Base Station And Cell; Radio Channels - Ericsson EF738 Service Manual

System Introduction

Base Station and Cell

A base station is able to communicate with any mobile telephone within a certain
area close to it. This area is called a cell.
The base station contains channel units. Each channel unit is equipped with a radio
transmitter, a radio receiver, and a control unit. The control unit is used for data
communication with the MSC and for data signalling with the mobile telephones on
the radio path. A channel uses two separate radio frequencies, one for transmission
by the mobile telephone and one for transmission by the base station. Such a chan-
nel is called a duplex channel. The distance between these two frequencies, the
duplex distance, is always the same and is 45 MHz.
All base stations within a service area are connected to the MSC via dedicated lines
in the public telephone network.

Radio Channels

Two different types of radio channels exist in a base station:
• voice channels (VC)
• control channels (CC)
The majority of channel units are voice channel units. Such a voice channel unit is
engaged in carrying one telephone call at a time. Depending on how many simulta-
neous calls a base station is required to handle, the number of
voice-channel units in some base stations may be only a few, while in others up to a
hundred or more.
There is normally only one control channel in each cell. A mobile telephone being
within a cell, and not in the conversation state, is always tuned to the control chan-
nel of this cell. The telephone supervises the continuous data stream.
The total number of channels available in a mobile telephone system is limited. As
the radio waves only reach so far from the base station, however, identical channel
frequencies can be used by several base stations, provided the distance between
them is sufficient to prevent interference. This method is used to increase the capac-
ity of the system.
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Figure 2: Channel Unit