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Is it possible to break out the i and q of a complex WiFi signal? If I have a laptop with a standard WiFi card in it, could I break out the i and q in analog, or upon sampling in an FPGA via digital means?

It must be possible since it is being done on a WiFi receiver, right?

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The in-phase and quadrature components are easy to break out. You just need to demodulate using in-phase and quadrature local oscillators. See the diagram below.

enter image description here

The received signal $r(t,k)$ is mixed with a $\cos$ and a $-\sin$ wave of the appropriate frequency, and then low-pass filtered.

The actual IEEE 802.11 modulation schemes used are a little more complicated than indicated, but not overly so.

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  • $\begingroup$ This was sort of what I was expecting. Where I get wrapped around the axle is the local oscillator. Since it is free running (wrt the transmitter), won't that effect the results? I know that it works since a receiver WiFi laptop has the same issue, but it seems like they would need to sync up somehow. So in theory this could all be done external to an ADC with components? $\endgroup$ – toozie21 May 6 '13 at 12:51
  • $\begingroup$ @toozie21 The local oscillator is synchronized with the received signal using, usually, some form of phase-locked loop. As JasonR says, there's lots happening in your laptop... doing it all individually is a little complicated. $\endgroup$ – Peter K. May 6 '13 at 14:08
  • $\begingroup$ @toozie21 You can do it with a free running local oscillator, you'll just have some carrier offset. $\endgroup$ – Jim Clay May 6 '13 at 16:11
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In theory, it can be possible to tap off some point in the analog chain to do what you want. Often, receivers have a superheterodyne structure, where the RF signal of interest is downconverted to some fixed intermediate frequency, after which further signal processing is performed to demodulate the signal and obtain the information encoded in it. That would probably be the best place to capture the view of the signal that you're looking for.

In practice, however, this is not likely to be possible. Modern WiFi receivers are highly integrated. It is likely that the receiver in your laptop consists of an antenna and a single-chip integrated circuit that performs all of the responsibilities of the receiver, analog and digital. Thus, there wouldn't be any spot to tap in order to get the view of the receiver that you want; everything is within a single integrated circuit package.

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  • $\begingroup$ All good points! I actually was hoping to leave a WiFi receiver out of the equation. Either break it apart with analog components (if it could be done with a mixer, and stuff like that), or via an FPGA if it was possible. $\endgroup$ – toozie21 May 6 '13 at 12:46

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