Re: [Discuss-gnuradio] Discuss-gnuradio Digest, Vol 196, Issue 21

[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
>