A method is provided for performing, at a wireless device, data transmission in a wireless communication system. The wireless device receives a configuration including a plurality of virtual cell groups, each of the virtual cell groups including two or more serving cells which share an identical virtual cell identification (ID). The wireless device also receives discovery signals scrambled with the identical virtual cell ID and system information including a physical random access channel (PRACH) configuration, from master cells of each of the virtual cell groups. The wireless device selects a cell based on the discovery signals and system information, and performs a random access channel (RACH) procedure and data transmission by using the virtual cell ID of the selected cell.