Tata Communications

GSM & CDMA Technology in Mobile Telephony

Signal Training SchoolBSF

What is Tele-Communication ?

• Tele means DISTANCE and Communication means INFORMATION TRANSFER• So transfer of information between two or more entities which may be far apart is called as Tele-Communication

Types of Communication tech.

• Wireline• Wireless

Communication Path• Wireline

– Physical layout of wires or optical fiber cables between entities . A physical contact is essential between the peer entities for communication

• Wireless– NO Physical connectivity is required . Communication is

done through radio links ( Electro Magnetic Waves ) and repeaters ( CELL SITES) are placed in between peer entities for efficient communication

What does one expects from

Wireless ?

• Lower cost • Roaming• Better service and coverage• NO dropped calls • Enhanced Privacy• NO Speech Clipping• NO Echo

Wireless• It is type of communication technology where

electromagnetic waves carry the signal (voice and data) on whole or part of communication path.

• Wireless telecommunication involves converting an audio signal into a Frequency (RF) signal and broadcasting it using radiating devices called antennas.

It all started like this• First telephone (photophone) –

Alexander Bell, 1880

• The first car mounted radio

telephone – 1921

Going further• 1946 – First commercial mobile radio-

telephone service by Bell and AT&T in Saint Louis, USA. Half duplex(PTT)

• 1973 – First handheld cellular phone – Motorola.

• First cellular net

Bahrein 1978

Generation Gap

• Generation #1 – Analog [routines for sending voice]

• All systems are incompatible

• No international roaming

• Little capacity – cannot accommodate masses of subscribers

Generation Gap(2)

• Generation #2 – digital [voice encoding]

• Increased capacity

• More security

• Compatibility

• Can use TDMA or CDMA for increasing capacity

History of GSM - I– 1982 Group Spéciale Mobile formed (origin of

term GSM)– 1897 Initial Memorandum of Understanding

(MoU) signed by network operators representing 12 countries

• coordinate introduction of GSM• agree time scales• plan the introduction of services• coordinate routing, billing and tariffs

– 1988 - Validation & trials

History of GSM - II– 1989 - European Telecommunications

Standard Institute (ETSI) formed– 1991 - Launch delayed due to lack of

mobiles– 1992 - Officially launched– 1993 - Commercial services start outside

Europe– 2003 - Over 200 countries– 2004 - Over 1 billion subscribers?– Ref: http://www.gsmworld.com

Motivations for GSM

• Political Considerations– Impending EU unification– Deregulation of mobile telephony

• Economic Considerations– single market– cost benefits through economies of scale– potential for export

Back to GenerationsBack to Generations

• Generation #2.5 – packet-switching

• Connection to the internet is paid by packets and not by connection time.

• Connection to internet is cheaper and faster [up to 56KBps]

• The service name is GPRS – General Packet Radio Services

The future is now

• Generation #3 Generation #3

• Permanent web connection at 2Mbps Permanent web connection at 2Mbps

• Internet, phone and media: 3 in 1Internet, phone and media: 3 in 1

• The standard based on GSM is called The standard based on GSM is called UMTS. Not yet implemented.UMTS. Not yet implemented.

• The EDGE standard is the development The EDGE standard is the development of GSM towards 3G. of GSM towards 3G.

But what’s cellular?

HLR, VLR, AC, EIR

MSC

PSTN

BS

MSC

ARCHITECTURE

CELL

• CELL is the geographical area covered by a cell site .

• All the receivers (handsets) in this particular area respond to this cell site i.e. they receives and transmits signal to a particular cell only

Cellular principles

• Frequency reuse – same frequency in

many cell sites

• Cellular expansion – easy to add new cells

• Handover – moving between cells

• Roaming between networks

CELL STRUCTURE AND FREQUENCY CELL STRUCTURE AND FREQUENCY ALLOCATIONALLOCATION

Architecture of a GSM Network

SD

Mobile Station

BTS

MSC/VLR

SIM MEBSC

Base StationSubsystem

GMSC

Network Subsystem

AUCEIR HLR

Other Networks

Note: Interfaces have been omitted for clarity purposes.

