multiplexing schemes

Faculty of Computing,Engineering & Technology

CommunicationsCOMMS (CE700038-2)

Alison L CarringtonC203

[email protected]/alg1

2008/9

Multiplexing, (FDM, TDM, CDM)& mobile communications

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2 Overview & Objectives

IntroductionMultiplexing Schemes

FDM, TDM & CDM

History/future of Mobile communication systems

1G, 2G, 3G & 4G?

Mobile Multiple Access SchemesAdvantages and disadvantages

FDMA, TDMA, CDMA & Combinations

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3 Introduction (1)

Multiplexing: name given to techniques which allow more than one message to be transferred via the same communication channel.Channel: could be a transmission line

Twisted pairCo-axial cableA radio systemFibre optic cableEtc..

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4 Introduction (2)

Channel will offer a specified bandwidth, which is available for a time, t, where t->∞.With reference to the channel there are 3 ‘degrees of freedom’

Bandwidth or frequencyTimeCode

ChannelBL BH

Bandwidth

freq

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5 Introduction (3)

Multiplexing are techniques whichallow k users to occupy a channelfor the duration in time, that the

channel is availablefreq

time

code

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6 Multiplexing

It costs significant amounts to change a telephone system, not least the cost of the “construction”Hence the more calls you can pump down a cable the more profitable the cable becomesTelco’s (Telephone companies) have developed elaborate multiplexing schemesThe schemes can be divided into three categories

Frequency Division Multiplexing (FDM)Time Division Multiplexing (TDM)Code Division Multiplexing (CDM)

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7 Sharing a medium

Time division multiplexing brought digital technology to mobile communicationsRecall, “multiplexing describes how several users can share the same medium with minimum or no interference” [Schiller 2003]In mobile communications multiplexing can be applied in 4 dimensions

SpaceFrequencyTimeCode

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8 Space Division Multiplexing (SDM)

Premise that if we have entities wishing to communicate using a single channel, then as long as we space them far enough apart interference will not occurTo reduce further, the risk of interference place guard space between the frequency spaces

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9 Frequency Division Multiplexing (FDM)

Divides the available frequency into non-overlapping bands with guard spaces between to avoid overlapping (adjacent channel interference)Receiver only has to know the frequency to tune in toUsed in analogue systems

f

Chan

nel f 1

Chan

nel f 2

C hann

e lf 3

Chan

nel f 4

Chan

nel f 5

Chan

nel f 6

t

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10 Time Division Multiplexing (TDM)

Allows access to entire frequency bandwidth but for a limited amount of timeAll senders use same frequency in at different timeIf two transmissions overlap known as co-channel interferencePrecise clock synchronisation required

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11 Combining FDM/TDM

By allowing a channel to use a certain frequency for a certain period of time more efficient use of resource is achievedMore robust against interference and tappingThis is the scheme used by GSM between the handset and base station

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12 Combining FDM/TDM

Requires coordination between the different sendersTwo senders will interfere if they select the same frequencyTo avoid this the senders hop between frequencies:

if the hop is fast enough the period of interference may be so small that if the coding of the data signal is sufficient to allow the receiver to recover the data the interference is deemed acceptable

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13 Code Division Multiplexing (CDM)

All channels use the same frequency, however, each channel is given its own unique codeEach code must be sufficiently orthogonal to allow appropriate guard spacesLarge range of codes provides significant expansion, security, etc

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14 Code Division Multiplexing (CDM)

Highly complex schemeReceiver has to know the code & be able to separate out other traffic on different codes which appear as background noiseReceiver & transmitter must be synchronised to provide correct decodingAll signals must reach the receiver with relatively equal strength or the receiver will not be able to distinguish between them

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15 Mobile Introduction

Public mobile radio services developed during the 1950’s

With a limited coverage area With a service available to a limited number of subscribers.

The rapid development of radio and electronic technology made possible the development of cellular systems during the 1980’s. During the 1990’s, digital cellular radio was introducedDuring the 2000’s truly multimedia tether less communications will be introducedThis section provides a review of these developments

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16 Radio Band Classification

100-10km3-30kHzVLFVery Low frequency

10-1m30-300MHzVHFVery high frequency

1000 -10m3-30MHzHFHigh frequency

1-0.1cm30-300GHzEHFExtra high frequency

10-1cm3-30GHzSHFSuper high frequency

100-10m300-3000MHzUHFUltra high frequency

1000-100m300-3000kHzMFMedium frequency

10-1km30-300kHzLFLow frequency

λ RangeFreq RangeABBClassification

The frequency & corresponding wavelength may bedetermined using c = f λ, with c = 3x108m/s, f = frequency in Hz& λ = wavelength in metres.

