As the single frequency block is shared in time domain between UL and DL the transmission in TDD is not continuous. All UL transmission need to be on hold while any downlink resource it is used and the other way around.
Switching between transmission directions has a small hardware delay (for both UE and NodeB) and needs to be compensated. To control the switching between the UL and DL a guard period GP is allocated which compensates for the maximum propagation delay of interfering components.
Description of the switching mechanism:In LTE TDD there is maximally one UL->DL and one DL->UL transition per 5ms period (half frame)
UL-> DL transition
Is done for all intra-cell UEs by the process of time alignment. The NodeB instructs each UE to use a specific time offset so that all UEs signals are aligned when they arrive at Node-B. So UL is synchronous very similar to FDD (intra cell case only!)
As shown in above figure there are 7 frame configurations, according to different DL/UL partition.
1. Downlink / Uplink ratio can vary from 1/3 (Frame configuration = 0) to 8/1 (Frame configuration = 1), depending on the service requirements of the carrier
2. Frame always starts with a downlink subframe, used for advertising the frame descriptor, PCFICH and PDCCH. UE hence learns the frame structure in this subframe.
3. 3rd frame is always used for uplink
4. When switching from downlink to uplink, there is need for a special switching subframe. No special subframe is used when switching from uplink to downlink.
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