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

Marcus D. Leech patchvonbraun at gmail.com
Tue Feb 26 10:13:06 EST 2019

On 02/26/2019 10:01 AM, Johannes Demel via USRP-users wrote:
> Hi all,
> this discussion started on discuss-gnuradio in case anyone wonders.
> Andy, thanks a lot for your detailed answers!
> And thanks Marcus for confirming these assumptions.
> So, now I only need to worry about spatial separation of my antennas.
> Now, the scenario is as follows, I want my system to work in TDD mode in
> the 3.7GHz band. I want continuous reception and occasional transmit bursts.
> Does UHD allow to use the TX/RX port only and do automatic switching? I
> send a burst to a USRP with a EOB flag and it does the switching
> automatically?
> I could potentially use 2 antennas but a one antenna system would be
> more elegant and I hope it would prevent any accidents with high
> transmit powers and antennas that are not separated far enough.
> I want to use GNU Radio and I hope I can use it as is or with minimal
> changes.
UHD takes care of TX/RX sequencing in the case where both RX and TX are 
configured for the same antenna.

> Ideally, the USRP does automatic switching and I'd either not receive
> samples during transmission on my receive chain or just the noise that
> happens to be sampled in the RX chain. I can handle echos as well. What
> I want to avoid is to implement logic that does the switching.
> Cheers
> Johannes
> Am 26.02.19 um 15:40 schrieb Marcus D. Leech via USRP-users:
>> 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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