Barcelona Airport is 12 km out from the city. There are various ways of getting from the airport to the city centre.
Barcelona has two international railway stations: Sants and França.
To reach the UAB you may take two train lines, RENFE or Ferrocarrils Catalans (FGC). The main stations are located on Plaça Catalunya which is at the very center of Barcelona and which is also a stop for several metro lines.
If you come with RENFE you will have to drop at the Cerdanyola-Universitat Autònoma Station and there a free shuttle bus will take you to the Campus. You will have to drop on the second bus stop and cross the "Facultat de Lletres" downstairs until you find the Facultat de Ciències Econòmiques i Empresarials which is the Faculty where the Departament d’Economia and Econòmica and IDEA are located.
The Ferrocarrils Catalans (FGC) have several lines all departing from the same platform at Plaça Catalunya. Only two are suitable to reach the UAB, S2 destination Sabadell and S55 destination UAB. The whole journey lasts roughly 30 minutes; depending on the departure station. There are trains approximately every ten minutes. Once at the campus follow the mass of people going downstairs and bearing left, down a wooden ramp until you reach Building B, Facultat de Ciències Econòmiques i Empresarials.