the microcells (but use different channels to those allocated to the microcells). However, it would enable those people moving along the road to be handled by the umbrella cell and experience fewer handovers than if they had to pass from one microcell to the next. 256 Newnes Guide to Radio and Communications Technology Transmissions both ways Basic radio communications systems use a single channel and what is known as a press to talk system, where the user presses a button or 'pressel' on the microphone to talk. This system is known as simplex as it uses a single channel. For a phone system a full duplex system is required where it is possible to speak in both directions at the same time. There are two main ways in which this can be achieved. The first is to transmit in one direction on one frequency and simultaneously transmit in the other direction on another. To achieve this there must be sufficient frequency separation and very good filters to make sure that the transmitter does not interfere with the receiver. This system is known as Frequency Division Duplex (FDD). Frequency separation to prevent interference between the two transmissions Uplink Downlink channel channel Frequency (a) Frequency division duplex (FDD) Time separation (guard time) to allow for time delays (signal propagation, switching times, etc.) Uplink Downlink Uplink Downlink (b) Time division duplex (TDD) Figure 10.2 Representation of frequency and time division duplex Cellular telecommunications 257 The other system uses only a single frequency and can be employed where digital or data systems are used. This requires the analogue audio signal to be digitized. A single frequency is used for the radio frequency signal and short packets of data are sent first in one direction and then the other. As these data bursts are relatively short the user does not notice the short delay introduced by the fact that the digitized speech signal is not sent immediately. This type of system is known as Time Division Duplex (TDD). It is often necessary to d