US Patent 9397875 Carrier grouping in multicarrier communications

A wireless device receives messages comprising parameters of carriers and carrier groups. Downlink transmissions on a first carrier and at least one second carrier are substantially time aligned with each other. The wireless device receives time alignment commands comprising a time adjustment value and an index identifying a carrier group. Uplink transmissions on a first carrier group and a second carrier group employ different synchronization signals on different carriers as timing reference and are adjusted in response to different time alignment commands.