ss of each other's filters, downlink transmis- sion occurring at the same time that the neighboring BTS is receiving would cause very severe BTS-to-BTS interference. Even though the user equipment (UE)-to-UE interfer- ence scenario is also possible, they are considered much less damaging with a lower likelihood. For the above reasons, downlink and uplink need to take place simultaneously in all base stations, and switching should take place very accurately at the same time. There- fore, TDD networks have strict requirements for synchronization and timing alignment of downlink and uplink symbols for all networks operating on the same band. All opera- tors should preferably synchronize their networks and use the same TDD frame config- uration between the uplink and downlink. The TDD base station synchronization can be obtained from Global Positioning System (GPS) or via the transport network with the Timing over Packet (ToP) solution. 8.7.1 Main Interference Scenarios in TDD System The main interference scenarios in TDD networks are BTS-to-BTS interference and UE-to-UE interference. Figure 8.11 illustrates both interference cases when Cell 1 and Cell 2 are not synchronized. In interference case A, Cell 2 transmission to UE2 over- laps with the reception of Cell 1 from UE1. If the two BTSs are close enough, then the interference level can be SO high that the uplink of Cell 1 is completely blocked by the simultaneous Cell 2 transmission. Cell 1 Downlink Uplink Figure 8.11 BTS-to-BTS and UE-to-UE interference due to timing misalignment within operator's own band. Cell 2 Downlink Uplink Interference case A Cell 1 Interference Cell 2 case B 8.7 Network Synchronization and Coordination Requirements 15 kHz 5.0 ms 30 kHz 5.0 ms 30 kHz 2.5 ms 30 kHz 2.5 ms 30 kHz 2.0 ms 120 kHz 1.25 ms DDDSU 120 kHz 1.0 ms Figure 8.12 Typical TDD frame configuration options (D = Downlink slot, S = Special slot, U= Uplink slot). In interference case B, the UE1 transmission interferes with the UE2 reception, and if UE1 and UE2 are close enoug