(FDMA). In this technology individual user resource allocation is achieved by using different carrier frequency channels. In FDMA a mo- bile network assigns to a mobile phone a 'channel' within an allocated frequency band for the duration of its voice call. For example, a 1G mobile network may use its allocated 10 MHz of radio spectrum and offer 400 voice channels each with a 25 kHz bandwidth. With a frequency reuse factor of 7, 57 voice channels will be available in each cell in this system. An FDMA system, serving these 57 mobiles in one cell is illustrated in Figure 10.14. Using FDMA technology, each mobile phone active on the network in a particular geo- graphic area is assigned a downlink frequency channel and an uplink frequency channel which it exclusively uses for the duration of its call. After the call is ended, the channel is released and may then be assigned to another mobile for a new call. Broadband Telecommunications Technologies and Management Example 10.2 An example of an FDMA system is the Advanced Mobile Phone System (AMPS) standard. In AMPS, an operator's available bandwidth is divided into several parts for frequency reuse, and then each part is subdivided into frequency bands of 30 kHz, each of which is then used for one connection between a mobile user and a base station. One such AMPS operator has been allocated 8.4 MHz of spectrum (4.2 MHz for downlink and 4.2 MHz for uplink). How many voice channels can be offered in each cell if the frequency reuse factor is 7? Answer The maximum number of users which can be simultaneously connected to an AMPS system in each cell is calculated to be 20. AMPS system's frequency utilisation efficiency in terms of voice channels per megahertz of bandwidth per cell is 20/4.2 2 4.8 voice channels/MHz/cell. This is illustrated in Figure 10.15. Mobile 1 Mobile 2 Frequency Mobile 3 Mobile 57 Base station Mobile 5 Mobile 4 Figure 10.14 FDMA as used in a typical 1G system Spectrum given one operator : e.g. 4.2 MHz (downlink) 600 kHz (Frequency reuse factor = 7) 3