Since UL and DL use the same carrier, additional interference issues may arise in TDD that are not encountered in FDD. In particular, downlink transmission may interfere with uplink transmissions and vice versa as illustrated in Figure below.
To provide good coverage, base station antennas are usually placed at elevated positions. As a result, there is often line-of-sight propagation between base stations.
Uplink traffic in base station 1 can therefore be severely interfered by downlink traffic of an adjacent base station 2, as shown in figure above.
Similar to the interference between base stations, there can be interference between terminals, see figure above. The occurrence of this is more stochastic in the sense that it depends on the location and activity of the terminals. Hot spot areas are examples of where UE-to-UE interference could be a problem.
Both of these additional sources of interference between UL and DL can be avoided if UL and DL do not occur simultaneously in different cells. The most common, simple and robust approach is to synchronize all base stations. For example with GPS, so that the radio frame starts at the same time in all cells. Additionally, the allocation of sub-frames to UL and DL should then be the same in all cells.
Timing synchronization and use of the same allocation of sub-frames to UL and DL alone may not be sufficient. While base stations 1 and 2 may change from transmit to receive exactly at the same time, the radio signals sent out previously still propagate in the air.
Due to the propagation distance between base station 1 and 2, the receiver of base station 1 may be turned on while the downlink transmissions from base station 2 still arrive at base station 1, potentially disturbing any uplink transmission. Therefore, extra guard time between switching from transmit (DL) to receive (UL) is required. The level of interference of adjacent (and more distant) base stations depends on the antenna heights, the antenna pattern, the antenna tilt, and the terrain conditions.