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

Johannes Demel demel at ant.uni-bremen.de
Tue Feb 26 10:01:03 EST 2019

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 
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 

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.


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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