+

PSTN

PLMN

Internet

Mobile Station (MS)• Mobile Equipment

– Fixed– Portable

• International Mobile Equipment Identity (IMEI) number

• Subscriber Identity Module (SIM)• Personal Identification Number (PIN)• International Mobile Subscriber Identity (IMSI)

number• Enables access to subscribed services• Smart card

Base Transceiver Station - BTS– Usually referred to as the Base Station– Provides the interface to the network for the

MS– Handles all communications with the MS– Less “intelligent” than analogue equivalent– “intelligence” now deployed on MS

• for example, when to perform a handover

– Transmitting power determines cell size

Base Station Controller - BSC– Controls Base Stations

• up to several hundred depending on manufacturer

– Manages radio channels• allocation and release

– Coordinates Handover– Physical location may vary– Abis interface

• between BSC and BTS

Network SubSystem(NSS)• Nerve Centre of entire GSM network• Manages all

• call processing• subscriber related functions

• Contains – the core switching component– a number of databases– gateways to other networks

• Uses Signalling System Number 7 (SS7)

Mobile Switching Centre (MSC)– Performs all switching/exchange functions– Handles

– registration– authentication– location updating

– A GSM network must have at least one MSC– May connect to other networks

• Gateway MSC (GMSC)

Home Location Register (HLR)– Administrative information for all subscribers

• IMSI number• actual phone number• permitted supplementary services• current location i.e. which VLR subscriber is

currently registered with• parameters for authentication and ciphering

– One HLR per GSM PLMN

Visitor Location Register (VLR)• Contains data on all MSs currently in the

area served by the MSC– permanent data (identical to that in HLR)– Consulted during

• call establishment• caller authentication

– Usually integrated with MSC so that geographic area covered by both coincides• signaling requirements simplified considerably

Equipment Identity Register (EIR)• Maintains lists of IMEI numbers of all valid

and invalid equipment for the network– IMEI - International Mobile Equipment Identity

• An IMEI may be invalid if– stolen– not approved for use on the network, possibly

due to some defect

• EIR consulted during registration/call setup

Authentication Centre (AUC)– Protected database– Stores all algorithms used for authentication

purposes– Knows which one has been issued to the

subscriber (stored on SIM card)– provides HLR or VLR with parameters for

completing authentication

Other Network Components

• Operations & Maintenance Centre

• Intelligent Networking

• Billing Centre

• SMS Gateway

Techniques Used in Wireless

•Global System of Mobile Communication (GSM)

• Wireless in Local Loop (WLL)

Wireless Communications Specifications

Digital Cellular Telephones

Standard

CDMA GSM

IS-95Global System for Mobile

Communication

Code Division Multiple Access  

Mobile Frequency Rx: 869-894 Rx: 925-960

Range (MHz) Tx: 824-849 Tx: 880-915

Multiple Access

CDMA/FDM TDMA/FDMMethod

Duplex Method FDD FDD

Number of Channels 20  124

Users Per Channels 15-50 8

Channel Spacing 1250 kHz 200 kHz

Modulation QPSK/OQPSK

GMSK

(0.3 Gaussian Filter)

8-PSK (EDGE only)

Channel Bit Rate 1.2288 Mb/s 270.833 kb/s

Cellular

Standard

Peak Data Rate

Typical Data Rate Connection ModulationFamily (kbits/s)

GSM

GSM-CSD 9.6/14.4 9.6 Circuit Switched GMSK

HS-CSD 28.8/43/2 28.8 Circuit Switched GMSK

GPRS 115/171 50 Packet Switched GMSK

EDGE 384/513 115 Packet Switched 8-PSK

UMTS

FDD 384/2000 144 Packet Switched QPSK

TDD 384/2000 144 Packet Switched QPSK

CDMAOne

IS-95A 14.4 14.4 Circuit Switched QPSK

IS-95B 64/115 56 Packet Switched QPSK

CDMA2000

IX 144/307 130 Packet Switched QPSK

IX EV 2000 N/K Packet Switched QPSK

TDMA CSD 9.6 9.6 Circuit Switched DQ/4PSK

PDC i-mode 9.5 9.6 Packet Switched DQ/4PSK

Data Rates Supported by Various Cellular Standards

Multiple Access Techniques

Multiple Access means the same frequency range i.e. bandwidth is used

by all the users

But there is no interference between the

users because each user is is talking to its respective counterpart in a unique code assigned to it which is different

from the other users

• In Frequency Division Multiple Access (FDMA), the frequency band is divided in slots. Each user gets one frequency slot assigned that is used at will.

• It could be compared to AM or FM broadcasting radio where each station has a frequency assigned.

CH 3

CH 4 Frequency

Time

Po

we

r

CH 1

CH 2

• In Time Division Multiple Access (TDMA), the frequency band is not partitioned but users are allowed to use it only in predefined intervals of time, one at a time.

• Each caller is assigned a specific time slot for transmission• Many conversations are multiplexed into a single channel

• Cha

nnel

1Cha

nnel

2Cha

nnel

3Cha

nnel

4Frequency

Time

Po

we

r

How GSM works ?

