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Introduction of GSM Radio Transmission

Introduction of GSM Radio Transmission: Wireless topologies are essentially identical to the same traditional cabled network topologies. Only the transmission media is different. 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 and the Base Transceiver Station and the information must to be adapted to be carried over 2Mbit/s PCM transmission through 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.
Introduction of GSM Radio Transmission
GSM Radio Transmission

Frequency Ranges of GSM
Frequency range allocation for GSM network
Frequency Range for GSM

Uplink refers to a signal flow from Mobile Station (MS) to Base Transceiver Station (BTS) and the downlink refers to a signal flow from Base Transceiver Station (BTS) to Mobile Station (MS). The simultaneous use of separate uplink and downlink frequencies enables communication in both the transmit (TX) and the receive (RX) directions. The radio carrier frequencies are arranged in pairs and the difference between these two frequencies (uplinkdownlink) is called the Duplex Frequency.

The frequency ranges are divided into carrier frequencies spaced at 200kHz. In GSM 900 the duplex frequency (the difference between uplink and downlink frequencies) is 45 MHz.In GSM 1800 it is 95 MHz. The lowest and highest channels are not used to avoid interference with services using neighbouring frequencies, both in GSM 900 and GSM 1800.The total number of carriers in GSM 900 is 124, whereas in GSM 1800 the number of carriers is 374.

The devices in the Base Transceiver Station (BTS) that transmit and receive the radio signals in each of the GSM channels (uplink and downlink together) are known as Transceivers (TRX).The radio transmission in GSM networks is based on digital technology. Digital transmission in GSM is implemented using two methods known as Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). In short I would like to write about the FDMA and TDMA.

Frequency Division Multiple Access (FDMA)
FDM is a technique of deriving two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum
to each of the individual channels.FDMA is the use of frequency division to provide multiple and simultaneous transmissions to a single transponder.

Frequency Division Multiple Access (FDMA) refers to the fact that each Base Transceiver Station is allocated different radio frequency channels. Mobile phones in adjacent cells (or in the same cell) can operate at the same time but are separated according to frequency. The
FDMA method is employed by using multiple carrier frequencies, 124 in GSM 900 and 374 in GSM 1800.

Time Division Multiple Access (TDMA)

TDMA is a communications technique that uses a common channel (multipoint of broadcast) for communications among multiple users by allocating unique time slots to
different users. TDMA is used extensively in satellite systems, LANs, physical security
systems, and combat-net radio systems.GSM became the global system for mobile communications. It uses TDMA with the addition of frequency hopping.

TDM is digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum (i.e., bit stream) by interleaving pulses representing bits from different channels.

Time Division Multiple Access (TDMA), as the name suggests, is a method of sharing a resource (in this case a radio frequency) between multiple users, by allocating a specific time (known as a time slot) for each user. This is in contrast to the analogue mobile systems where one radio frequency is used by a single user for the duration of the conversation.

In Time Division Multiple Access (TDMA) systems each user either receives or transmits bursts of information only in the allocated time slot. These time slots are allocated for speech only when a user has set up the call however, some timeslots are used to provide signalling and location updates etc. between calls.

Time Division Multiple Access (TDMA) divides one radio frequency channel into consecutive periods of time, each one called a "TDMA Frame".The TDMA timeslots are called "Physical Channels"

The radio carrier signal between the Mobile Station and the BTS is divided into a continuous stream of timeslots which in turn are transmitted in a continuous stream of TDMA frames.
Subscriber switches on his mobile phone and receives a call. This simple act of switching on the phone involves the following steps:

➥The mobile scans all the radio frequencies and measures them.
➥It selects the frequency with the best quality and tunes to it. With the help of a synchronisation signal in a TDMA frame, the mobile synchronises itself to the network.

The synchronisation information required by this process is broadcast by the network and analysed by the mobile.Registration and authentication are the next steps and these consist of the following operations: 

➥A point to point connection must be set up. The mobile station makes a request for a channel to establish the connection.
➥The network acknowledges the request and allocates a channel. The mobile receives and reads this information.
➥The mobile then moves to the allocated (dedicated) channel for further transactions with the network. The next steps are registration and authentication.

This is a simplified description of the process, but it conveys the idea that there are many functions involved in the air interface to enable a mobile user to have conversation. Each one of these functions requires a separate "logical channel" as the data contents are different. Some of them are Uplink , others are Downlink and some are Bi-directional.

GSM Modulation Process
GSM uses digital techniques where the speech and control information are represented by 0s and 1s. How is it possible to transmit digital information over an analogue radio interface?

The digital values 0 and 1 are used to change one of t he characteristics of an analogue radio signal in a predetermined way. By altering the characteristic of a radio signal for every bit in the digital signal, we can "translate" an analogue signal into a bit stream in the frequency domain.

This technique is called modulation. Analogue signals have three basic properties: Amplitude, Frequency, Phase. Therefore, there are basically three types of modulation process in common use:-

➥Amplitude Modulation: Amplitude modulation is the process in which the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal.
➥Frequency Modulation: Frequency modulation is the process in which the frequency of a carrier wave is varied in accordance with some characteristic of the modulating signal.
➥Phase Modulation: Phase modulation is the process in which the phase of a carrier wave is varied in accordance with some characteristic of the modulating signal.

Frequency and Amplitude Modulation in GSM
Modulation Process in GSM

GSM uses a phase modulation technique over the air interface known as Gaussian Minimum Shift Keying (GMSK). At the GSM air interface, the bit rate is approximately 270Kbits/s. At this bit rate, the duration of one bit is 3.69 ms, i.e. the value of the bit requires 3.69 ms of transmission time. GMSK changes the phase of the analogue radio signal depending on whether the bit to be transmitted is a 0 or a 1.
GSM Phase Modulation for example
Phase Modulation

The radio air interface has to cope with many problems such as variable signal strength due to the presence of obstacles along the way, radio frequencies reflecting from buildings, mountains etc. with different relative time delays and interference from other radio sources. With such levels of interference, complex equalisation techniques are required with GMSK.

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