TDD offers flexible deployments without requiring a pair of spectrum resources. For TDD deployments in general, interference between UL and DL including both basestation-to-basestation and UE-to-UE interference needs to be considered. One example includes layered heterogeneous network deployments, where it may be of interest to consider different uplink-downlink configurations in different cells. Also of interest are deployments involving different carriers deployed by different operators in the same band and employing either the same or different uplink-downlink configurations, where possible interference may include adjacent channel interference as well as co-channel interference such as remote basestation-to-basestation interference. Currently, LTE TDD allows for asymmetric UL-DL allocations by providing seven different semi-statically configured uplink-downlink configurations. These allocations can provide between 40% and 90% DL subframes. The semi-static allocation may or may not match the instantaneous traffic situation. The current mechanism for adapting UL-DL allocation is based on the system information change procedure. Additional mechanisms could include e.g. dynamic allocation of subframes to UL or DL.

The scope of this study item is given in [2].