An implementation of downlink asynchronous HARQ for LTE TDD system

The HARQ enables the error correction of the unsuccessfully decoded transport block, but it requires redundancy retransmissions and delay in receiving an acknowledgement. In order to provide the flexibility of scheduling, the DL of LTE system uses the asynchronous HARQ so that retransmissions occur at any time. Generally in the FDD, the HARQ round trip time (RTT) which is time interval between initial transmission and retransmission is fixed. In the TDD, UL/DL subframe is separated in time domain by UL-DL configuration, so the HARQ RTT is different according to each subframe and that would be longer than the FDD. Especially when retransmissions occur, if the HARQ RTT increases, the QoS requirement of the retransmitted packet would not be satisfied by increased end-to-end latency. In this paper, we suggest the method of implementing DL asynchronous HARQ in LTE TDD system that the eNB can apply by UL-DL configuration as minimizing the HARQ RTT, and analyze in aspect of the HARQ RTT.