The GSM radio interface

The radio interface [13] is the interface between the mobile stations and the fixed infrastructure. It is one of the most important interfaces of the GSM system.The Radio Interface (Um) is split into several channels: traffic channels and signaling channels. The traffic channels carry user data. The signaling channels carry management and control information.

Frequency allocation:

Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:

The band 890-915 Mhz has been allocated for the uplink direction (transmitting from the mobile station to the base station)FDMA and TDMA

The band 935-960 Mhz has been allocated for the downlink direction (transmitting from the base station to the mobile station).

Access Scheme :

The multiple access scheme defines how different simultaneous communications, among different mobile stations located in some different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM.

Using FDMA, a frequency is assigned to a user. So, the larger the number of users in a FDMA system, the larger the number of available frequencies must be.

The limited available radio spectrum and the fact that a user will not free its assigned frequency until he does not need it longer, explain why the number of users in a FDMA system can be "quickly" limited.

On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing the common channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA is used with a FDMA structure.

In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 125 carrier frequencies spaced one from each other by a 200 khz frequency band.

Normally a 25 Mhz frequency band can provide 125 carrier frequencies. Each carrier frequency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in a TDMA system. A TDMA frame is formed with 8 bursts. Each of the eight bursts, that form a TDMA frame, is then assigned to a single user.

Structure:

A channel corresponds to the recurrence of one burst every frame. It is defined by its frequency and the position of its corresponding burst within a TDMA frame. In GSM, there are two types of channels

• The traffic channels used to transport speech and data information.
• The control channels used for management messages and some channel maintenance tasks.