What Is Gsm

Overview

The GSM system utilises new technologies efficiently. A collection of innovative technological features demonstrates the engineering force behind the system. The use of certain technological features is motivated by the benefit that is offered. The variety of the level of technologies is wide; from small details hidden deep in the system to revolutionary new concepts at the top level of the system, visible to all users.

The world is becoming digitalised. Analogue systems in many fields of technology are being replaced by a modern digital alternative. One of the main improvements when compared to the first generation cellular systems is that GSM is a digital system. We shall take a closer look in this module what it means that the system is digital, and what benefits it offers.

A key feature in any cellular mobile network is that several users can communicate without disturbing each other. As a matter of fact, the frequency band that is allocated for mobile communications is always limited and a lot of effort is spent to develop systems that can serve as many users as possible in a given band. GSM uses a combination of two distinct methods to achieve these tight requirements.

Digitalisation

GSM is a digital system. Let's take a closer look at what it means, if a signal is analogue or digital.

An analogue signal means in principle, that the signal can have any values at any time. A good example is a traditional thermometer outside your window. The range of the meter is typically from -40 Celsius to +40 Celsius and the reading can be anything between these values. In theory, if we had eyesight well enough, we could distinguish arbitrary small differences in the temperature. If the reading of the thermometer were converted to an electrical form, such as voltage, we would have an analogue signal.

In a mobile communications network, part of the transmission connection uses a radio link and another part uses 2Mbit/s PCM links. Radio transmission is used between the Mobile Station (MS) and the Base Transceiver Station (BTS) and the information must be adapted to be carried over 2Mbit/s PCM transmission links throughout the remainder of the network.

In a mobile communications network, part of the transmission connection uses a radio link and another part uses 2Mbit/s PCM links. Radio transmission is used between the Mobile Station (MS) and the Base Transceiver Station (BTS) and the information must be adapted to be carried over 2Mbit/s PCM transmission links throughout the remainder of the network.

The radio link is the most vulnerable part of the connection and a great deal of work is needed to ensure its high quality and reliable operation. This issue will be discussed in other modules.

The frequency ranges of GSM 900 and GSM 1800 are indicated below:

Note that the uplink refers to a signal flow from mobile station to Base Transceiver Station (BTS) and the downlink refers to a signal flow from Base Transceiver Station (BTS) to mobile station. The use of separate uplink and downlink frequencies enables simultaneous communication in both directions. The radio carrier frequencies are arranged in pairs and the difference between any pair of frequencies (uplink-downlink) is called the Duplex Frequency.

The frequency bands utilised in GSM are centered on carrier frequencies spaced 200 kHz apart. As an example, the following table shows the allocation of frequencies in GSM 900:

Channel Uplink signal (MHz) Downlink signal (MHz)

1 890.1 - 890.3

(890.2 -centre freq.) 935.1 - 935.3

(935.2 -centre freq.)

2 890.4 (centre freq.) 935.4 (centre freq.)

3 890.6 (centre freq.) 935.6 (centre freq.)

... ... ...

124 914.8 (centre freq.) 959.8 (centre freq.

In GSM 900 the duplex frequency (the difference between uplink and downlink frequencies)