Part of the requirements for ‘evolved UTRAN’ is the ability to cope with various spectrum allocations from much less than 5 MHz to much more than 5 MHz, accommodating future 3G spectrum allocations. Ultimately, the maximum achievable data rate available should be 100 Mb/s in 20 MHz. Multicarrier technology should then allow a smooth migration from 1.25-MHz bandwidth to 20-MHz through 5, 10 and 15-MHz steps.
To increase this flexibility, the E-UTRAN air interface supports both frequency division duplex (FDD) and time division duplex (TDD) modes of operation (Figure 3.35). Moreover, most of the design parameters are common to TDD and FDD modes to reduce the complexity of the terminal.
TDD mode often requires synchronization between various transmitters and receivers in different cells. The reason why is discussed in the following and can easily be understood when looking at Figure 3.36. Two problem cases can be observed, in which either: