ges in spectrum to traffic capacity with demand. match capacity demands. Can be transmitted over longer dis- Longer GP must be introduced to accommodate Distance tances without any limitation of the effect of multipath and propagation delay. frame structure. For a given size of spectrum, FDD Due to more overhead in frame structure, to Throughput gives better peak throughput due to meet the same performance as FDD, TDD re- less overhead frame structure. quires more radio sites for a given spectrum. Because the radios are continuously switching UL and DL are in separate bands; Synchroniz- between UL and DL within the same band, ac- hence no need for phase alignment. ation curate phase alignment is mandatory. Because UL and DL transmission bands are different and predefined, Multiple input multiple output (MIMO), beam Features features like multiple antenna forming, beam alignment, and SO on are easier to Support schemes, beam formation, and beam implement with TDD. alignment are not efficient for FDD. To understand the need for frequency accuracy, consider the example of the handover process between cell sites. In FDD-based systems, the handset, or user equipment (UE), acquires two fre- quency channels from the base station (BS): one for transmission and the other for reception. If the BS has an accurate source of frequency, then the radios will be very closely tuned to the allocated, expected frequency and the UE will be locked onto them. When the UE moves from its current BS to a neighboring BS, as shown in Figure 10-4, it needs to acquire two new frequency channels from the neighboring BS for the call to seamlessly continue. The radio in the UE switches to the new channels and tries to continue the call. If the frequency off- set between adjacent base stations is too large (more than 50 ppb), the UE radio will not be able to lock onto the frequency being used by the new BS, and the call will be dropped. f1 + 50pbb (((2))) NodeB f2 + 50pbb PRC/PRS Backhaul (((2))) NodeB Figure 10-4 Call Handover Scenario-FDD This