Link Level Investigation of ACK/NACK Bundling for LTE TDD

In LTE TDD there is typically no one-to-one association between UL and DL subframes, and for DL heavy asymmetries with more DL than UL subframes, ACK/NACK reports for multiple DL subframes need to be transmitted in an UL subframe. To improve uplink control channel performance, ACK/NACK bundling, where multiple ACK- NACKs are combined to a single ACK/NACK response for several DL subframes, is supported. In this paper, we study the impact on downlink performance of ACK/NACK bundling on link-level for LTE TDD systems both for SIMO and MIMO systems. It is found that the expected loss due to ACK/NACK bundling is rather small and hence bundling can be a good solution in many scenarios.