GSM Interception Using USRP and OpenBTS

I have a working OpenBTS with two RFX1800 and 52Mhz clock on a Pre- configured Ubuntu Linux netbook. I need it reconfigured to receive only, and to intercept and decipher radio traffic and record from both BTS and phones. A5.1 deciphering required. Need a customized front end GUI for user to easily enter values. In other words its called a Passive GSM interception system and/or also a Active GSM Interception system. Its intercepts traffic between the BTS and the phone and records and deciphers it using a A5.1 decipher unit which is connected to the system.