I am an extreme beginner with SDR. I am trying to use the 820T2 & SDR device to capture uplink gsm packets sent by my phone to the nearest Base Station. I have an Rx Monitor App installed on my phone to see my Uplink and Downlink channel frequency (904MHz and 949MHz respectively). I am using grgsm_livemon on a Ubuntu VM installed on VMWare to scan for packets. I am able to capture downlink data easily via gr_gsm. However, I see no signals on the frequency monitor of gr_gsm in the uplink frequency. I tried to move a few steps within the band to find any peaks, but unfortunately no result.

I also tried getting a call on my phone and then I see some instantaneous peaks throughout the whole Uplink spectrum for unknown reasons. (This is totally opposite of what I thought, I believed a GSM uses TDM and FDM for multiplexing multiple users. And a particular user would be assigned a static frequency and one or multiple time-slots for communication, and hence I should see instantaneous peaks at my uplink frequency only). Also, I am unable to capture any packets from these instantaneous peaks.

I would appreciate it if I get to know what am I missing to capture the uplink data or any tweaks to do it. In case you need any more information, Let me know. Thanks in advance!

EDIT: So I am sharing a couple of screenshots to have a better understanding of the problem. a) Network Details normal. Here the Frequency hopping is off which is not seen in SS. UE Network Details normal

b) grgsm output during Call. The peak is prominent but I couldn't take an SS as it's too fast. Frequency spectrtum during call

c) Network details when I picked up the call UE during call

d) grgsm DOWNLINK output

downlink without call

  • 1
    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Nov 23 '20 at 17:29
  • 2
    $\begingroup$ A) are you sure your phone is using only gsm900 and not switching to another band /mode? B) are you talking? The phone doesn't transmit much when you're silent, just 2-5 packets/second. Listen to the breakthrough with a cheap amplified loudspeaker to be sure. $\endgroup$
    – tomnexus
    Nov 23 '20 at 21:39
  • $\begingroup$ yes, it is at GSM900 both with and without call as you can see in SS above. Also, the only thing that changes is that Frequency Hopping is turned ON. Edit: Also I tried with a different operator, so you can see a change in Uplink and downlink frequency as 891 and 936 resp. B) Talking does gives some signals but no data is captured. And can elaborate the last line. I am sorry but what do you mean by breakthrough? $\endgroup$
    – anonymous
    Nov 24 '20 at 9:14

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