GSM Using TDMA - Seminar Report


Since the evolution of man he has always tried learn and gain as much as possible for himself and his community, to relate him self to the various cultures castes and creeds of mankind. This Quest for realizing and exploring has lead to inventive thinking and a constant probation and to relate to each other throughout the world they have fore bared the gap of communication to expunge one such a minuscule but major hurdle a common Global System for communications was founded communication on the move with a bit to bit elaboration was analyzed and the forth presentation bears the facts of a Global System For Mobile communication and on which basis it is employed along with the assets & environment   .            
Ø  1st Generation cellular mobiles phones used analog channels.
Ø  2nd Generation cellular mobiles use digital channels.

 1st generation analog cellular mobile communication contained of analog channels, each cell has bunch of different freq channels & each costumer is given a particular frequency channel.

 2nd generation digital cellular mobile communications constituted of digital channels, a number of costumers share a particular frequency channel dynamically through TDMA.

ü  Basic Cell Structure
Be it digital or cellular, the regions are always divided into hexagons and the MU is connected to BS as usual.

Ø  2nd Generation constitutes of GSM.
Ø  GSM is not a software but a standardization group.
It was established in 1982 to create common European mobile standard.
Cellular industry is the most rapidly increasing industry currently there are millions of cellular subscribers.

ü  What  does  G S M  work on ?
Ø  GSM works on TDMA (Time Division Multiple Access).
Ø  All stations transmit data on same frequency but their transmissions are separated in time.

 The TDMA system is designed for use in a range of environments and situations, from hand portable use in a downtown office to a mobile user traveling at high speed on the freeway

Ø  Time Division Multiple Access
Ø   TDMA has 2M channels of bandwidth  say x hertz.
Ø  Reverse channel->f, f+x, f+2x,……f +(M-1)x
Ø  Forward channel->f, f+x, f+2x,……f +(M-1)x + x
Ø  x = Bandwidth
Ø  f = center frequency of lowest frequency channel.
Ø  When connection is setup it is full duplex and the user has two frequencies allotted to him.
Ø  f , f +x.
Half of the frequency is for forward channel and half of it is for reverse channel.
Forward channel: The channel traversing from the base station to mobile unit.
Reverse channel: The channel traversing from the mobile unit to base station.
Hence we can say that the physical channel is subdivided into number of sub channels.

ü  Basic operations
Ø  To send voice traffic on GSM (TDMA) mobile we need to,
Ø  Perform analog to digital conversion,
Ø  Error detection and correction,
Ø  Channel access has to be done properly,
Eg :- PCM , delta modulation.
Since in TDMA, data is sent in the form of frames.
For proper access a memory location holds the information about the time slots allotted to each user.

ü  TDMA design considerations.
Ø  Number of logical channels.
Ø  Maximum cell radius.
Ø  Frequency.
Ø  Maximum vehicle speed ( Vm ).
Ø  Maximum coding delay.
Ø  Maximum delay spread ( Xm ).
Ø  Bandwidth.
Usually 835km, 900Mhz,Vm==250km/hr, 20 ms to avoid adding unduly delays Xm==10;this is the difference in propagation delay among different multi path signals arriving at the same antenna

Not to exceed 200khz,corresponding to 25khz per channel.

ü  Advantages of T D M A.
Ø  Easily adapted to voice and data traffic.
Ø  Has a data rate of expandable multiples of 64kbps (up to120Mbps).
Ø  Enables various services including multimedia and video conferencing.
Ø  No interference.
Ø  Extended battery life and talk time.
Ø  TDMA is the only technology that offers an efficient utilization of hierarchical cell structures ( HCS’s  ) offering Pico, micro, and Macro cells .
Þ      Various services like fax, voice band data, short message service (SMS).
Þ      Extended battery life since the mobile is only transmitting a portion of the time (from 1/3 to 1/10) of the time during conversations.
Þ      HCSs allow coverage for the system to be tailored to support specific traffic and service needs. By using this approach, system capacities of more than 40-times AMPS can be achieved in a cost-efficient way.
Ø  TDMA allows service compatibility with the use of dual mode handsets.
Ø  TDMA has increased resistance to multi path distortion.
Ø   TDMA allows handoff to/from an analog AMPS channel.
Ø  The most widely deployed digital technology at the present time is TDMA.

ü  TDMA  frame format
ü  GSM network

Ø  GSM provides recommendations, not requirements.
Ø  The GSM specifications define the functions and interface requirements in detail but do not address the hardware.
Þ      TDMA functions (that is; to provide connection between two remote MU) is the basic function of GSM and to achieve that we need physical and mechanical support proper routing, control and processing of data.
Þ      Not specifying hardware makes it easier on the designer so that he can buy his components according to his choice.

ü  GSM network elements.

Ø  The GSM network is divided into three major systems:
1)       The switching system (SS).
2)       The base station system (BSS).
3)       The operation and support system (OSS).
·         GSM inter working unit (GIWU)
1)       Gateway mobile services switching center (GMSC)
2)       Authentication center (AUC)
3)       Home location register (HLR)
4)       Visitor location register (VLR); mobile services switching center (MSC)
5)       Equipment identity register (EIR)
6)       Base Station Controllers (BSCs) and the base transceiver stations (BTSs).Message center (MXE)
Ø  The switching system (SS) :
Ø  It is responsible for performing call processing and subscriber-related functions.
Ø  It constitutes of

Ø  Home location Register (HLR):
Þ      The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
Ø   Mobile Services Switching Center (MSC):
Þ      The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
Ø  VLR Visitor Location Register:
Þ      The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Ø All radio-related functions are performed in the BSS (Base Station Service) it consists of

Ø   Authentication center (AUC):
A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

Ø  Equipment Identity Register (EIR):
The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

Ø  Base Station Controllers-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 BSCs are served by an MSC.

Ø  Base Transceiver Stations 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 BTSs are controlled by a BSC.
Ø  The Operation and Support System-OSS
Ø  The implementation of OMC is called the operation and support system (OSS).
Ø  MXE: Message center
Ø   MSN:  Mobile service node
Ø   GMSC: Gateway mobile services switching center
Ø  GIWU: GSM inter working unit

The operations and maintenance center (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 entity from which the network operator monitors 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.

ü  Advantages and disadvantages.
Ø  GSM carries all the advantages and disadvantages of TDMA.

Ø  The most prominent advantage can be stated as the application of TDMA through proper protocols and standards thus making it feasible.

Ø  Only disadvantage is that vacant time slots are encountered whenever a particular MU is idle but this we can overcome using synchronous TDMA.

ü  Personal communication services (PCS) for GSM

Frequency band: The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).
Þ      Duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies channel separation .The separation between adjacent carrier frequencies. In GSM, this is 200 KHz .
 A channel has two frequencies, 80 MHz apart.
Þ      Modulation: Gaussian minimum shift keying (GMSK).
Þ      Transmission rate— 270 kbps.
Þ      Access method— TDMA
Speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC
Modulation—Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).

Transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.

Access method—GSM utilizes the time division multiple access (TDMA) concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.

Speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.

ü  Facts about GSM

Ø  Right now, GSM is the accepted cellular standard in Europe; South America; North America; Southeast Asia etc.
Ø  Scandinavia first deployed cellular services for commercial use.
Ø  Call waiting is not supported by GSM.

This paper resolute the evolution of inventive thinking along with human requirements to meet the challenges of urban global communications.
The major disadvantage of TDMA is the non-availability of the call-waiting feature. Which can be overcome
by Asynchronous TDMA.
 The organization of asynchronous TDMA is done through artificial intelligent Single input Memory Modules commonly known as SIMM.

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