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Electronic Engineering

George Alexander

ELE 31EMT/EMC Engineering Management

Mobile Cellular Telephones – an Overview

Prepared by Dr Jean Armstrong with additional input from Michael Feramez

13 March, 2006

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Cellular Generations

1G Basic Mobility Basic Services Incompatibility

2G Advanced Mobility (Roaming) More Services (Data) Towards Global Solution

3G Seamless Roaming Service Concepts & Models Global Radio Access Global Solution

1980 1990 2000

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StandardisationThe uniform GSM standard in European countries has enabled globalisation of mobile communications.ITU had a dream to specify one common global radio interface technology.ITU harmonisation effort was done under the name FLPMTS (Future Public Land Mobile Telephony System) and later under IMT-2000.In 1999, ITU approved an industry standard for third-generation (3G) wireless networks.

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GPRS (2.5G)General Packet Radio ServiceEnables high-speed wireless internet and other data communicationsMore than four times capacity of conventional GSMPacket data service -> subscribers always connected and on line

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3G Key Requirements Services

Within IMT-2000, the ITU has defined the following key requirements for 3G services: improved system capacity (traffic handling), backward compatibility with second-

generation (2G) systems, multimedia support (higher data speed), and high speed packet data services as shown on

the next slide.

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High-Speed Packet Data Services

2 Mbps in fixed or in-building environments (very short distances, in the order of metres)384 kbps in pedestrian or urban environments144 kbps in wide area mobile environmentsVariable data rates in large geographic area systems (satellite)

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Mobile Cellular Telephones

Mobile phones use radio waves to transmit and receive voice signalsUseable electromagnetic spectrum is a limited resource with frequency allocations for broadcast television, radio, military applications etcmobile phones could only have widespread application with the idea of frequency reuse

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Mobile phone systems without frequency reuse

Mobile phone systems without frequency reuse had large high powered transmitters at the cell site mounted on high towers and covered a large area. Relatively few channels (<20 ) were availablethe frequencies were not reused nearby

cell site

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Cellular mobile phones

The frequencies F1 are reused in non adjacent cells

F1 F2

F1

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Frequency reuse schemesMany cellular systems are designed with the available channels divided into 7 groups For equally spaced cell sites in flat terrain this results in hexagonal shaped cellsin practice cell shapes depend on the terrain and the distribution of users

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Cochannel interference

A user may experience co-channel interference from users in other cells which have been allocated the same frequenciesFor the hexagonal pattern shown below each cell has 6 interfering cells distance 4.6R away and other more distant interfering cells

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Aspects of telephone systems

A cellular mobile phone systems has many of the same functions as the the wire based system (POTS) but these are often more complexIn addition it has a number of additional functions such as ‘handover’ which occurs when a user moves from one cell site to another

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What do we know about the POTS network?

CE customer equipment e.g. telephoneSN switching node e.g. telephone exchangeTL transmission link e.g. pair of copper wires from

customer to local exchange

CE

SNSN

SN SN

TLTL

TL

TL

TLCE

CE

TL

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Transmission

In POTS the transmission of voice signals is in analogue form along a pair of copper wires from the customer’s premises to the local exchangeMany methods of transmission are used between exchanges

analogue digital copper wire optical fibre microwave radio link

CE

SNSN

SN SN

TLTL

TL

TL

TLCE

CE

TL

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Transmission in mobile phone systems

transmission between cell site and mobile is by radio analogue or digital depending on system

transmission from cell site through the network may use optical fibre, copper wire, or microwave radio

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Radio Transmission in the mobile network

Mobile telephones have frequency allocations around 800 - 900MHz.The wavelength in free space at 900MHz is 0.33 metresMultipath effects cause fading as well as the direct signal from the cell site to

the antenna there may be a number of reflected signals

if the path lengths differ by half a wavelength they may cancel and a fade occurs

a mobile unit travelling at 24km/h in a fading environment will experience about 15 nulls per second. reflected as well as direct

signals may reach antenna

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Signalling:How does the network know which phones to connect?

