 25 dB at 3 GHz on the webpage,
> > but
> > the datasheet indicates 20 dB is actually typical and 17 dB is
> > minimum.
> > UBX-160 board design, if not done right, can degrade that.
> > 
> > 17 dB of isolation is does not cover the 35 dB difference between
> > max
> > TX power and max Rx input power.
> > 
> > 
> > > Or
> > > does the USRP take care of that automatically? e.g. turn down RX
> > > gain
> > > during transmission?
> > 
> > I don't think so.
> > 
> > 
> > > Even if a transmission is received at the same receiver, I'd
> > > expect
> > > my
> > > RX DSP chain to just demodulate that burst and later on this
> > > packet
> > > would be discarded.
> > 
> > That's my experience with WBX-120 boards installed in a working
> > system: the radio can hear itself.
> > 
> > 
> > >   Also, I'd envision a block that 'blanks' my RX
> > > samples whenever TX is expected.
> > 
> > It's easy enough to throw away these packets at the MAC layer.
> > Receiving them is also a rudimentary check for a continuous built
> > in
> > test to detect if the transmitter has completely failed.
> > 
> > 
> > > All in all the questions are:
> > > - do I need to implement some logic in GR to turn down RX gain
> > > (or
> > > shut
> > > off RX) during a TX burst?
> > 
> > Given the numbers above, I would think you need to keep the gain
> > down.
> > 
> > 
> > > - does UHD take care of that?
> > 
> > I don't think so.
> > 
> > 
> > > - If I need to take care of that, how did you guys handle that
> > > problem?
> > 
> > Don't run the RX chain at full gain.
> > 
> > Put an LNA and limiter out in front of the USRP's Rx port.
> > 
> > And if you're using one antenna for both Tx and Rx, on the TX/RX
> > and
> > RX2 ports respectively, you'll need a PIN diode RF T/R switch out
> > in
> > front of the USRP as well.
> > 
> > Regards,
> > Andy
> > 
> > > Cheers
> > > Johannes
> > >