A method and apparatus for sending uplink control information (UCI) by a multi-mode wireless transmit/receive unit (WTRU) capable of operating on multiple component carriers of a plurality of radio access technologies (RATs) for multi-RAT operation are disclosed. The multi-mode WTRU may generate first UCI pertaining to a first RAT and second UCI pertaining to a second RAT. The WTRU may then send at least part of the first UCI and least part of the second UCI via a feedback channel on a component carrier of a second RAT. The first RAT may be Long Term Evolution (LTE) and the second RAT may be LTE-Advanced (LTE-A), or vice versa. Each RAT may operate on a different frequency. The UCI of the RATs may be multiplexed onto a carrier of any one of the RATs. The UCI bits for a pair of, or multiple, serving cells may be jointly encoded.