When someone wishes to make a call they lift the telephone receiver which sends a signal to the exchange

CE

SNSN

SN SN

TLTL

TL

TL

TLCE

CE

TL

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Establishing a call between two phones on different local exchanges

The calling party is often called the A party and thecalled party the B party

CEA SN1 SN2 CEB

time

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signalling between the telephone and the local exchange is customer network signalling

transmitted along wire pair must be easy for telephone to generate

signalling between exchanges is network signalling

Message sequence diagram for telephone call

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Signalling in the mobile telephone network

Signalling in the mobile network is much more difficult the customers move a dedicated channel is not available between

each telephone and a fixed local exchange

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Switching in POTS

In response to the signalling sequence a connection is made between the calling (A) party and the called (B) party: this requires switching of the call

CE

SNSN

SN SN

TLTL

TL

TL

TLCE

CE

TL

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switching is based on switching matrices

Switching

Inlet 1 is connected to outlet 3

Inlet 2 is connected to outlet 1

1 2 3 . . . . . N

N Outlets

1

2...M

MInlets

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Multistage switching

Most switching nodes have a series of switching stages.

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Switching for the mobile network

switching is more complex switching as mobile moves from one cell to

another switching from cell site into the mobile

network switching from mobile network to POTS

network if required

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Busy hour traffic

it would be too expensive to design the telephone network to cope with every possible traffic loadnetworks are usually designed to give a certain probability of a call being blocked during the ‘busy hour’the usual design rule for the fixed telephone network is that there should be a probability of 0.02 of blocking of a call during busy hour

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Traffic in the mobile network

the traffic characteristics are different for the mobile network peak loads on arterial roads during the rush

hour peak loads in the city during the day

need to consider the probability of a telephone moving between cellssophisticated planning is required to achieve the best performancechannel allocations my be changed between cells so that resources are moved to t he cells which are busy at any particular time

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More about transmissionin the POTS

Connection from the telephone to the local exchange is two wire

transmission is baseband and is analogue: no modulation

the microphone generates a voltage across the two wires which is proportional to the sound input

Connections between exchanges - separate paths for transmission in each direction

used to be four wires - pair for each direction now could be coaxial cable, microwave radio link,

optical fibre etc, but distinct separate transmission channel for each direction

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Aspects of telephone system

transmissionsignallingswitchingtraffic

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Transmission: frequency range of voice signals

Human ear can hear frequencies in range 20-16000Hz approxMost of the energy is concentrated between 1KHz and 4KHzInternational standard for telephony: only frequencies in range 300Hz to 3400Hz transmitted

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Transmission: frequency division multiplexing

when there are a number of trunks and significant distance between exchanges, a number of voice signals are multiplexed onto one carrierthe speech signal is bandlimited to 300Hz to 3400Hz. This signal is used to modulate a carrier. Single sideband modulation is used

300 3400 Frequency

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Modulation in analogue mobile phones

analogue mobile phones use frequency modulationdifferent carrier frequencies are used for different mobile phones within the same cell

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Digital transmission in POTS

speech is transmitted in analogue form from handset to local exchange

usually at local exchange converted to digital form sampled 8000 times per second each sample 8 bit word resultant bit rate 64kbits/sec

digital signals quality does not depend on distance compatible with computers more easily switched can be multiplexed using time division multiplexing

analogue signal

samples taken 8000 times per secondeach sample is converted into an eightbit binary number

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Time Division Multiplexing

Many media, such as coaxial cable and optical fibre, have the capacity to carry much more information than one telephone call.In the past, frequency division multiplexing (FDM) was common. With FDM different carrier frequencies were used for different telephone channelswith time division multiplexing (TDM) different time slots are allocated to different calls

A B C D A B C D

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Multiplexing in the GSM system

The GSM digital telephone network uses a combination of FDM and TDMThe available bandwidth is divided up into 200kHz bandsEach carrier frequency supports one direction of transmission for up to eight simultaneous telephone callsThese eight calls share the frequency using time division multiplexing

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GSM network Components

MobileStation (MS)

BSC

BTS

BaseStationSystem(BSS)

Mobile SwitchingCentre(MSC)

PSTNand other

networks

BSCBase station controller

BTS Basestation transceiver

OnlyGatewayMSCs have connectionto other networks

PSTN - public switched telephone networkPLMN - public lands mobile network

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Relationship of network components to cells

