GSM Communication System

1).Introduction:

GSM (Global System for mobile communication) is a term known world wide for the digital cellular communication. In 1982 this term was introduced by the CEPT. It created a group, Groupe Special Mobile(GSM), which was provided with the task to design a cellular system that can operate in the 900MHz frequency region. [1]

In the early 1980’s, most of the telephone systems were analog rather than digital. This had a lot of disadvantages because it was unable to handle the increasing number of customers in a cost effective manner. As a result the digital technology was introduced which would be helpful in increasing the capacity of handling the customers.

The GSM network was started in 1992, and in 1993 more than one million subscribers made calls through GSM networks. In the beginning, the GSM network was popular in Europe, but started to spread itself outside Europe. [1]

The majority of the GSM networks operate in the 900MHz band, but some operate in 1800MHz band too like (GSM1800) PCN, (DCS1800), and in the United States 1900MHz band (GSM 1900). The basic architecture of these systems is identical but they differ only in the radio frequencies used. [1]

There are about 380 million subscribers of the GSM technology in all the three frequency bands(900 MHz, 1800 MHz, 1900 MHz). In total there are 373 networks in 142 countries in operation.

The GSM technology has been a revelation as it has made wireless communication easily accessible.

2).The GSM network:

The GSM network is divided into three different parts: [2]

• The switching system(SS)

• The base station system(BSS)

• The operation and support system(OSS)

2.1).The switching system:

The switching system is responsible for performing call processing and subscriber related functions. [2]

2.2).The base station system:

It is responsible for performing all radio related functions. It consist of base station controller(BSC) and base transceiver station(BTS). [2]

2.2.1).BSC:

The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSC are served by an MSC. [2]

2.2.2).BTS:

The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTS are controlled by a BSC. [2]

2.3).The operation and support system:

The operations and maintenance centre (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional part from which the network operator and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations. [2]

3).Multiple accessing techniques:

There are different major multiple accessing techniques in GSM, but we will describe just two of them:

3.1). Frequency division multiple access (FDMA):

FDMA is one of the most widely used multiple access techniques. In this technique, the frequency band is divided in to channels of equal bandwidth so that the conversation is carried on different frequencies. In FDMA, one specific frequency is allotted to every caller. This cause overload situations in case of high demands. The channel bandwidth is narrow usually 30KHz. [2]

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FDMA systems include C-Netz. In C-Netz, two frequency bands of 4.44MHz are further divided into 222 individual communication channels at 20 KHz bandwidth. The hardware components in FDMA are simple, but still each channel needs its own transceiving unit. Also, the tolerance required for the high-frequency network and the linearity of the amplifier in the transmitter stage of the base station are high. It is because a large number of channels need to be amplified and transmitted together. [1]

3.1.1). Advantages: [4]

• No network timing is required.

• Channel assignment is simple.

3.1.2). Disadvantages: [4]

• Power levels must be closely checked to obtain efficient use of transponder RF output power.

• Inter-modulation difficulties require power back-off as the number of RF carrier increase.

3.2). Time division multiple access (TDMA):

Time Division Multiple Access or TDMA is a method used to enable multiple stations to transmit on the same frequency but on different time slots. TDMA systems alleviated the channel capacity issue by dividing a single radio channel into timeslots and then allocating a timeslot to a user. TDMA system have three timeslots per channel. To use these timeslots, the analog voice has to be converted to digital. [3]

A digital wireless transmission technology, TDMA is used by the digital system Global System for Moblile Communications as a cellular phone technology to fit more cellular users, in sequence, on a single radio without interference from other callers. This form of technology is called multiplexing. A call is placed and sounds are digitized and then the information is broken into data packets which given a unique time assignment. TDMA systems operate in either the 800-MHz or 1900-MHz frequency bands.

3.2.1).Advantages: [4]

• Shares single carrier frequency with multiple users.

• Slots can be assigned on demand.

• Less stringent power control due to reduced inter-user interference.

3.2.2).Disadvantages: [4]

• Higher synchronization overhead.

• Equalization necessary for high data rates.

• Slot allocation complexity.

• Interference with other devices.

4). Summary:

In this report we have tried to cover the basic principles of GSM technology. We explained the basic architecture of GSM along with the multiple accessing techniques involved. We also tried to discuss the advantages and disadvantages of each accessing technique.