[USRP-users] TDD mode with USRPs in GNU Radio

Marcus D. Leech patchvonbraun at gmail.com
Tue Feb 26 09:40:17 EST 2019

On 02/26/2019 08:41 AM, Andy Walls via USRP-users wrote:
> 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.
RX is protected by TWO layers of RF switch, not just one.  That gives 
you about 30-40dB of isolation.

>> 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: "discuss-gnuradio at gnu.org" <discuss-gnuradio at gnu.org>
>>>> 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
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