Half Duplex application


So I am trying to do half duplex communication with the LimeSDR and a patch antenna.
I was wondering if the LimeSDR had the series of SDR peripherals and GPIO for this half duplex purpose.

I am assuming I would need some sort of RF switch and would have to turn off my HPA before switch to RX.

If anyone could give me some clarification on how to go about doing half duplex with the LimeSDR, then that’d be amazing.

I believe GPIO board is what you are looking for, but I don’t know in details how to use it for that purpose :wink:

The GPIO pins on the LimeSDR can be accessed for read and write via Limesuite API. It should be pretty easy to control RX LNA, TX PA and other required sequencing via th API.

The add on GPIO board would be good to use too as it performs logic voltage level shifting and also has onboard relay drivers with back emf protection. It will help protect the gpio pins on the LimeSDR from potential damage.

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You basically have these options:

  1. Use multiple antennas. Easiest way. Mute TX by powering down PAD.
  2. Use external circulator. Passive element, have narrow band and about 20-30 dB attenuation between RX and TX. There are rumors that some RF guru even achieved full duplex single antenna operation with circulators and some ancient magic.
  3. Use external switch. Wire switch to spare GPIO and toggle switch simultaneously with PAD.
  4. Utilize LimeSDR on-board elements. LimeSDR have build-in external loopback switches between TX2 and LNAH and antenna ports. If you experienced enough, you can re-wire them to be used as antenna switches. But this is really hard stuff, and 99% chance of breaking something. Just for information you can study LimeSDR schematics https://github.com/myriadrf/LimeSDR-USB/raw/67e715500d8a11096e675ccad5579350a0d38bd7/hardware/socket/1v4/Project%20Outputs%20for%20LimeSDR-USB_1v4s_LMS031pad/LimeSDR-USB_1v4s_schematic_r4.PDF - look at IC4 and IC5 elements.
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Thank you!

Do you have any suggestions for an external switch to accomplish this? I am trying to do the same thing and have looked all over for information on a simple tx/rx rf switch to shunt the RX when TX input is sensed.

You are probably looking for something like this or something from this list.

As for using GPIO to control such switch - I am not sure if setting GPIO pin at the same time at which onboard PA will be enabled is a simple task. As far as I know GPIO API does not let you set some pin at specific hardware timestamp. Actually, in our application we have used external power detector to produce a signal used to control RF switch.

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Thanks! That minicircuit switch should fit the bill. May I ask what you’re using your half-duplex circuit for? Are you doing simplex work?

Actually, do you (or anyone) know of a SP2T RF Switch where the default unpowered state is to close the circuit to RF-Common for RF1 or RF2? Basically, to simplify the circuit, I am looking for a switch where I can have the control signal ON when the radio is transmitting and OFF (Rx) when it is not so it defaults to RX.

We were implementing high speed, bi-directional OFDM-based TDD radio link for maritime environment. It was typical half-duplex application with the same omnidirectional antenna being used both for TX and RX purposes. It was actually based on USRP N210, not LimeSDR. Because we couldn’t open the case without loosing the warranty, we have decided to use external RF power detector in order to control RF switch. People here are suggesting that in case of LimeSDR you can simply use GPIO pin for that purpose, which sound right, but I’m afraid it’s not so simple, as it would require modifing FPGA code or something like that to set the pin at the same time at which internal LimeSDR power amplifier is being enabled.

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