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I have a complex input signal of 300 ms at baseband. I have already performed acquisition on this signal and acquired four GPS satellites and have the code phase and Doppler frequency of each of them.

Now for tracking I am following Kai Borre's MATLAB code for a GPS software receiver. But, this code is written for a real input signal at IF (intermediate frequency).

Now I need my signal (which is currently on baseband (zero-IF)), to be at 1.5 MHz IF (intermediate frequency).

Is it practically possible to change the complex signal at baseband to a real signal at IF (1.5 MHz) without losing the information on imaginary part? My sampling rate is 5 MHz.

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  • $\begingroup$ Mix it down numerically to complex baseband! That's but a multiplication with $e^{-j2\pi f_{IF} t}$; the values of $f_{IF}$ and $t$ both depending on both the IF frequency and sample rate. $\endgroup$ Jul 2, 2018 at 10:23
  • $\begingroup$ Thanks, I already have the signal at complex baseband. but I have a code for a signal at IF frequency. Now I have two choices: 1: either I bring my baseband signal to IF which I think is practically not possible. 2: I make changes in the code so that it works for baseband signal. Which RF frontend can I use to receive signals at IF frequency? I tried RTLSDR and USRP frontends and both gives output at baseband. $\endgroup$
    – Priyanka.P
    Jul 2, 2018 at 11:46
  • $\begingroup$ well, then mix up by multiplication with $e^{j2\pi f_{IF}t}$ (you might need to interpolate to a sufficient sampling rate first), and then throw away imaginary part. $\endgroup$ Jul 2, 2018 at 11:47
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    $\begingroup$ @Priyanka.P can you edit your question to tell us exactly what you're doing when mixing, and how that fails? $\endgroup$ Jul 2, 2018 at 18:06
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    $\begingroup$ Hello Marcus, I have edited my question. My baseband sampling rate is 5 MHz and I need 1.5 MHz IF. $\endgroup$
    – Priyanka.P
    Jul 3, 2018 at 9:20

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There are lots of examples on the web for GPS signal tracking at baseband. Look for "SoftGNSS" on GitHub.

As far as converting from complex baseband to real IF, it's the opposite as what receivers normally do. You can certainly modulate a signal from complex baseband to real IF by multiplying the real part by cos2pift and the imaginary part by -sin2pift and then add them together, like normal transmitters do.

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