In LTE, the UL is synchronous which means that the eNodeB controls the transmission timing. Due to the round-trip propagation delay, the transmission start in the UE depends on the UE – eNodeB distance in order to get all uplink receptions aligned in the base station receiver. In FDD, timing alignment messages are sent by the eNodeB to instruct the UE to advance its timing.
However in TDD, this timing advance should not result in a collision with a downlink transmission to the same terminal. Sufficient guard time can prevent self-interference.
Switching from DL to UL
At the switching of DL to UL, the guard period is to be chosen large enough to:
Note here that the guard period needed to avoid interference from other sites is not related to the cell radius, whereas the guard period requirement to avoid UE self-interference is directly related to the cell radius.
Switching from UL to DL
At the switch from UL to DL, the guard period is to be chosen large enough to:
The guard period at the switch from UL to DL is hence mainly related to the required switching time.
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