ulations which may limit transmission power levels. In contrast to fixed (wired) communications where usually a dedicated link exists between a transmitter and a receiver, terrestrial wireless communications are carried out in the 'open'. Broadband Telecommunications Technologies and Management Mobile 1 Mobile 2 Base station Mobile 5 Mobile 4 Figure 10.4 A wireless star network Any such link, whether a frequency channel or a time slot is a 'public' resource and is shared between a large number of end-users. A big challenge of wireless communications is how to use this limited resource in an efficient way. Duplex Modes Most wireless telecommunications systems operate in a full duplex mode. That is, a channel carries information from a central node to an end-user, and another channel is used for the reverse communications. In this way, both central and subscriber devices transmit and receive data. Usually communications from the central node to the end-user are called downlink, and the reverse uplink, transmissions. Duplex operation is possible using orthogonality in either frequency or time domains. In the frequency division duplex (FDD) mode two different frequency channels are used for the downlink and uplink transmissions. In contrast, time division duplex (TDD) mode transmis- sions in the downlink and uplink are carried within the same frequency band, but during dif- ferent time frame channels. FDD and TDD modes of operation are illustrated for a terrestrial mobile telecommunications system in Figure 10.5. Frequency Bands FDD and TDD uplink and downlink transmissions use a number of frequency bands in the 450 MHz to 5.85 GHz range. Many of these frequency bands are internationally used to allow the same devices to be operable in many countries. There also exist frequencies bands which are only used in a particular country/region. Moreover, some frequencies are specified for FDD and some for TDD operation. A list of frequencies used for mobile communications and the region where they are used is given Figure 10