Identifying rogue GSM base stations by intercepting downlink beacon channels

1. A method comprising:

utilizing a computer system to perform a plurality of operations including:

operating on a signal {x(n)} to generate a plurality of baseband signals based on a corresponding plurality of overlapping frequency bands having distinct center frequencies, wherein each of the baseband signals is generated by frequency-translating the signal {x(n)} based on a center frequency of the corresponding frequency band to obtain a corresponding frequency-translated signal and by low-pass filtering the corresponding frequency-translated signal;

identifying which one or more of the overlapping frequency bands are spectrally occupied;

analyzing the baseband signal corresponding to each of the one or more spectrally-occupied frequency bands in order to classify the spectrally-occupied frequency band as not containing a GSM signal, as containing a GMSK-modulated GSM signal, or as containing a PSK-modulated GSM signal;

estimating a respective value of carrier frequency offset for each of the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding baseband signal;

applying a frequency correction to each of the one or more baseband signals that correspond to the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding respective estimated value of carrier frequency offset; and

identifying one of the one or more frequency corrected signals as corresponding to a GSM beacon channel by determining which of the one or more frequency corrected signals includes a known extended training sequence and a known frequency correction burst.

2. The method of claim 1 , wherein the plurality of operations also include computing the signal {x(n)}, wherein said computing includes:

receiving an input signal that includes a mixture of the first signal transmitted by a rogue base station and a second signal from a victim base station; and

removing the second signal from the input signal to obtain the signal {x(n)}.

3. The method of claim 1 , wherein the plurality of operations also include:

extracting information regarding a rogue base station from the frequency corrected signal that corresponds to the GSM beacon channel.

4. The method of claim 3 , wherein said extracted information includes a base station color code and a network color code of the rogue base station.

5. The method of claim 3 , wherein the plurality of operations also include:

displaying the extracted information via a display device.

6. The method of claim 3 , wherein said extracting the information includes:

extracting a synchronization burst from the frequency corrected signal that corresponds to the GSM beacon channel; and

performing equalization, demodulation and decoding on the synchronization burst.

7. The method of claim 1 , wherein said analyzing the baseband signal corresponding to each of the one or more spectrally-occupied frequency bands includes:

computing correlations between downsampled versions of the baseband signal and each of a plurality of templates, wherein the templates include GMSK and 8-PSK modulated versions of a set of GSM training sequences;

determining whether one of the correlations includes a periodic series of peaks having magnitude greater than a first threshold and having interpeak distance corresponding to a known constant value.

8. The method of claim 7 , wherein said analyzing the baseband signal also includes:

in response to determining that one of the correlations includes the periodic series of peaks, classifying the baseband signal as being GMSK modulated or 8-PSK modulated based on whether the template that gives the correlation including the periodic series of peaks is GMSK modulated or 8-PSK modulated.

9. The method of claim 7 , wherein said analyzing the baseband signal also includes:

computing a magnitude spectrum of a fourth power of the baseband signal;

determining whether the magnitude spectrum includes three peaks that have magnitude larger than a second threshold and that conform to a predetermined pattern of interpeak distances.

10. The method of claim 1 , wherein said estimating the respective value of the carrier frequency offset for each spectrally-occupied frequency band includes:

removing frequency correction bursts from the corresponding baseband signal by (a) computing an occupied bandwidth of the baseband signal in each of a sliding series of windows that cover the baseband signal, and (b) discarding portions of the baseband signal that correspond to a subset of the windows having occupied bandwidth lower than a threshold.

11. The method of claim 1 , wherein said estimating the respective value of the carrier frequency offset for each spectrally-occupied frequency band includes:

maximizing a cyclic correlation between a magnitude spectrum of the corresponding baseband signal and a magnitude spectrum of a reference GMSK signal.

12. The method of claim 1 , wherein the plurality of operations also include:

applying a low pass filter to each of the one or more frequency corrected signals prior to said identifying one of the one or more frequency corrected signals as corresponding to a GSM beacon channel.

