WHAT ARE THE REQUIRED GUARD PERIODS BETWEEN UL AND DL FOR LTE TDD MODE OF OPERATION?

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:

• Allow the base station to switch from transmission to reception.

• Allow a cell edge terminal to receive the last DL sub-frame, switch from reception to transmission and start the timing aligned UL transmission. This equals to the maximum cell round trip time (RTT) plus terminal switching time. For a 100 Km cell, the cell RTT alone is 667us.

• Avoid eNodeB-to-eNodeB interference from other sites due to propagation delays and possibly synchronization errors.

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:

• Allow the base station to switch from reception to transmission.

• Allow a UE close to base station to complete UL transmission, switch from transmission to reception and start receiving the first DL sub-frame.

• Allow interference from other eNode Bs due to synchronization errors

The guard period at the switch from UL to DL is hence mainly related to the required switching time.