Technology for forming carrier aggregation timing advance groups in a heterogeneous network (HetNet) is disclosed. One method comprises assigning at least a first component carrier cell to one of a first timing advance group and a second timing advance group. At least a second component carrier cell is assigned to one of the first timing advance group and the second timing advance group. A separate timing advance index value is selected for each of the first and second timing advance groups. The timing advance index value is used to refer to the timing advance group in signaling in the HetNet.