 is referred as frequency error-the difference between the actual frequency used by the BS and the frequency that the BS should be using. The limit in frequency error needs to be maintained across the BS to deliver a better user experience, such as successful handover processing, cell throughput, and SO forth. To ensure that all the connected BSs are within the frequency synchroniza- tion limits, a common source is used for frequency and data clock generation. Additionally, the frequency of the carrier output of the transmitter must also be accurate to guaran- tee there is no overlap of signal that would cause interference between neighboring base stations. This interference and crosstalk between overlapping signals would impair the voice quality and generally degrade the user experience. In TDD-based systems, the UE and BS must be not only frequency synchronized but phase syn- chronized as well. In TDD, to allow full-duplex operation, the uplink and downlink share the same frequency channel but utilize different time-slots. Accurate phase/time synchronization is critical for error-free communications between UE and BS, as both radios alternate between transmit and receive. If either end loses synchronization, their transmit time-slots overlap, and the two ends talk all over the top of one another. As shown in Figure 10-5, accurate phase/time synchronization is also critical to reducing service outage at the point of handover from one BS to the adjoining BS. Furthermore, if neighboring base stations are not synchronized and use different uplink and downlink slots on the same channel, then interference occurs at the overlapping edges between cells. Time domain interference during handover UE and BS not time synchronized UE subframes D U D D D U D D (((2))) eNodeB U D D D U D Base station subframes Time domain synchronized between UE and BS UE subframes Backhaul UDDDUDDD (((2))) eNodeB Base station subframes Figure 10-5 Call Handover Scenario-TDD The preceding section mentions that TDD uses a guard time to prevent 