• GSM works on the principle of FDMA + TDMA

• This means the bandwidth allotted is firstly divided accordingly cell i.e. each cell region works on different frequency allotted to it and each cell frequency is different from the adjacent cells.

This is FDMA

Frequency Re-UseFrequency Re-UseA frequency (channel) can be used again within an FDMA or TDMA network, but cells using the same frequency must be separated by an appropriate distance. Adjacent cells must be assigned a different set of frequencies. For example, a cell using frequency A must not be adjacent to another cell using frequency A.

So each cell works at a particular frequency and now this frequency is divided in time slots i.e. each user is given a specified time in which data of that respective user is transferred . This is TDMA

Wireless in Local Loop

• WLL is also a standard evolved and used in USA,It is emerging because of its advantages over GSM.

• WLL uses CDMA as communication technique

CDMA

• Code Division Multiple Access

• All users share the same frequency all the time

• To pick out the signal of specific user, this signal is modulated with a unique code sequence.

CDMA CODE DIVISION MULTIPLE ACCESS

Code Division means the voice analog signal is converted into its corresponding digital signal but this digital signal is accordingly a unique code assigned to that user.

SPREAD SPECTRUM TECHNIQUE

CDMA works on the principle SPREAD SPECTRUM Technique, which means that it spreads the information contained in a particular signal of interest over a much greater bandwidth than the original signal i.e. large range of frequencies are used for information transfer

Due to Spread Spectrum Technique

• Capacity increases of 8-10 times that of gsm system

• Improved call quality,with better and more consistent sound .

• Simplified system planning through the use of same frequency in every sector of cell

• Enhanced privacy• Improved coverage

Cell Structure & Frequency allocation of CDMA

Each CELL-SITE in a CDMA network can use all available frequencies. Adjacent Cells can transmit at the same frequency because users are separated by Code Channels

GSM Vs CDMA

GSM• Works on TDMA +FDMA

• Different cell works on different frequencies

• Due to change in frequency from one cell to another complex hand off

• Due to Complex procedure Call interference is more likely

WLL• Works on CDMA with

Spread Spectrum Technique

• Universal frequency use

• Soft Handoff tech leads to lower call drops

• Rake receiver technology leads low call interference and .

Hands-Off

Handoff is the process of transferring a call from one cell to another. This is necessary to continue the call as the phone travels.

Hands-Off in GSM

TDMA and FDMA systems use a hard handoff when the mobile is moving from one cell site to another. These technologies do not allow for any type of make-before-break handoff. A hard handoff can increase the likelihood of a dropped call. A hard handoff requires the mobile to break the connection with the old BTS prior to making the connection with the new one. Hard handoffs are also called “Break-Before-Make

•Hard Hands-OffBreak before

Make

Soft Hands-OffMake before Break

CDMA

Call Interference leads to Disturbance , Low Speech, Echo

Signals sent over the air can take a direct path to the receiver, or they can bounce off objects and then travel to the receiver. These different paths, called multi-paths, can result in the receiver getting several versions of the same signal but at slightly different times. Multi-paths can cause a loss of signal through cancellation in other technologies

GSM

When a signal travels to a handset through different paths, in GSM handset, it will pick the first signal reaching to it, no matter how weak that signal is. Thus leading to Call Disturbance, Low Speech and some times Echo

Rake Receiver in CDMA

• CDMA's rake receiver is multiple receivers in one. The rake receiver identifies the three strongest multi-path signals and combines them to produce one very strong signal. The rake receiver therefore uses multi path to reduce the power the transmitter must send.

• Both the MOBILE and the CELL SITES use rake receivers

Power Adjustments in GSM

• If all mobiles transmitted at the same power level, the base station would receive unnecessarily strong signals from mobiles nearby and extremely weak signals from mobiles that are far away. This would reduce the capacity of the system.

• This problem is called the NEAR-FAR problem

Power Adjustment in CDMA

• Power control is a CDMA feature that enables mobiles to adjust the power at which they transmit. This ensures that the base station receives all signals at the appropriate power. The CDMA network independently controls the power at which each mobile transmits.

• Both forward and reverse links use power control techniques.

ADVANTAGES

• CDMA capacity is ten to twenty times that of analog systems, and it's up to four times that of TDMA.

• CDMA's universal frequency reuse

• CDMA users are separated by code

• CDMA's soft handoff leads to low or no call drops

• The Rake Receiver leads to better voice quality

• NO INTERFERENCE

• Power control keeps power at an optimal level.

• Power control helps the network dynamically expand the coverage area.

• CDMA's wide band signal reduces fading.

CDMA technology has numerous advantages including

• Coverage • Capacity • Clarity • Cost • Compatibility

• Coding provide the ability to cover more users for the same amount of available power used in other systems.

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