overlap between transmis- sion and reception. The guard time should be sufficiently long to allow the signal from the UE to reach the base station before the base station shuts down its receiver and switches to transmit mode. Consequently, the guard time is decided based on two main factors: Propagation delay: This is proportional to the distance separating the UE and the BS. The BS informs the UE when to start the uplink transmission. The greater the distance between UE and base station, the longer the transmitter must wait before it receives a reply to its transmission, because the receiver must wait for all the data in that time-slot to arrive before beginning to transmit a reply. Switching time: The time required to switch between transmit and receive mode. The network operator configures the guard time based on the cell size. The preceding paragraphs confirm that every adjacent BS must be in strict phase/time synchroniza- tion with all its neighbors (for example, to allow smooth handover of any UE between them). For that reason, the standards organizations have agreed that TDD requires a phase alignment of less than 3 us between base stations to function correctly. This requirement is satisfied by synchronizing all the base stations to a common phase and time ref- erence, SO the operator makes sure every BS is aligned to within +1.5 us of a commonly agreed time reference. This common time standard is normally Coordinated Universal Time (UTC), SO if every BS is within +1.5 us of UTC, then every BS will be within 3 us of all its neighbors. This phase/time alignment to UTC is delivered either through the back-haul network or via timing radio signals re- ceived by a global navigation satellite system (GNSS) receiver. The BS in turn continuously aligns the phase/time on the UE using the radio signals. If the UE is not able to align its time with the BS, it needs to reattach to the BS and regain its synchronization. These requirements for timing are explained with more detail in the following section "Timing Re- q