A base station transmits subframes of downlink carriers in a primary cell group and downlink carriers in a secondary cell group in substantial time alignment with each other. A wireless device transmits subframes of uplink carriers in the primary cell group in substantially time alignment with each other. The wireless device transmits subframes of uplink carriers in the secondary cell group in substantially time alignment with each other. The subframes transmission timing for uplink carriers in the primary cell group and in the secondary cell group employ different synchronization signals as a timing reference and different timing advance commands.