Page 42 - From GMS to LTE

28 From GSM to LTE-Advanced Pro and 5G

Burst, 577s

01234567012 34567 012

TDMA frame, 4.615 ms
Figure 1.22 A GSM TDMA frame.

allocated timeslot number 2 for a voice call, then the mobile device will send and receive
only during this burst. Afterward, it has to wait until the next frame before it is allowed
to send again.
By combining the two multiple access schemes it is possible to approximately
calculate the total capacity of a base station. For the following example, it is assumed
that the base station is split into three sectors and each sector is covered by an inde-
pendent cell. Each cell is typically equipped with three transmitters and receivers
(transceivers). In each sector, 3 × 8 = 24 timeslots are thus available. Two timeslots
are usually assigned for signaling purposes, which leaves 22 timeslots per sector for
user channels. Let us further assume that four or more timeslots are used for the
packet‐switched GPRS service (see Chapter 2). Therefore, 18 timeslots are left for
voice calls per sector, which amounts to 54 channels for all sectors of the base station.
In other words, this means that 54 subscribers per base station can communicate
simultaneously.
A single BTS, however, provides service to a much higher number of subscribers, as
all of them do not communicate at the same time. Mobile operators, therefore, base
their network dimensioning on a theoretical call profile model in which the number of
minutes per hour that a subscriber statistically uses the system is one of the most
important parameters. A commonly used value for the number of minutes per hour
that a subscriber uses the system is 3. This means that a base station is able to provide
service to 20 times the number of active subscribers. In this example, a base station with
54 channels is, therefore, able to provide service to about 1080 subscribers.
This number is quite realistic as the following calculation shows: Telefonica O2
Germany had a subscriber base of about 20 million in 2014 [20]. If this value is divided
by the number of subscribers per cell, the total number of base stations required to
serve such a large subscriber base can be determined. With our estimation above, the
number of base stations required for the network would be about 18,500. This value is
in line with the numbers published by the operator [20].
Each burst of a TDMA frame is divided into a number of different sections as shown
in Figure 1.23. Each burst ends with a guard time in which no data is sent. This is neces-
sary because the distance of the different subscribers from the base station can change
while they are active. As airwaves propagate ‘only’ through space at the speed of light, the
signal of a faraway subscriber takes a longer time to reach the base station compared to
that of a subscriber who is closer to the base station. To prevent any overlap, guard times
were introduced. These parts of the burst are very short, as the network actively controls
the timing advance of the mobile device. More about this topic can be found below.
The training sequence in the middle of the burst always contains the same bit pattern.
It is used to compensate for interference caused, for example, by reflection, absorption
and multipath propagation. On the receiver side, these effects are countered by