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17 Worldwide cellular subscriber growth

0

200

400

600

800

1000

1200

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Subs

crib

ers

[mill

ion]

Note that the curve starts to flatten in 2000

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/

http://www.cellular-news.com/story/30361.php

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18 Cellular subscribers per region (June 2002)

Asia Pacific; 36,9

Europe; 36,4

Americas (incl. USA/Canada);

22

Africa; 3,1

Middle East; 1,6


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19 Medium Access Control (MAC)

Whilst SDM, FDM, TDM, CDM describe how the medium is accessed at the physical layer, how the selected multiplexing scheme is “regulated”is called the Medium Access scheme (equivalent to OSI Layer 2 the Data Link Layer)In mobile this layer is divided between the Logical Link Control (2b) and the MAC (2a)

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20 Medium Access Control (MAC)

Why can we not simply use proven data MAC’ssuch ac CSMA/CD used on ethernet?

On a fixed wire, the propagation etc, is a known factor, the sender is responsible for detecting collisions, etc. If collision occurs everyone using the medium will be awareIn wireless networks attenuation, etc means signal decreases as it propagates out from the transmitter, therefore a collision may occur but will not be detected by the sender

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21 The role of the Access Scheme

Clearly conventional digital access schemes cannot be transferred to mobileEach access scheme has its own solutionMobile networks use a combination of the schemes to overcome the problems

Space Division Multiple AccessFrequency Division Multiple AccessTime Division Multiple AccessCode Division Multiple Access

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22 Telephone Network

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23 Cellular Communication System

Cellular telephones are personally portable devices that may be used in motor vehicles or by pedestrians communicating by radio-wave at 800-900-MHz band

they permit a significant degree of mobility within a defined serving region that may be hundreds of square kilometers in area.

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24 Cellular Telephone System

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25 Cellular Coverage

The geographic area to be served by a cellular radio system is broken up into smaller geographic areas, or cells.Uniform hexagons most frequently are employed to represent these cells on maps and diagrams;In practice, though, radio-waves do not confine themselves to hexagonal areas, so that the actual cells have irregular shapes.All communication with a mobile or portable instrument within a given cell is made to the base station that serves the cell.

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26 Cellular Migration Paths

First Major Migration PathI Gen, 80’s, ETACS (C-450,NMT-450..), (FDMA), AnalogueII Gen 90’s - GSM, II.5 Gen - GPRS, EDGE, (TDMA) DigitalIII Gen, 00’s, W-CDMA , (CDMA), All Digital

Second Major Migration PathI Gen, 80’s, AMPS, (FDMA), AnalogueII Gen, 90’s, IS-54 (TDMA), IS-95 (CDMA), DigitalIII Gen, 00’s, Cdma2000 (CDMA), All Digital

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27 Mobile Communication #1

Two aspects of mobility:user mobility: users communicate (wireless) “anytime, anywhere, with anyone”device portability: devices can be connected anytime, anywhere to the network

Example Wireless vs MobileStationary PCLaptop in a hotelWireless LAN inhistoric buildingPersonal Digital Assistant

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The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks:

local area networks: standardization of IEEE 802.11,ETSI (HIPERLAN)Internet: Mobile IP extension of the internet protocol IPwide area networks: e.g., internetworking of GSM and ISDN

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Started with Mobile phonesWhich were voice onlyLimited battery lifeLimited roaming capabilityLimited qualityUnsecured

Advent of Digital phonesAllowed for better use of this phone technologyAllowed the user to roam and receive calls anywhereCall were encryptedThe use of digital communications allowed data calls

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Roaming capabilityBrought forward technologyDual/Tri Band phonesSatellite phonesExpensive about £1.20 a minute for a world phone call

This allowed the user to move from the office/home environment and still be connected.Laptops

As computers got smaller it was now possible to carry it with you

As the computer is available the data was required for it

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Data UseGSM phones allowed 9.6 Kbps data communications

Enough for email and simple file transfers

SMS messaging is the most popular data useVoice calls are coming to the peak of the popularity

Additional avenues of revenue are requiredIncreased data is a obvious choiceCharge the user for the packet receivedCharge for the services they are accessingi.e. Football results service

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32 Personal Communication Services (PCS)

We will soon have the ability for anyone to access digital information like the Internet.Unlike the Internet, there will be value added service from day one

Video on DemandPaying your credit card billOrdering services

Value added services will be the primary goal of the PCS

This will be needed to pay for the infrastructure and licenses paid for

Each user will be able to view the information as they want it

Central control will not be put upon on the usersDiffering levels of hardware capability will effect the end presentation

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33 Applications

Vehiclestransmission of news, road condition, weather, music via DABpersonal communication using GSMposition via GPSlocal ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancyvehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance

Emergenciesearly transmission of patient data to the hospital, current status, first diagnosisreplacement of a fixed infrastructure in case of earthquakes,hurricanes, fire etc.crisis, war, ...

