i'm sorry for 4 years gap... was searching for the same and i found that it's possible to capture the traffic but for that u'll ned a RF generator that can generate signals stronger than the BTS..... but there is one more thing if you are looking to sniff traffic of one perticular mobile than you'll just need a signal strong enough for that mobile to connect to you (you have to be close to the mobile)..... for more info PM me may be we can work together i'm searching for informations tooo....