Gsm & Cdma Technology In Mobile Telephony

GSM & CDMA Technology in Mobile Telephony

Signal Training School BSF

Tata Communications

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 TeleCommunication

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 radiotelephone 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 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 Permanent web connection at 2Mbps Internet, phone and media: 3 in 1 The standard based on GSM is called UMTS. Not yet implemented. • The EDGE standard is the development of GSM towards 3G.

But what’s cellular?

BS

PSTN

HLR, VLR, AC, EIR

MSC

ARCHITECTURE

MSC

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 ALLOCATION

Architecture of a GSM Network Mobile Station

ME

SIM

Base Station Subsystem

BTS

BSC

Network Subsystem

MSC/ VLR

Other Networks

GMSC

PSTN

PLMN

EIR SD

HLR

AUC

Internet

+

Note: Interfaces have been omitted for clarity purposes.

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 CDMA

GSM

IS-95

Global System for Mobile Communication

Standard

Code Division Multiple Access

Mobile Frequency

Rx: 869-894

Rx: 925-960

Range (MHz)

Tx: 824-849

Tx: 880-915

Method

CDMA/FDM

TDMA/FDM

Duplex Method

FDD

FDD

Number of Channels

20

124

Users Per Channels

15-50

8

Channel Spacing

1250 kHz

200 kHz

Multiple Access

GMSK (0.3 Gaussian Filter) Modulation

QPSK/OQPSK

8-PSK (EDGE only)

Channel Bit Rate

1.2288 Mb/s

270.833 kb/s

Data Rates Supported by Various Cellular Standards Cellular Family

Peak Data Rate Standard

(kbits/s)

Typical Data Rate

Connection

Modulation

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

FDD

384/2000

144

Packet Switched

QPSK

TDD

384/2000

144

Packet Switched

QPSK

IS-95A

14.4

14.4

Circuit Switched

QPSK

IS-95B

64/115

56

Packet Switched

QPSK

IX

144/307

130

Packet Switched

QPSK

CDMA2000

IX EV

2000

N/K

Packet Switched

QPSK

TDMA

CSD

9.6

9.6

Circuit Switched

DQπ/4PSK

PDC

i-mode

9.5

9.6

Packet Switched

DQπ/4PSK

GSM

UMTS

CDMAOne

Multiple Access Techniques Mu lt ip le Ac cess 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

4 CH

3 CH

2 CH

CH

1

Ti m

e

Power

• In Fr eq ue ncy Div isi on Mu lti pl e Access (FD MA) , 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.

Frequency

• 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

l4 ne

Ch an

Ch a

nn

el

3

l2 ne

an

Ch

Ch

an

ne

Frequency

l1

Ti m

e

Power

•

How GSM works ? • GSM works on the principle of FD MA + TDM A • 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 FDM A

Frequency Re-Use A 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 . Thi s is TD MA

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 Co de Divis ion 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 “Bre ak-Bef ore-M ake

•Hard Hands-Off

Break before Make

CDMA

Soft Hands-Off Make before Break

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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