Link

http://www.digi.com/technology/rf-articles/rf-basics

http://www.ni.com/white-paper/3992/en/

http://www.vodafone.com/content/index/about/sustainability/mpmh/how_mobiles_work/base_stations.html

http://www.tutorialspoint.com/gsm/gsm_base_station_subsystem.htm