Page 41 - From GMS to LTE

Global System for Mobile Communications (GSM) 27

Adjacent cells that
have to send on a different
frequency

Neighbor cells
that are further
away
Figure 1.20 Cellular structure of a GSM network.

Figure 1.21 Sectorized cell
configurations.
3 1
2 1
2

sites. Therefore, only a limited number of different frequencies can be used per base
station to increase capacity.
To increase the capacity of a base station, the coverage area is usually split into two or
three sectors, as shown in Figure 1.21, which are then covered on different frequencies
by a dedicated transmitter. This allows a better reuse of frequencies in two‐dimensional
space than is the case where only a single frequency is used for the whole base station.
Each sector of the base station, therefore, forms its own independent cell.

1.7.3 The GSM Air Interface
The transmission path between the BTS and the mobile device is referred to, in the
GSM specifications, as the air interface or the Um interface. To allow the base station to
communicate with several subscribers simultaneously, two methods are used. The first
method is frequency division multiple access (FDMA), which means that users com-
municate with the base station on different frequencies. The second method used is
time division multiple access (TDMA). GSM uses carrier frequencies with a bandwidth
of 200 kHz over which up to eight subscribers can communicate with the base station
simultaneously as shown in Figure 1.22.
Subscribers are time multiplexed by dividing the carrier into frames with durations of
4.615 milliseconds. Each frame contains eight physically independent timeslots, each
for communication with a different subscriber. The time frame of a timeslot is called a
burst and the burst duration is 577 microseconds. For example, if a mobile device is