13. A non-transitory computer-readable memory medium storing program instructions, wherein the program instructions, when executed by a computer system, cause the computer system to: operate on a signal {x(n)} to generate a plurality of baseband signals based on a corresponding plurality of overlapping frequency bands having distinct center frequencies, wherein each of the baseband signals is generated by frequency-translating the signal {x(n)} based on a center frequency of the corresponding frequency band to obtain a corresponding frequency-translated signal and by low-pass filtering the corresponding frequency-translated signal;

identify which one or more of the overlapping frequency bands are spectrally occupied;

analyze the baseband signal corresponding to each of the one or more spectrally-occupied frequency bands in order to classify the spectrally-occupied frequency band as not containing a GSM signal, as containing a GMSK-modulated GSM signal, or as containing a PSK-modulated GSM signal;

estimate a respective value of carrier frequency offset for each of the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding baseband signal;

apply a frequency correction to each of the one or more baseband signals that correspond to the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding respective estimated value of carrier frequency offset; and

identify one of the one or more frequency corrected signals as corresponding to a GSM beacon channel by determining which of the one or more frequency corrected signals includes a known extended training sequence and a known frequency correction burst.

14. The memory medium of claim 13 , wherein the program instructions, when executed by the computer system, further cause the computer system to compute the signal {x(n)}, wherein said computing includes:

receiving an input signal that includes a mixture of the first signal transmitted by a rogue base station and a second signal from a victim base station; and

removing the second signal from the input signal to obtain the signal {x(n)}.

15. The memory medium of claim 13 , wherein the program instructions, when executed by the computer system, further cause the computer system to:

extract information regarding a rogue base station from the frequency corrected signal that corresponds to the GSM beacon channel.

16. The memory medium of claim 15 , wherein said extracted information includes a base station color code and a network color code of the rogue base station.

17. The memory medium of claim 15 , wherein said extracting the information includes:

extracting a synchronization burst from the frequency corrected signal that corresponds to the GSM beacon channel; and

performing equalization, demodulation and decoding on the synchronization burst.

18. A computer system comprising:

a processor; and

memory that stores program instructions, wherein the program instructions, when executed by the processor, cause the processor to:

operate on a signal {x(n)} to generate a plurality of baseband signals based on a corresponding plurality of overlapping frequency bands having distinct center frequencies, wherein each of the baseband signals is generated by frequency-translating the signal {x(n)} based on a center frequency of the corresponding frequency band to obtain a corresponding frequency-translated signal and by low-pass filtering the corresponding frequency-translated signal;

identify which one or more of the overlapping frequency bands are spectrally occupied;

analyze the baseband signal corresponding to each of the one or more spectrally-occupied frequency bands in order to classify the spectrally-occupied frequency band as not containing a GSM signal, as containing a GMSK-modulated GSM signal, or as containing a PSK-modulated GSM signal;

estimate a respective value of carrier frequency offset for each of the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding baseband signal;

apply a frequency correction to each of the one or more baseband signals that correspond to the one or more spectrally-occupied frequency bands that have been classified as containing a GMSK-modulated GSM signal, based on the corresponding respective estimated value of carrier frequency offset; and

identify one of the one or more frequency corrected signals as corresponding to a GSM beacon channel by determining which of the one or more frequency corrected signals includes a known extended training sequence and a known frequency correction burst.

19. The computer system of claim 18 , wherein the program instructions, when executed by the processor, further cause the processor to:

extract information regarding a rogue base station from the frequency corrected signal that corresponds to the GSM beacon channel.

20. The computer system of claim 18 , wherein the program instructions, when executed by the processor, further cause the processor to compute the signal {x(n)}, wherein said computing includes:

receiving an input signal that includes a mixture of the first signal transmitted by a rogue base station and a second signal from a victim base station; and

removing the second signal from the input signal to obtain the signal {x(n)}.