GSM or Global System for Mobile Communication is a technology that’s widely used in mobile communications, especially mobile phones. This technology utilizes microwave and signal transmission divided by time, so that the signal information sent will arrive at the destination. The GSM standard for mobile communications as well as mobile technology is deployed more than its counterparts around the world, like CDMA. At this time we will discuss how to track a cell phone by using theDoppler effect, in other words we will make it easier to know the whereabouts of a person just by having information such as cell phone numbers.
GSM Network Architecture
Typical GSM network architecture is divided into 3 parts:
All elements of the network at the top form a PLMN (Public Land Mobile Network).
Picture 1. GSM network architecture
Mobile Station or MS is a device used by the customer for making phone calls. This device consists of:
Base Station System or BSS consists of:
Network Sub System or NSS consists of:
GSM Layers
There are 3 layers in the GSM network:
Picture 2. Illustration of how GSM works
How Doppler Works
Doppler is a change in the frequency or wavelength of a wave source that is received by the observer. This is the Doppler effect formula which is not affected by wind:
Where:
Doppler effect formula which is influenced by the wind:
Where:
This is the illustration of Doppler effect:
Picture 3. Doppler effect illust
From the above picture, there are 3 persons: A, B and C. A is the person in the middle who could detect the source of the wave/sound from B or C. Because the wave/sound that came from B or C travels in a certain frequency and distance, the Aperson could distinct the source of the wave/sound.
Concept
In this article, we are proposing a GSM radar using the Doppler effect, where the Doppler effect itself will be used to listen for the mobile phone uplink. There are some literature and references that mention about the Doppler effect being used to identify a signal if the Doppler effect is combined with the right filter processing according to the signal characteristic being transmitted.
Research
1. OpenBTS Installation
This article won’t go further step by step on this OpenBTS installation until it could be used, because there are already a lot of tutorials which cover the installation process. For this research, we are using USRP N200 from Ettus Research. But as we proceed using OpenBTS with USRP N200, we realize that there is an anomaly in the signal transmitted by USRP N200. So, we are using a spectrum analyzer to figure out and find a solution for the signal anomaly. This is the setup we are using:
Picture 4. Using spectrum analyzer to figure out USRP N200 signal anomaly
Picture 5. Signal anomaly as seen on spectrum analyzer
As you can see from the picture above, the signal generated by USRP N200 looks like a horn and the noise is quite high. The possible cause for that anomaly is USRP N200 clock is not accurate, and the solution for that is by adding a filter, so the final result will be a correct GSM modulation like this picture:
Picture 6. Correct GSM modulation after adding a filter
2. Doppler Design
After doing some research on Doppler design, we found out that some design is not capable for a frequency of 900 MHz, but we have a workaround and modified existing Doppler design so it capable of reaching 900 MHz and even higher. This is the block diagram for modified Doppler design (courtesy of Ramsey):
Picture 7. Modified
Picture 8. Tracking mobile phone illustration
Conclusion
From the above explanation, we could conclude that the Doppler effect could be used to lookup the position of a device transmitting a signal in a certain frequency. We could take this research further to detect any kind of living creature (e.g. endangered species) that in some way is transmitting a signal in a certain frequency, as long as we have the sound sample of that creature.
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