In a random access procedure, a downlink random access response indicates at least a timing advance and an explicit indication identifying one component carrier or cell (CC/Cell) of a plurality of component carriers or cells, or a group of CCs/Cells, to which the timing advance applies. The eNB or UE then operate their radio on the indicated one CC/Cell synchronous with the timing advance. In various embodiments the random access response may also indicate an uplink resource granted to the user equipment, which lies on that same one or group of CCs/Cells. The explicit indication may be a single bit indicating a timing advance group; a CC/Cell/group index whose presence in the response is indicated by a flag bit; a CC/Cell/group index in the response is known from the response's format whether it includes such an index, where the format indication can be via a bit in the response itself or RRC signaling or implicit from the RACH and cross-carrier scheduling being configured.