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

[Johannes Demel]

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?

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?
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.

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/

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

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

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
>>
>