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.
Within a radio frame, the transmission direction changes several times between downlink and uplink.
In special subframe DL to UL switching happens. Special subframe includes DL,UL and a guard period.
Due to the different signal transit times between the eNodeB and the various mobile stations, a timing advance mechanism involving a time gap called “guard period” is needed when the transmission direction switches from downlink to uplink. However, no guard period is needed when the transmission direction switches from uplink to downlink.
In the uplink,as shown in above figure the greater the distance between the eNodeB and the mobile station, the earlier the mobile station must start transmitting. This helps ensure that all signals reach the eNodeB in a frame-synchronous manner. When switching from downlink to uplink, a guard period is inserted between the DwPTS and UpPTS field in each special subframe. The duration of the guard period is configured by the network, based on the cell size. The maximum possible guard period length of ten OFDM symbols allows cell sizes with a radius of 100 km.
When switching from uplink to downlink there is no need for a guard period, since the uplink signals all arrive at the eNodeB in a frame-synchronous fashion - thanks to the timing advance mechanism - and the downlink data is also transmitted in the form of a frame-synchronous OFDMA signal.
Not my creation, its copied from here http://communicationanddsp.blogspot.in/2014/01/why-special-subframe-is-needed-in-lte.html but was explained bey nicely so just copied.
