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Using an inexpensive SDR (such as the USRP or Per Vices Phi), I want to process two or more communication protocols at the same time (e.g. WiFi and GSM). Is it possible to Tx (or Rx) both at the same time, like FDMA, or would they need to do some fast switching between modulations and frequencies, like TDMA? I'm not sure if the receiver would be able to accept it though since it is not expecting TDMA.

So I think FDMA would be the best approach, but like I said the question is whether or not it's possible to construct a waveform that can transmit both protocols simultaneously. I think it may also depend on frequency separation, combined bandwidths, mutual sample rate, etc.

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  • $\begingroup$ Do you want to transmit WiFi and GSM at the RF frequencies defined in the respective standards or can you transmit at arbitrary frequencies? $\endgroup$ – Deve Jul 13 '12 at 13:32
  • $\begingroup$ Each protocol at its standard band. And WiFi and GSM are examples, they could be anything (e.g. receive WiFi and ATSC). I've seen papers about collocated protocols like WiFi and Bluetooth working together. $\endgroup$ – wrapperapps Jul 13 '12 at 13:37
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    $\begingroup$ At a theoretical level- yes, you can. In fact it is trivially easy to do. You simply create the two signals individually and then add them together. $\endgroup$ – Jim Clay Jul 13 '12 at 13:56
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    $\begingroup$ The problem with doing it is a hardware and, to a lesser extent, software problem. For instance, can your hardware transmit at two completely separate frequencies? Unless it has been specifically created to do so, probably not. Because it is a hardware and software problem, and not a signal processing problem, I think that this should be migrated to the electronics board. $\endgroup$ – Jim Clay Jul 13 '12 at 13:58
  • $\begingroup$ When they are added together, how will they up-converted by the mixer/BPFs to their respective frequencies, which may be relatively far apart? Is that the main practical limitation of a theoretically possible process? $\endgroup$ – wrapperapps Jul 13 '12 at 14:01
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Theoretically it's possible to do a frequency multiplex of two different protocols with one SDR transmitter. However, the practical main limitation is the speed of the digital-to-analog converter (DAC). E.g. for a WLAN and GSM band the channels that are closest on the frequency axis are at 1800 MHz and 2412 MHz, respectively. For generating such a signal you'd need a sampling rate of over 1200 MSa/s. Even if you find an SDR device that is equipped with an DAC that fast, it will be very challenging to filter the aliasing spectra generated by the DAC in order to avoid out-of-band radiation. So your sampling rate has to be even higher in practice.

If you want to transmit two independent data streams at bands very close to another, OFDM can be advantageous because of it's narrow-band characteristic. If the modulation is synchronized, the bands can even overlap. The latter is an case without practical relevance, though.

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  • $\begingroup$ Thanks for providing specific limitations, as well as elaborating on OFDM use. $\endgroup$ – wrapperapps Jul 13 '12 at 14:21
  • $\begingroup$ @wrapperapps Please note that I've corrected a mistake. $\endgroup$ – Deve Jul 13 '12 at 14:29
  • $\begingroup$ Twice the combined bandwidth, makes sense. $\endgroup$ – wrapperapps Jul 13 '12 at 14:31
  • $\begingroup$ @wrapperapps why is it twice the combined bandwidth? $\endgroup$ – James Graham Oct 8 '17 at 13:59
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Given sufficient input bandwidth to cover all your signals, this depends strictly on how much processor performance each filter, demodulator and/or modulator takes. If you can run 20 filters and demodulators simultaneously in 20 threads in real-time on your processor(s), and the modulation and power of each signal provides a sufficient S/N ratio, you should be able to receive 20 signals at the same time.

This is not that much different from running 20 radios, each tuned to a completely different station, but all attached to the same antenna. Just the digital equivalent (or simulation), given sufficient processor performance and input bandwidth.

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