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I've seen a few references to full duplex SDR radios, and I'm wondering how can one transmit and receive at the same time without overwhelming the receiver, and without heavy filtering such as the cavity filters used in repeaters.

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    $\begingroup$ I'm not sure that this question has good premise... In the detailed description of the product mentioned in the post they do in fact mention very heavy filtering. They also mention that the receiver and transmitter have separate filters as well. From what I can see, there is no reason why they couldn't be able to do heavy filtering on the receiver so that it's not overwhelmed by the transmitter. $\endgroup$ – AndrejaKo Dec 11 '13 at 21:40
  • $\begingroup$ @AndrejaKo That still doesn't make sense. What are they filtering? If the receiver works from, say, 1.8MHz to 30MHz, and the transmitter works from 1.8MHz to 30MHz, then how would they filter out the transmitter, given that the receiver still has to receive at the frequency the transmitter is currently transmitting at? $\endgroup$ – Adam Davis Dec 11 '13 at 21:43
  • $\begingroup$ Actually, now that you mention it, they did that they're filtering outside interference, so signal from within the band should be received. I'll see if I can dig something up... $\endgroup$ – AndrejaKo Dec 11 '13 at 21:51
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    $\begingroup$ Totally superficial: if you know what you're transmitting, you can subtract it from what you're receiving and whatever's left is the incoming signal. That's similar to how noise-canceling headsets work. $\endgroup$ – Pete NU9W Dec 11 '13 at 22:14
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    $\begingroup$ @PeteNU9W So if you're transmitting 100W, then there's no danger that the receiver will be overloaded? Are you assuming these are on different antennas? Do the systems require additional separation, or can I have the transmitter output and receiver input connected together to the same antenna and expect nothing will go wrong, and I'll be able to receive both the transmission and all other signals I'm looking for? If this is the case, why do repeaters bother with cavity filters at all? Isn't this the same problem they have? $\endgroup$ – Adam Davis Dec 11 '13 at 22:18
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One method to achieve this is to invert, attenuate, and delay the transmitted signal and feed it back into the receiver. This will cancel out most of the power of the transmission, so the input end of the receiver isn't swamped by the RF from the transmitter.

http://sing.stanford.edu/fullduplex/

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This will have to be actively managed, but the report suggests that it works for ultrawide bandwidth signals.

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The bladeRF is one such SDR radio that supports full duplex, i.e. simultaneous signal transmit and receive.

The short answer is that in its case, as I suspect in many other cases with a "raw" SDR unit, they could not be used "out of the box" for full-duplex communications. I.e. the receive circuity will be swamped unless the user takes their own care to prevent this.

If you take a look at this forum thread, Cavity Duplexer needed? you will find the first reply says basically:

  • the bladeRF isn't a great choice in situations where one would typically use a cavity duplexer, e.g. repeater
  • "using separate RX and TX antennas is entirely doable, especially if the antennas are in each other's nulls and they're operating far enough apart frequency-wise … Filters are, as always, a good investment, but they don't need to be anywhere near as sharp as a cavity" — i.e. you could sort of use it anyway, but you're relying on external arrangements for preventing receiver desense/overload

Likewise in the case of the Flex radio systems you mention in your question. For example in Is the 6500 a true 'full duplex' radio you find answers such as:

  • "appears to me the answer is yes... If you have sufficient isolation between your RX and TX antennas"
  • " I am not sure it will be possible with the 6500. With the 6700 you have the two SCUs and a compliment of BPFs for both of them."
  • "The radio hardware has some capabilities that you would consider full duplex. There are limits to what can or that you would want to do and we have not yet spent any time looking into this. For the immediate future, we are muting receivers when transmitting…"

So again, it's clear that there is no silver bullet inside either the bladeRF or the Flex radio SDRs which handles the tight filtering needed when transmitting near a receiver. The 6700 model radio mentioned in comparison to the Flex unit would possibly be able to serve as a cross-band repeater with its separate band pass filters, but not within the same band if the TX signal has any significant level of amplification.


However, this is not to say that having an SDR with full-duplex capability is worthless — especially if you were to use it as test equipment rather than a transceiver. A vector network analyzer like the VNWA3 for example is quite similar to a full-duplex SDR, architecturally speaking. With the right software and perhaps an external bridge or similar, I would think something like the bladeRF could be useful for measuring filters and much much more.

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