System and method for cellular call monitoring using downlink channel correlation
Methods and systems for monitoring communication of mobile communication terminals, such as cellular phones. An off-air monitoring system reconstructs both directions of a call conducted between two mobile terminals by receiving only downlink channels. The off-air monitoring system receives multiple downlink transmissions transmitted by base stations in a wireless communication network. The system uses the received downlink transmissions to reconstruct the bidirectional call content of calls conducted between wireless terminals. In order to reconstruct the bidirectional content of a given call, the system identifies and correlates the two downlink transmissions of that call from among the multiple received downlink transmissions.
1. A method for reconstructing bidirectional call content of calls conducted between wireless communication terminals, the method being exclusively based on downlink transmissions without a need to receive any uplink channels, the method comprising:
receiving, by a monitoring system, multiple downlink transmissions that have been or are being transmitted from at least one base station or satellite of at least one wireless communication network to wireless communication terminals of the at least one wireless communication network;
identifying, by the monitoring system, among the multiple downlink transmissions, a pair of first and second downlink transmissions that carry respective first and second opposite directions of a given terminal-to-terminal call, wherein the monitoring system has no prior identification of pairs of downlink transmissions; and
reconstructing, by the monitoring system, a bidirectional content of the given call from the identified first and second downlink transmissions.
2. The method according to claim 1 , wherein identifying the first and second downlink transmissions comprises extracting first signaling information from the first downlink transmission, extracting second signaling information from the second downlink transmission, and finding a correlation between the first and second signaling information.
3. The method according to claim 2 , wherein extracting the first and second signaling information comprises extracting a first identifier from the first downlink transmission and a second identifier from the second downlink transmission, and wherein finding the correlation comprises correlating the first and second identifiers with a same communication terminal that participates in the given call.
4. The method according to claim 1 , wherein identifying the first and second downlink transmissions comprises identifying a call-progress signaling pattern that is correlative in time in the first and second downlink transmissions.
5. The method according to claim 1 , wherein identifying the first and second downlink transmissions comprises extracting first voice content from the first downlink transmission and second voice content from the second downlink transmission, and finding a correlation between the first and second voice contents.
6. The method according to claim 5 , wherein finding the correlation comprises detecting a time correlation between voice activity periods in the first voice content and silence periods in the second voice content.
7. The method according to claim 1 , wherein receiving the downlink transmissions comprises receiving the first downlink transmission from a first base station, and receiving the second downlink transmission from a second base station that is different from the first base station.
8. The method according to claim 1 , wherein receiving the downlink transmissions comprises receiving the first downlink transmission from a first wireless communication network, and receiving the second downlink transmission from a second wireless communication network that is different from the first wireless communication network.
9. The method according to claim 1 , wherein receiving the downlink transmissions comprises receiving the first downlink transmission using a first receiver, and receiving the second downlink transmission using a second receiver that is separate from the first receiver.
10. The method according to claim 1 , wherein reconstructing the content comprises reconstructing the given call in real-time while the given call is in progress.
11. The method according to claim 1 , wherein reconstructing the content comprises reconstructing the given call off-line after the given call ends.
12. The method according to claim 1 , wherein identifying the first and second downlink transmissions comprises applying multiple different correlation criteria to the multiple downlink transmissions.
13. The method according to claim 12 , wherein applying the multiple different correlation criteria comprises applying a first correlation criterion to the multiple downlink transmissions so as to identify a set of candidate pairs of the downlink transmissions, and applying a second correlation criterion to the set of the candidate pairs so as to identify the first and second downlink transmissions.
14. An apparatus for reconstructing bidirectional call content of calls conducted between wireless communication terminals, the reconstruction being exclusively based on downlink transmissions without a need to receive any uplink channels, the apparatus comprising:
at least one receiver, which is configured to receive multiple downlink transmissions that have been or are being transmitted from at least one base station or satellite of at least one wireless communication network to wireless communication terminals of the at least one wireless communication network; and
a processor, which is configured to identify, among the multiple downlink transmissions, a pair of first and second downlink transmissions that carry respective first and second opposite directions of a given terminal-to-terminal call, and to reconstruct a bidirectional content of the given call from the identified first and second downlink transmissions, wherein the processor has no prior identification of pairs of downlink transmissions.
15. The apparatus according to claim 14 , wherein the processor is configured to extract first signaling information from the first downlink transmission, to extract second signaling information from the second downlink transmission, and to find a correlation between the first and second signaling information.
16. The apparatus according to claim 15 , wherein the processor is configured to extract a first identifier from the first downlink transmission and a second identifier from the second downlink transmission, and to correlate the first and second identifiers with a same communication terminal that participates in the given call.
17. The apparatus according to claim 14 , wherein the processor is configured to identify the first and second downlink transmissions by identifying a call-progress signaling pattern that is correlative in time in the first and second downlink transmissions.
18. The apparatus according to claim 14 , wherein the processor is configured to extract first voice content from the first downlink transmission and second voice content from the second downlink transmission, and to find a correlation between the first and second voice contents.
19. The apparatus according to claim 18 , wherein the processor is configured to find the correlation by detecting a time correlation between voice activity periods in the first voice content and silence periods in the second voice content.
20. The apparatus according to claim 14 , wherein the at least one receiver is configured to receive the first downlink transmission from a first base station, and to receive the second downlink transmission from a second base station that is different from the first base station.