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34 Typical Application – Road Traffic

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35 Mobile & Wireless Services: Best Connected

GSM 53 kbit/sBluetooth 500 kbit/s

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36 Evolution path of cellular communication

1G

2G

2.5G

3G

4G

Analogue Digital Multimedia1980’s 1990’s 2000 +

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37 Tutorial - For Each Generation 1-4G define:

Strengths

Weaknesses

Technical Specification

Reasons to Upgrade

?G

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38 Solution: 1G Technology #1

StrengthsFairly High Range – Up to 50 miles from BSUses FDMA to increase potential usersCell based network allows for the same frequency to be reused within different cells

WeaknessesHigh Power Usage – Required large battery Finite amount of possible phone numbers provided by the serviceNo counteraction for noise, or scanning due to being an analogue signalInterference with radioLimited capacity due to available spectrumCalls disconnected due to handover – no priorityVoice only trafficRoaming was impossible due to different standardsExpansion difficult – frequency planning

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Technical SpecificationsRange of 50 kmAMPS (advanced mobile phone system) base Uses a hexagonal cell based networkAMPS Operates on a signal range of 800 MHz Analogue AMPS, TACS, NMT, FDMATACS uses 900 MHz, 25 kHz channels, 1000 channels

Reasons to UpgradeHigh power usageThe amount of phones trying to be used exceeded the possible usageAnalogue Technology prone to scanning or noise (security)Increased demandSmaller devicesTracking of deviceInterference limited also improved error checkingAbility to expand the networkRequirement for standard design rather than many (e.g. tacs, amps)

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StrengthsVoice data can be compressed allowing greater throughput Less power intensive – longer battery life, small batteriesDigital error checking removes noiseIntroduction of SMS and email availability on mobile handsets (digital data services)Harder to intercept digital signals and reduced scanningLess signal power needed on handset so cells can be made smallerClearer voice dataGSM allows signal roamingStandards based allows roamingProvides securityExpanded capacity – digital based more efficient

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WeaknessesDigital signals can produce dropouts rather than a static noiseSmaller cells cause some phones to struggle to receive a reasonable signal strengthLoss of tone on voice Limited data on control channelDesigned for voiceStill not one standard throughout the globeDifficult to expand network – freq planning required

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Technical SpecificationsGSM (European standard)Operates in 4 different signal ranges (900 MHz or 1800 MHz for European and 850 MHZ or 1900 MHZ on the American Continent.25 MHz bandwidth is divided into 124 carrier frequency channels, each spaced 200 kHz apart.Uses TDMA (Time Division Multiplexing) to give 8 full rate or 16 half rate speech channels per radio frequency channel.Transmission power of 2 watt in 850/900 and 1 watt in 1800/ 1900.22 km range on GSM

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Uses a GMSK (Gaussian minimum-shift keying) a modulation which is a continuous phase frequency shift keying which reduces interference from cross channelIntroduced the SIM (Subscriber Identity module) to contain the users subscription information and phone book.

Reasons to UpgradeSupport more users, higher data rates are requiredUse of more data centric applicationsRequirement for a single global standard

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44 Solution: 2.5G Technology #1

StrengthsFrees up control channelHigher data rate then 2G and 1G due to dedicated channelsGPRS

Added the application of WAPEnabled to work within the GSM technology framework with little in the way of change Data is sent in packets thus lowering the resource requirementsCapable of switching between voice and data communications and also providing simultaneous data and voice transferPacket and voice roaming possible

EDGEImproved air interface technologyQOS supportedBuilds on GPRS technology3 times faster than GPRSImproved forward error checking

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WeaknessesTo use GPRS you need a GSM deviceStill not a global standardDue to backward compatibility system not designed optimally Because channels dedicated to packets there are less voice channel availableNo 2.5G evolution in USAUplink and downlink is still symmetrical

