rtain HARQ process, then Tround-trip later, the same HARQ process is considered for retransmission. In LTE, the UE receives uplink HARQ feedback from the eNB on PHICH. PHICH received in subframe k relates to the transmission in the subframe k - 4. If the UE receives NACK on PHICH (but does not receive a grant), a non-adaptive retransmission is automatically trig- gered. LTE-based LAA uses asynchronous HARQ in the uplink. To enable asynchronous HARQ, the eNB needs to know the HARQ process that the UE is using when performing a transmission/retransmission to correctly choose the soft-buffer where the received transmis- sion should be combined. Hence, the eNB needs to indicate to which HARQ process a grant is related and the redundancy version that should be used SO that the UE uses the correct HARQ process with the correct redundancy version when performing a transmission or retransmission. Therefore, with the uplink asynchronous HARQ protocol, all transmissions or retransmissions are scheduled via (e)PDCCH. The process index is indicated in the HARQ process index field in the uplink grant. For synchronous uplink HARQ, the number of HARQ processes is not explicitly specified. Instead, the supported number is derived from the HARQ timing. For asynchronous uplink HARQ, a maximum number of HARQ processes may need to be specified. The exact number of uplink HARQ processes in the eNB may be left to implementation [12]. In synchronous uplink HARQ, the UE can expect to receive uplink grants at a specified time as the HARQ process follows a fixed pattern. It should be noted that the UE monitors (e)PDCCH once in every HARQ RTT even if the UE has received ACK in PHICH. With asynchronous uplink HARQ, the UE does not know when to expect grants as the eNB may send them at any time. In addition, if HARQ buffer is not flushed, the UE would never stop monitoring (e)PDCCH according to the current mechanism. Therefore, with asynchronous uplink HARQ, the DRX behavior needs to be modified. In carrier aggregation, the same DRX operatio