mitting. Naturally this type of spectrum allocation is known as unpaired spectrum. When using TDD, it is necessary to synchronize the base stations SO that they transmit at the same time. This prevents a base station in an adjacent cell transmitting when a weaker mobile is transmitting. It is found that each type of system has its advantages. TDD is most suited to use with small cells where high data rates are needed, whereas FDD is better suited to covering wider areas. The architecture required for UMTS is not the same as that required for GSM although work has been undertaken to ensure that the maximum reuse possible is available. In this way the investment required for the new infrastructure is reduced to a minimum. In the new system a new radio access network known as a UTRAN (UMTS Terrestrial Radio Access Network) is used. One of the major elements of the UTRAN is the base station or Node B as it is called. This communicates over a radio link with the mobile handsets and back into the network, as shown in Figure 10.11, over what is termed an Lub-interface. The Node B provides all the Node B Node B network Node B Node B UTRAN network Figure 10.11 The UMTS Terrestrial Radio Access Network 278 Newnes Guide to Radio and Communications Technology RF processing to enable the transmission and reception of signals to and from the handsets. The Node Bs are controlled by what is known as an RNC or Radio Network Controller. This is broadly equivalent to the base station controller found in GSM. It controls several Node Bs. It manages the resources, allocating the capacity for the data channels, enables facilities such as call set-up, and provides switching and traffic routing. Although it appears similar to a BSC it also includes some of the functionality of the MSC. This enables the handovers between cells to be handled solely by the UTRAN. In this way it is not necessary to involve the core network, even when cell handovers between different RNCs are required. This makes handovers much easier to manage. The RNC then