HARQ Feedback for Carrier Aggregation in LTE-A TDD

Carrier aggregation (CA) is introduced in Long Term Evolution-Advanced (LTE-A) to provide larger bandwidth and higher peak rate for a single user. L1 control signaling design is very critical for implementing CA, and Ack/Nack feedback for CA is particularly challenging in time division duplexing (TDD) mode due to the potential large UL/DL asymmetry and limited transmit power in UL. This paper addresses some key issues for Ack/Nack feedback in TDD CA, namely the Ack/Nack bundling schemes, the number of feedback bits to be supported, and the discontinuous transmission (DTX) state feedback. For the last issue, the concept of bundled DTX feedback is proposed. The impacts of different feedback options on DL throughput are evaluated through simple theoretical analysis and extensive system level simulations, and some design principles are proposed, which can be used as guidance for concrete L1 design.