Cellurar Radio Systems

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47 Cellular radio systems

PART 1 (47.1 – 47.3.5)
47.1Introduction
Cellular radio systems are by far the most common of all public mobile telephone networks, the earlier (pre-cellular) networks now all being in decline. The basic principles of cellular systems were established by Bell Laboratories in 1949, but it was not until the early 1980s that technology allowed real commercial networks to be built and service offered to the public. Systems were developed at different times in different countries and subject to a variety of different constraints such as frequency band, channel spacing etc. As a result, a number of different and incompatible cellular standards are in use throughout the world, and the more important standards are summarised later in this chapter. Work is already well in hand to specify and develop second generation cellular systems, for which the opportunity is being taken to develop common standards and systems across several countries. One such notable system, GSM, has been developed in Europe, and is described in some detail later in this chapter. 47.2Principles of operation

47.2.1 Network configuration
In a cellular radio system, the area to be covered is divided up into a number of small areas called cells, with one radio base station (BS) positioned to give radio coverage of each cell. Each base station is connected by a fixed link to a mobile services switching centre (MSC), which is generally a digital telephone exchange with special software to handle the mobility aspects of its users. Most cellular networks consist of a number of MSCs each with their own BSs, and interconnected by means of fixed links. The MSCs interconnect to the public switched telephone network (PSTN) for both outgoing calls to, and incoming calls from fixed telephones. Figure 47.1 shows a typical network arrangement. [pic]

Figure 47.1 Cellular network configuration
A cellular network will be allocated a number of radio frequencies, or channels, for use across its coverage area, this number being dependent upon the amount of spectrum made available by the licensing authority and the channel spacing of the technical standard used by the network. The radio channels are grouped together into a number of channel sets, and these sets are then allocated to the cells, one set per cell, on a regular basis across the whole coverage area. Each channel will therefore be re-used many times by the network. The method of radio planning and allocation of channels to the cells is described later in this chapter. 47.2.2Signalling

Generally, one radio channel is set aside in each cell to carry signalling information between the network and mobile stations. In the land to mobile (L-M) direction, overhead information about the operating parameters of the network, including an area identifier code, is broadcast to all mobiles located in the cell's coverage area. In addition specific commands are transmitted to individual mobiles in order to control call setup and mobiles' location updating. In the mobile to land (M-L) direction, the signalling channel is used by the mobiles to carry location updating information, mobile originated call setup requests, and responses to land originated call setup requests. 47.2.3Location registration

When a mobile is not engaged in a call, it tunes to the signalling channel of the cell in which it is located and monitors the L-M signalling information. As the mobile moves around the network, from time to time it will need to retune to the signalling channel of another cell when the signal from the current cell falls below an acceptable threshold. When the mobile retunes in this way, it reads the overhead information broadcast by the new cell and updates the operating parameters as necessary. It also checks the location information being broadcast by the new cell and, if this differs from the previous cell, the...
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