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    Networked Worlds:Telepho

    ny,Broadband

    Wireless

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    Convergence andChange

    Local TelephonesLong Distance Telephones

    CableRadio & TVInternetWireless

    Networks .. .

    Introduction

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    Local TelephoneServiceNatural

    Monopoly?Declining AverageCostsLow MarginalCostsRight-of-WayIssuesNetwork

    Effects

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    Local Service

    A B

    C

    Geographic Division ofMarkets

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    Regulation

    Common Carrier StatusLocal Telephone

    Service

    FCC, Utilities Commissions, Local Franchise Boards

    Reasonable and nondiscriminatory rates

    Limited liability.

    Open Access Rules

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    History

    1876 Telephone patented

    1878 Exchanges1889 Automated Exchanges1895 Main AT&T Patents expire; rates fall

    50% by 19071902 1002 cities had service; 451 had at least two

    providers1900s Bell Labs created; company patents

    improvements; company develops longdistance; competitors decline

    Local TelephoneService

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    History1910 Telephones become common carriers but not of each other. States introduce

    regulation1913 Consent decree: telegraph, local service,

    long distance separate monopolies1920s Bell Labs wins Nobel Prize1934 FCC created1947 Bell Labs invents transistor & wins

    second Nobel Prize1949 First government antitrust suit1956 First antitrust suit ends: AT&T blocked

    from competing outside telephony.1950s Snapshot: Ma Bell

    Local TelephoneService

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    History 1960s Bell Labs discovers cosmic microwave

    background, wins third Nobel Prize

    1960s Competitors fight AT&T on right tocompete on equipment, long distance

    1974 Second government antitrust suit1984 Breakup; competition in long distance and

    equipment; Baby Bells.

    1996 Telecommunications Act introducesLocal Competition

    Local TelephoneService

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    Create"Competition"?Regulating Consumer

    PricesRegulating

    Competitors' Costs

    vs.

    Does "OpenAccess"

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    Long DistanceServiceNatural

    Monopoly?Declining AverageCostsLow Marginal

    Costs

    Right-of-WayIssuesNetwork

    Congestion!

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    Long Distance

    A B

    C

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    Cable

    NaturalMonopoly?Declining AverageCostsLow Marginal

    CostsRight-of-WayIssuesNetworkEffects

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    Cable

    A

    Geographic Division ofMarkets

    Neighborhood-by

    B

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    Cable1940s Local service for rural areas

    1950s 1960s FCC regulates cable to carry localstations, protect UHF, preventsiphoning, and fund public serviceprograms

    1972 Consensus Agreement liberalizesrestrictions while micromanagingwhat cable stations can do.

    1970s Consensus Agreement becomesunmanageable

    1977-79 Court challenges erode FCCs abilityto require cable to carry local signals,prevent siphoning.

    1980 Deregulation

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    1984 Cable Communications Policy Act lets FCC waiverate regulation where effective competitionexists. Local government cannot grant exclusive

    licenses.Open access: Must carry content from competitors

    on common carrier basis.

    1992 Cable TV Consumer Protection & Competition

    Act lets local authorities set prices for basic service inmost cases. Act ends state regulation of non- basicservice.

    Local governments cannot charge exorbitant fees.

    Cable

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    TV & Radio

    NaturalMonopoly?

    Declining AverageCostsLow MarginalCostsRight-of-Way

    IssuesNetwork

    LimitedSpectrum

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    A BThe Internet

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    Internet ServiceNatural

    Monopoly?Just Like LongDistance?Network Effects?AT T v. C i ty o fPo r t l andStates Cannot Demand Open Access of Broadband

    FCC is studying the issue.

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    A B

    Network

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    Backbone

    LastMileLast 100Feet

    Middle Mile

    Chokepoints

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    The Last 100 feetTelephones

    Wireless

    Cable

    Satellite

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    Wireless

    Hedy Lamarr (1913-2000)

    DoesThisWomanLookFamiliar?

