A method implemented by a wireless transmit receive unit, WTRU, characterized by the method comprising: the WTRU receiving a radio resource control, RRC, connection reconfiguration message over a first component carrier, wherein the RRC connection reconfiguration message indicates configuration information for the WTRU to support a plurality of additional component carriers, wherein each of the plurality of additional component carriers is initially deactivated for the WTRU, and the RRC connection reconfiguration message indicates respective identifiers assigned to each of the plurality of additional component carriers for reference during subsequent component carrier activation; the WTRU receiving a medium access control, MAC, control element, CE, that indicates activation of one or more of the plurality of additional component carriers, wherein the MAC CE comprises a bit field indicative of which one or more of the plurality of component carriers are to be activated; the WTRU determining that a second component carrier is to be activated based on the bit field comprised in the MAC CE and on the respective identifier assigned to the second component carrier in the RRC connection reconfiguration message; the WTRU monitoring for physical downlink control channel, PDCCH, transmissions over the second component carrier based on the bit field comprised in the MAC CE and on the respective identifier assigned to the second component carrier in the RRC connection reconfiguration message; the WTRU receiving a PDCCH transmission over the second component carrier, the PDCCH transmission indicating scheduling information for a physical downlink shared channel, PDSCH, transmission over the second component carrier; and the WTRU receiving the PDSCH transmission over the second component carrier in accordance with the PDCCH transmission received over the second component carrier. the WTRU receiving a radio resource control, RRC, connection reconfiguration message over a first component carrier, wherein the RRC connection reconfiguration message indicates configuration information for the WTRU to support a plurality of additional component carriers, wherein each of the plurality of additional component carriers is initially deactivated for the WTRU, and the RRC connection reconfiguration message indicates respective identifiers assigned to each of the plurality of additional component carriers for reference during subsequent component carrier activation; the WTRU receiving a medium access control, MAC, control element, CE, that indicates activation of one or more of the plurality of additional component carriers, wherein the MAC CE comprises a bit field indicative of which one or more of the plurality of component carriers are to be activated; the WTRU determining that a second component carrier is to be activated based on the bit field comprised in the MAC CE and on the respective identifier assigned to the second component carrier in the RRC connection reconfiguration message; the WTRU monitoring for physical downlink control channel, PDCCH, transmissions over the second component carrier based on the bit field comprised in the MAC CE and on the respective identifier assigned to the second component carrier in the RRC connection reconfiguration message; the WTRU receiving a PDCCH transmission over the second component carrier, the PDCCH transmission indicating scheduling information for a physical downlink shared channel, PDSCH, transmission over the second component carrier; and the WTRU receiving the PDSCH transmission over the second component carrier in accordance with the PDCCH transmission received over the second component carrier.