Is it possible to detect a 3G mobile network without having the orthogonal code or the spreading code used in 3G?

By detect I mean pick up the radiated radio frequency from the mobile phone then see the peak frequencies then convert that to voltage.

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    \$\begingroup\$ What do you mean by "detect"? What kind of information are you looking to find? \$\endgroup\$ – Eugene Sh. May 28 '15 at 19:01
  • \$\begingroup\$ I added a description of that in the question Engene Sh. \$\endgroup\$ – user3052793 May 28 '15 at 20:09
  • \$\begingroup\$ So you want to detect some frequencies in the air. Match an antenna and receiver and measure the intensity. That's pretty much about it. \$\endgroup\$ – Eugene Sh. May 28 '15 at 20:11
  • \$\begingroup\$ Yes, you can pick it up on a spectrum analyser, or build a tuned frequency detector. \$\endgroup\$ – pjc50 May 28 '15 at 20:25

You might consider the RTL-SDR software, which typically uses an inexpensive tuner based on the RTL2832U chipset. You'd want to select a dongle/hardware that contains the frequency range of interest (for GSM, the Elonics E4000 would likely do alright). Consider the legality of scanning in your country, though. Note that in the US, "it is a federal crime to monitor cellular phone calls."

A basic build guide for a scanner built using this approach can be found on Adafruit. The general steps are:

  1. Get a dongle supported by RTL-SDR
  2. Install the Osmocom RTL-SDR libraries
  3. Try starting out with the GQRX software; which is the recommended starting software from the RTL-SDR site for Linux (unsure of your OS, but the link contains a Windows guide too).

The "RTLSDR Scanner" tool appears useful to analyze the spectral results from the scanner hardware (e.g. this is a software spectrum analyzer).

See also: Decoding unencrypted GSM traffic using RTL-SDR

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