ng sections, you saw that one of the factors influencing the length of the GP is the propagation delay between the UE and the eNB. Once the UE is connected, the eNB keeps estimat- ing the propagation delay, called the timing advance offset, . The offset needs to be calculated for each connected device to coordinate its transmission. A special subframe is used in TDD to ad- just the GP to ensure a smooth switch between downlink and uplink according to the received TA offset. As the UE receives the value of TA (TA is in the range of 0-1282 for LTE) from the eNB, it is im- portant that each device acquires the synchronization information before it starts data transmission. Each additional increment in the value of TA (0.5208 us) allows for approximately 156 meters extra round-trip distance between the UE and the eNB (0.5208 us at 300,000 km/s is 156 m). For 4G LTE, the value range of 0-1282 for TA allows for a cell radius up to 100 km. BS to BS Synchronization (TDD) The time synchronization between adjacent eNBs is important to avoid cell interference. When a UE is moving from one eNB to another, the relative timing difference between subframe timing boundary becomes important. It is related to the time duration of the CP and the propagation delay between the cells. For smaller cell sizes, this is about 3 us. Consequently, in TDD deployment scenarios from small cells up to full-sized macro cells, the standards specify a mandatory phase alignment within 3 us as the synchronization requirement. This phase synchronization accuracy is defined as applying at the eNB antenna connectors. In the case of a small cell or home eNB (HeNB), there is a technique whereby it derives its timing from the signal transmitted by a neighboring synchronized eNB. This is referred to as synchroniza- tion using network listening and is shown in Figure 10-13. This has an advantage in those envir- onments where obtaining a phase/time signal from another source might be difficult (such as GNSS signal inside the home). Sync eNB (((2))) eNodeB Ba