Berlin Schönefeld Airport (SXF) is about 18 km away from the city center. RE arrives at Alexanderplatz in about 22 minutes. Bus line number 171 connects the airport directly to the subway station "Rudow".
Berlin Tegel Airport (TXL) is about 8 km away from the center. There is a connection to the ring road and the public transport network with bus lines.
MORE INFO: http://www.berlin-airport.de/en/