4 Frequency

Every mobile network consists of several cells. A cell describes the geographical area that is serviced by one single base station (BS) (antenna) or one sector of a base station that is separated in different service areas (see below). Every base station serves at least one cell by providing radio channels to it.

There are different reasons for the cellular structure. One is that the signal field strength is more or less attenuated while propagation through the air. To cover a huge area with only one transmitter an enormous power would be necessary (cf. broadcast). The retransmission from the mobile station (MS) would be impossible due to low battery power.

The cell size is also limited by signal trip time. For example, in GSM the equalizer in the receiver can cope with delays up to 233 µs resulting in a maximum distance of 70 km. On the other hand, in Frequency Division Multiple Access (FDMA) networks, only a limited number of channels/frequencies is available, forcing to reuse them due to capacity reasons. If the frequencies are assigned properly to the cells they can be reused more often in the whole service area increasing the number of users that can be serviced. The frequency assignment is based on the calculation of interference from cells using the same frequency. Those cells have to have a certain distance not to interfere with each other.

How often the frequency can be reused in the service area increases with forming smaller cells, thus decreasing interference from cells with the same frequency or by improving system components in a way that they can deliver an acceptable quality of service although the distortions increase.

If cells are smaller this leads to an increased number of cell sites and costs. Possibilities to shorten the effect are suppression of signals during pauses of speech and sectorization of cells, in a way that every sector has one antenna only covering a certain angle around the cell site (e.g. 60° or 120°). In that way the mutual interference decreases and every single frequency can be reused more often (see below). The number of sites remains constant. If we also improve the coding of the transmitted signals (e.g. Enhanced Full Rate channels in GSM), a mobile network can keep connections even if larger distortions appear.

To avoid distortions caused by interference different cells within FDMA-networks (e.g. GSM) are assigned different frequencies. Because neighboring cells are assigned different frequencies, it is necessary for the mobile station (MS) to change its frequency when crossing a cell border. In addition, it needs - independent of the frequency assigned – to transfer the connection from the old to the new base station. If there is an ongoing connection this is called handover (cf. Chapter 7.2). The handovers increase the signaling traffic inside the network clearly, but they allow an area-wide coverage for millions of users.