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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