A user equipment (UE) is configured to perform a Hybrid Automatic Repeat Request (HARQ) in an un-licensed system. The UE includes at least one antenna configured to communicate with a base station. The UE also includes processing circuitry. The processing circuitry is configured to: send and receive data through the at least one antenna, in response to failing to decode a transport block transmitted by a first downlink carrier, transmit a negative acknowledgement message to the base station, and receive a re-transmission of the transport block from a second downlink carrier. The first downlink carrier comprises a unlicensed spectrum carrier and the second downlink carrier is different from the first downlink carrier.