This paper presents SPHERE, a Simulation Platform for HEterogeneous wiREless systems, and describes its motivation, methodology and implementation approach. This advanced system level simulation platform emulates simultaneously the transmission of GPRS, EDGE Multi-slot, HSDPA and WLAN at the packet level, which allows conducting novel investigations on common radio resource management for beyond 3G systems or on the optimization of radio resource management techniques. This paper presents the simulation platform, validates it and introduces its research potential.