Systems adopting Orthogonal Frequency Division Multiplexing (OFDM) require an eNodeB to receive signals from multiple User Equipment (UEs) “synchronously”, i.e., uplink signals of the multiple UEs should arrive at the eNodeB at the same time, so as to achieve orthogonality among uplink data transmitted at the same time by different UEs. Specifically, in implementation, it needs to be guaranteed that the difference between the time of arrival of data transmitted by different UEs to the eNodeB in the same uplink sub-frame is kept within the range of a Cyclic Prefix (CP). Since the UEs are in various distances from the eNodeB, transmission clock of each UE needs to be adjusted so that a UE far from the eNodeB starts to transmit earlier and a UE closer to the eNodeB starts to transmit later, and this process is referred to as uplink synchronization or timing alignment. To be specific, the eNodeB transmits a Timing Advance (TA) to each UE, and the UE decides the start time of an uplink sub-frame transmission according to the TA.
In a Frequency Division Duplex (FDD) system, the uplink transmission TA is only relevant to the position of the UE in the cell. The position of the UE in the cell determines the time taken by a radio signal to take a round trip between the UE and the eNodeB which is denoted as TRTT.
TRTT=2×D/C, where D is the distance between the UE and the eNodeB, C is the speed of light, i.e., C=3×108 m/s.
Propagation delay Tprop=0.5×TRTT=TRTT/2.
As shown in FIG. 1, downlink data transmitted by the eNodeB to the UE arrive at the UE after a propagation delay of Tprop=TRTT/2, thus a frame timing clock of the receiver of the UE is late than the clock of the eNodeB by TRTT/2.
Likewise, uplink data transmitted by the UE arrive at the eNodeB also after a delay of TRTT/2. In order to make the uplink data transmitted by the UE accurately fall in an uplink sub-frame reception window of the eNodeB, uplink data transmission should be started by the UE TRTT earlier than the downlink frame timing, and the amount of time in advance is the TA.
In a Time Division Duplex (TDD) system, due to the existence of uplink/downlink switching, besides the position of the UE in the cell, the timing advance is also relevant to the time taken by the eNodeB to switch from reception to transmission. Due to the uplink/downlink switching which is implemented by switching of radio frequency devices in the eNodeB and the UE, downlink-to-uplink guard period (TDU) and uplink-to-downlink guard period (TUD) are needed. The eNodeB does not transmit/receive data within the guard periods. Specifically, the value of each guard period is:TDU=max(TRTT+TUE,Rx-Tx,TeNodeB,Tx-Rx)  (1)TUD=max(TUE,Tx-Rx,TeNodeB,Rx-Tx)  (2)
where the TUE,Rx-Tx denotes the time taken by the UE to switch from reception to transmission; TeNodeB,Tx-Rx denotes the time taken by the eNodeB to switch from transmission to reception; TUE,Tx-Rx denotes the time taken by the UE to switch from transmission to reception; TeNodeB,Rx-Tx denotes the time taken by the eNodeB to switch from reception to transmission. The above four parameters are determined by performances of the eNodeB and the UE, and will be determined in the standards as a time template, e.g., as the typical values shown in Table 1 which are in unit of microsecond (μs)
TABLE 1TeNodeB, Tx-RxTUE, Rx-Tx TUE, Tx-Rx TeNodeB, Rx-Tx[μs][μs][μs][μs]10-1510-4010-2010-20
Existing LTE TDD systems have only one Guard Period (GP). The GP includes both the downlink-to-uplink guard period TDU and the uplink-to-downlink guard period TUD, i.e.,GP=TDU+TUD 
But in practice, TUD is implemented by advance of an uplink reception timing window at the eNodeB. As shown in FIG. 2, the time interval between the downlink transmission window and uplink reception window is TDU, and the time interval between the uplink reception window and the next downlink transmission window is TUD. Downlink data transmitted by the eNodeB are received by the UE after a delay of TRTT/2. For a UE, in order to make uplink data transmitted by the UE fall accurately in the reception window of the eNodeB, uplink transmission TA of the UE should be TTA=TRTT+TUD. That is to say, different from the FDD systems, timing advance of the UE in TDD systems depends not only on the position of the UE in the cell but also on the uplink-to-downlink guard period. In the prior art, there is no detailed solution for indicating timing advance in the TDD system.