r decoder processor Figure 10.2 A simple block diagram of a wireless communications system Broadband Telecommunications Technologies and Management modulation technique may be more superior, cost and complexity considerations may result in selection of a slightly inferior technique for a particular application. The decision generally depends on the required degree of efficiency and cost of implementation. The physical means for launching an electromagnetic wave into a communications medium differ considerably for wires, air or optical fibres. For example, a twisted pair of copper wires may be directly connected to a telephone; a coaxial cable requires a special connector; an optical signal needs a very accurate set-up for a laser light source to be connected to a fibre. Wireless systems use a number of different types of antennas depending on the application and the frequency of operation. In general an antenna's size and shape are dependent on the required gain and the frequency of operation, and the characteristics of the transmission medium. A number of antennas are shown in Figure 10.3. Antennas Telecommunications antennas are devices that convert electrical energy to electromagnetic energy and radiate them and vice versa. An antenna may be designed to radiate energy equally in all directions or to concentrate radiation towards a designed direction. Conversely, a receiver antenna converts radiated electromagnetic energy into an electrical current which may then be processed by the receiver electronics. The size and shape of antennas depend on the applications for which they are used. A directional antenna concentrates energy in a particular direction and therefore has a higher 'gain' compared with an omnidirectional antenna. The antennas in Figure 10.3 are (from left to right) for satellite, TV reception and a mobile communications base station. All these antennas are directional. Antennas are gen- erally characterised by their degree of directionality and the amount of electromagnetic energy they can radiate