Each cell has a BSSA number of BSS are connected to a MSC

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Allocation of radio spectrum:Frequency Division Multiplexing

The radio spectrum available for digital mobile phones is divided up into ‘channels’signals are modulated onto carriers which are spaced at 200kHzCertain frequencies are always allocated to uplinks (mobile to base station) and certain to downlinks (base station to mobile)The available frequencies may be allocated to different operators

frequency

uplink Buplink A downlink A downlink B

group of ‘channels’each of which is200kHz wide

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Time Division Multiplexing:physical channels

Each radio frequency carrier is modulated with a time division multiplexed signalThere are eight slots in a time division multiplexed frameone time slot of a TDMA frame on one carrier is a physical channel

TDMA Frame and contents when speechis being transmitted in a time slot

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Logical Channels

physical channels can be used for different types of logical channelsTraffic Channels (TCH) are used to carry encoded speech or user dataControl Channels are used to carry signalling and synchronization data broadcast control channels common control channels dedicated control channels

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Broadcast control Channels

Broadcast control channels are downlink and point-to-multipoint frequency correction channel

carries information to allow the MS to adjust the carrier frequency accurately

synchronization control channelcarries information for frame synchronization

broadcast control channelcarries general information relevant to that cell e.g. which frequencies are associated with this cell

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Common Control Channels

used to convey signalling information shared by all the mobiles in the cellpoint-to-point

Paging channelused to page the MS, downlink, point-to-point

random access channelused by MS to request allocation of a SDCCH, uplink, point-to-point

access grant channelused to allocate an SDCCH, downlink, point-to-point

SDCCH = stand-alone dedicated control channel

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Dedicated Control Channels

point-to-point, dedicated to signalling associated with one mobile

stand-alone dedicated control channelused for system signalling during a call set-up and before a traffic channel has been allocated. Up/downlink

slow associated control channelcarries information such as measurement reports from the mobile about received signal strengths from adjacent cells. Is carried in control slots of multiframe. up/downlink

fast associated control channelsteals slots from voice or data transmission. Used for example during handover

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Additional TopicsBroadbandADSLEDGEInternational telephone callsDECTCable ModemsATMBluetoothVOIPWLAN

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Broadband“Broadband communication consists of the technologies and equipment required to deliver packet-based voice, video and data services to end users”International Engineering Consortium

This provides much faster speeds than dial-up connections (max 56kbps) with the additional benefit of not tying up a phone line.

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ADSLAsymmetric Digital Subscriber Line“A modem technology that converts existing twisted- pair telephone lines into access paths for high-speed communication …” International Engineering Consortium

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EDGE - Enhanced Data-rates for Global Communication

Evolutionary path to 3G services for GSM and TDMA operatorsBuilds on General Packet Radio Service (GPRS) air interface and networksPhase 1 (Release’99 & 2002 deployment) supports best effort packet data at speeds up to about 384 kbpsPhase 2 (Release’2000 & 2003 deployment) will add Voice over IP capability

Universal Wireless Communications Consortium

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DECTDigital Enhanced Cordless Communication“A world-wide standard for short-range cordless mobility” – ETSIApplications such as domestic cordless phones.Cordless PABXs

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Cable Modems“Cable modems are devices that allow high-speed access to the internet via a cable television network”.International Engineering Consortium

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ATMAsynchronous Transfer Mode“A high-performance, cell-orientated switching and multiplexing technology that utilises fixed-length packets to carry different types of traffic”.International Engineering Consortium

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BluetoothShort range radio technologyEnables transmission of signals over short distances between telephones, computers and other devices.Eliminates the need for wires/cables.It is a global standard developed jointly by major telecommunications suppliers Intel, Nokia, Ericsson, Toshiba, IBM

Ref. http://www.ericsson.com/technology/

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VOIPUses internet to transmit voiceIs gradually replacing the traditional telephone network for transmitting voice.Some initial quality problems are being addressed

Ref. http://www.budde.com.au/

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WLANWireless Local Area NetworkComplements access technologies for cellular networksHigh data rates – up to 54MbpsUsed in indoor ‘hotspots’

Ref. http://www.ericsson.com/technology/

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Thanks for your attention