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I am trying to fully understand the physical layer of the GSM standard. For doing that I am looking at the E-GSM-900 band (mostly used in Europe). In my country these frequencies has been assigned to the network operators as follow:

Table headers: Block, Uplink, Downlink, Operator, Concession, Expiration

enter image description here

So I set my spectrum analyzer to see the Downlink frequencies for that band (925.1 - 959.9 MHz):

  • Central frequency: 940 MHz
  • Frequency span: 30 MHz

And I am able to see this: enter image description here enter image description here enter image description here

According to ETSI TS 136 211 V10.0.0 the modulations used are:

  • Downlink: OFDM
  • Uplink: SC-FDMA

And looking into the next spectrum intervals:

  • 930 - 935 MHz
  • 940 - 945 MHz
  • 950 - 955 MHz

It seems to me that it looks like OFDM modulated signals. But what is the first chunk preceding this OFDM blocks? Precisely the intervals:

  • 925 - 930 MHz
  • 940 - 945 MHz

What are they? Which signals are modulated there and in which modulation schema? I do not know if I am looking to the wrong standard (there is this huge mess between GSM-LTE bands), or if it is just my network analyzer configuration, or something else. I hope you shed some light on this.

EDIT: Added waterfall and time snapshot

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    \$\begingroup\$ Since no time duration is shown, also include the waterfall display. The spectra changes between idle negotiation and connection. \$\endgroup\$ Sep 13, 2017 at 17:32
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    \$\begingroup\$ Examine the time slots over a 120ms interval. \$\endgroup\$ Sep 13, 2017 at 17:38
  • \$\begingroup\$ Added! Very interesting note. I ve been observing the spectrum but I does not seem to change a lot. What I have seen is that yesterday the spectrum peaks where different (the no OFDM part) \$\endgroup\$
    – JVGD
    Sep 15, 2017 at 9:00
  • \$\begingroup\$ you got everything but a 120ms frame... ! like a 20s waterfall and a 10 us sweep, looking for all the caller channels in a 120 ms frame then sub frame should appear. then adjust trigger \$\endgroup\$ Sep 15, 2017 at 14:43

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