GSM Detector (operating and standby modes)

[electro_fran]

Hello everybody,
My objective is to design a GSM detector that detects mobile phones even if they are in standby mode. The first step is converting the GSM signal to IF signal using a downconverter. My IF signal is between 20 Mhz and 130 Mhz. My question is: how can I detect the GSM signal in this 20Mhz-130Mhz signal?
I only need to detect the presence of GSM signal in that IF signal and I don't need any processing.

 

[kripacharya]

You could use some sort of RSSI system with a settable threshold level. The tx signals from a nearby phone would be larger than that from a base station (assuming you are not near a BS).

I believe even in standby, the mobile phones are polled regularly by the BS, so be on the lookout for short bursts of transmission as well.

 

[BigBoss]

But GSM signals are variable carriers so the handset changes its carrier frequency depending on BS to which it has connected.(There are many BS around the handset)
So locking a carrier won't be continuously, after a while this carrier will have a different frequency in GSM band.You should track this frequency hopping but its data is sent in GSM datastream as encrypted so you cannot know this information.That's why locking to a single carrier is not a proper solution.
Instead, a banpass ( GSM band which you're interested in) system following a RF detector will give you more flexibility.There are many RF detector in the market that gives you Power Level of the signal strength.
https://www.analog.com/en/rfif-components/detectors/products/index.html#RMS_Detectors

 

[electro_fran]

@ kripacharya: Can you give me more detail about RSSI and how can I use it with my 20Mhz-130Mhz IF signal?

 

[kripacharya]

But GSM signals are variable carriers so the handset changes its carrier frequency depending on BS to which it has connected.(There are many BS around the handset)
So locking a carrier won't be continuously, after a while this carrier will have a different frequency in GSM band.You should track this frequency hopping but its data is sent in GSM datastream as encrypted so you cannot know this information.That's why locking to a single carrier is not a proper solution.
Instead, a banpass ( GSM band which you're interested in) system following a RF detector will give you more flexibility.There are many RF detector in the market that gives you Power Level of the signal strength.
https://www.analog.com/en/rfif-components/detectors/products/index.html#RMS_Detectors

This is all correct of course. However imho OP requires a more simplistic solution, which I inferred from the fact thats he's down converted the WHOLE gsm band, and is neither trying to decode a particular handset, or even lock onto a single carrier.

In addition, he wants to detect passive/ standby phones as well, which (i think) only talk on the control channel frequencies.

I think his best bet would be to further narrow the bandwidth to only the control channel frequencies, and monitor the signal strengths there.

What say you ? Good solution ?

 

[electro_fran]

Can you give me more detail about RSSI and how can I use it with my 20Mhz-130Mhz IF signal?

Please, Which circuitry shoud I add after the downconverter stage in order to be able to detect the existance of a GSM signal in my 20Mhz-130Mhz IF signal?

 

[electro_fran]

Is it OK to use an RF power detector to this purpose?