Uplink Control for Low Latency HARQ in TDD Carrier Aggregation

A higher data rate and lower latency are two main features of Long Term Evolution Advanced (LTE-A). In order to support a higher peak data rate, carrier aggregation (CA) is implemented into LTE-A systems to provide sufficient bandwidth for data transmission. However, there is a problem in achieving lower latency. Time Division Duplexing (TDD) systems may not have a sufficient number of uplink subframes to transmit downlink hybrid automatic repeat request (HARQ) feedback, which results in additional latency compared with Frequency Division Duplexing (FDD) systems. In TDD CA systems, the downlink HARQ round trip time (RTT) of a component carrier (CC) is related to its TDD configuration. In this paper, a low latency downlink HARQ feedback method for TDD CA systems is proposed to shorten the transmission delay. The serving eNB determines whether to allocate the physical uplink control channel (PUCCH) to a new CC by comparing the downlink HARQ RTT of that CC to that of the primary cell. If the downlink HARQ RTT can be reduced, the eNB will allocate the PUCCH to this new CC. By using this method, the downlink HARQ RTT of TDD CA systems can be significantly reduced.