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Technical SpecificationsFDM, FDMA and TDMAUsers can access more than 1 channel at a time hence the higher data rates. Data channels are shared.GPRS

QPSK modulation171 kbps theoretical maximum data rate, actual 30 – 40 Kbps

EDGE384 kbps theoretical, 80 – 100 actual. Increases down to change in coding schemes8PSK modulation

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Reasons to UpgradeStill not fully compatible globallyVoice and data still treated separately – require a standard so multimedia can be transmitted and received Higher data rates and increased capacityHigher security measures (CDMA is more difficult to intercept therefore more secure)Dynamically allocated direction of channel (TDM)

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W-CDMA replaced TDMA to increase the amount of users and allow higher speeds Strengths

3 different data rates based on distance. 2Mbps for fixed in building services (Pico cell), 384 kbps in urban environments (micro cell) and 144 in wide area mobile environments (macro cell) in FDD mode with a modulation of QPSKUMTS Incorporates the developments made for the GPRS and EDGE networksWithout the chipping code the data is essentially useless so a moderate level of security (scrambling codes can be used)Ability to extend network easily by adding cells or sectoring existing cellsCapacity not limited by bandwidth but other user interferenceHigher data rate then previously and also is flexible and variable depends on cell size, user mobility and requirements.Dynamic allocation of bandwidth, i.e. direction of flow (TDD)Power control can save battery

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WeaknessesStill not one single global standard i.e. frequencies, duplex, multiple access technique, nodes in infrastructure – due to backward compatibility issues Expensive licences – pass cost to user. Not fair competition i.e. not many providers (operators) difficult for new operatorsLack of coverage – too new hence migration to standard systemsAlways on = drain of batteryPower control – requires signalling therefore a drain on resources

Technical SpecificationsUses 3 main technical implications - UMTS (Europe), CDMA2000(America) and TD-SCDMA (China)UMTS uses ATM (Asynchronous Transfer Method) which allows circuit switched transfer of data using packetsUMTS using ATM also allows a high speed of data transfer up to 10Gbps and provides a QOS for the duration of packet transfer.

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Technical Spec continued….UMTS uses Wideband code division multiple access (W-CDMA) which uses 10x the current processing power of 2G to encode the signal and is done with QPSKW-CDMA supports two modes of operation TDD (time division duplex) and FDD (frequency division duplex) W-CDMA allows multiple users to communicate at the same time over the same frequency. Utilising “Chipping Codes”which is supplied by the base station to the device. The chipping code is used to identify the signals from the device and it can also be used to adjust the frequency of data transferred during the transfer.UMTS uses 5Mhz for the signal and CDMA uses only 200 KHz

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Reasons to upgrade1 standard required 1 device should connect to fixed network best/fast/lowest error rateReconfigure itself dynamicallySoftware radio

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52 Solution: Reasons for Future G???? #1

Higher data rates, more secure and more reliableSingle StandardOFDM (Orthogonal Frequency division modulation)QAM (Quadrature amplitude modulation)MIMO (Multiple input, multiple output) antenna arraysKiller app – What will be the new application that is a must for phone technologySingle device that can reconfigure itself dependant on access methodI.e. pda phone/laptop, personal organiser in one!Mp3 player

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53 Solution: Reasons for Future G???? #2

Beheshti, B.“Study of the technology migration path of the cellular wireless industry from 3G to 3.5G and beyond”;Long Island Systems, Applications and Technology, 2005. IEEE Conference6 May, 2005 Page(s):15 - 28

o

3.5G

and

beyond

Beheshti,

B.;

Long

Island

Systems,

Appl

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ns

54 Solution: Development of 4G - Future

VoicePacket

Switched

M3 Circuit Switched

Fixed Network:

ATM, IPv4/6, Diff Serv, MPLS

PSTN, ISDN xDSL

Wireless Personal Area Net (WPAN)

MobileAccess Network

(UTRAN)

Hierarchical Cell StructureM3

SatelliteAccess

Network

Bluetooth, WI-FI, WLAN, Cellular, SatelliteAd-Hoc

COMMS (CE700038-2) 2008/9

Mul

tiple

xing

, (FD

M, T

DM

, CD

M)

and

mob

ile c

omm

unic

atio

ns

55 Overlay Networks - the global goal

regional

metropolitan area

campus-based

in-house

verticalhandover

horizontalhandover

integration of heterogeneous fixed andmobile networks with varyingtransmission characteristics


multiplexing schemes

Documents