Method and apparatus supporting random access transmissions

A method and base station are disclosed. The base station comprises a transceiver and a processor. The base station transmits a system information block (SIB) to a wireless transmit/receive unit (WTRU), wherein the SIB indicates random access transmission uplink resources. After transmitting the SIB, the base station receives a random access preamble from the WTRU. After receiving the random access preamble, the base station receives, using one or more of the uplink resources indicated by the SIB, a first message from the WTRU, wherein the first message includes a first radio network temporary identifier (RNTI) of the WTRU. After receiving the first message, the base station transmits a second message to the WTRU, wherein the second message is derived from a second RNTI, the second RNTI being different than the first RNTI. The base station may be configured to communicate with the WTRU on a plurality of cells.