istinguish between the link from the mobile to the base station and the link from the base station to the mobile. The first, i.e. the link from the mobile to the base station, is often called the uplink or the reverse link as the signal is being transmitted up to the base station. The second, i.e. the link from the base station to the mobile, is known as the downlink or the forward link. Mobile phones Today's mobile phones or handsets are remarkable feats of electronic engineering. They contain a vast amount of functionality in a very small space. In fact the requirements for more functionality in smaller packages has forced the pace of technology development, particularly in the radio frequency arena. Phones contain a receiver, transmitter and all the logic and control circuitry (which will undoubtedly be based around a ERICSSON 12:35 4 ghi Figure 10.3 A typical mobile phone handset (courtesy Sony Ericsson) 258 Newnes Guide to Radio and Communications Technology microprocessor) required to maintain the call as well as an advanced user interface consisting of a keypad and display. To enable these to perform all the functions that are required and to send and receive all the correct codes and controls, a vast amount of software is required. Without this the phone would not be able to operate. There are a number of different topologies that can be adopted for phones. The first generation phones typically used a double conversion superhet, converting down from the incoming signal frequency to a first IF that was usually around 45 MHz and then to a second IF at either 10.7 MHz or 455 kHz. However, today's phones generally convert right down to the baseband frequency where the demodulation takes place. Obvi- ously the transmit side is the inverse of the receiver. This approach significantly simplifies the design, and hence there is less circuitry. The local oscillator signal is generated using a frequency synthesizer. In this way the internal processing and logic in the phone can accurately and easily control the tran