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    FrequencyHopping

    Inventor &Movie Star

    Any girl can be glamorous.All you have to do is stand stilland look stupid.

    Hedy Lamarr (1913-2000)

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    Technologyireless

    Wires vs. Broadcast

    Radio, Cellular,& Time Division

    Multiplexing

    Smart Transmitters

    Frequency Hopping

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    NetworkExternalities?

    WithinCountries?AcrossCountries?

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    Picking Winners

    -- Orphaned Users

    -- Reaching Equilibrium Efficiently

    - Duplicated Development

    - Fragmented/Technically InferiorWinners

    Issues

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    Picking Winners, ctd.

    -- Network Effects

    -- Choosing Wrong

    -- Antitrust Dangers

    Issues

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    SchumpeterianCompetitionExport

    Markets

    Issues

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    Wireless

    USA

    FCC RegulationFirst Generation

    Second Generation

    Advanced Mobile Phone

    Standard (AMPS)

    GSM, DAMPS/TDMA

    Qualcomm CDMA

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    Wireless

    USA

    Third Generation

    CDMA 2000

    and

    W-CDMA

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    Wireless

    EuropeFirst Generation:

    National Bodies

    National Systems:

    TACS (UK), RC-2000 (France),Netz B (Germany), RTMS (Italy),NMT-450 (everywhere else).

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    Wireless Europe

    Second Generation:

    European TelecommunicationsStandards Institute (ETSI)

    Standard for Mobile (GSM).

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    Wireless Europe

    GSM Conquers The World !!!

    105 countries

    Not Western Hemisphere

    60% of Worldwide sales.

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    Third Generation:

    W-CDMA

    Backward Compatible to GSM

    Incompatible With Qualcomm

    CDMA

    Less Capacity, More Interference!

    Wireless Europe

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    Third Generation:

    QualComm/Ericcson Treaty

    Standards vs. Patents

    Ericcson Buys QualComms

    Hardware Business

    Wireless Europe

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    Third Generation:

    QualComm/Ericcson Treaty

    Wasteful Patents

    Are Cross-Licenses Good for

    Consumers?

    Wireless Europe

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    Wireless

    World

    Third Generation: CDMA2000 vs.W-CDMA:

    China

    Korea

    Japan

    Is it a Small World (After All)?

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    Wireless

    Implications

    Antitrust Policy:

    - Schumpeterian Competition

    - Innovation Cycle vs. CourtDockets

    - Innovation Cycle vs. TippingTime

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    Evolution of Cellular Systems

    Introduction

    1st Generation cellular systems

    2nd Generation cellular systems 3rd Generation cellular systems

    Ben Slimane [email protected]

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    Multiple Access/Multiplexing

    Methods Frequency Division Multiple Access

    (FDMA)

    Time Division Multiple Access (TDMA) Frequency-Hop Code Division Multiple

    Access (FH-CDMA)

    Direct Sequence-Code Division MultipleAccess (DS-CDMA)

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    Cellular System Evolution

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    1G Cellular Systems

    Appeared in late 1970s and deployed inearly 1980s

    All based on analog techniquesAll used FDMA and FM modulation

    Date rate: 8-10 kbps

    Low system capacity (reuseN=7) Large cells with omni-directional base

    station antennas

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    1G: NMT

    1981Nordic Mobile Telephone

    First generation analog technology

    NMT450 and NMT900 Free standard ready 1973, 1977

    Network open 1981 in Sweden and Norway

    Based on FDMA

    Channel bandwidth: 25/12.5 kHz Total number of channels: 1999

    Analog traffic channel, digital control channel

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    1G: AMPS

    Advanced Mobile Phone System (AMPS)

    Appeared late 1970s,

    First deployed in 1983, US, South America,China, and Australia

    Based on FDMA

    Channel bandwidth: 30 kHz Total number of channels: 832 channels

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    1G: AMPS

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    1G versus 2G

    Digital traffic channels first-generation systemsare almost purely analog; second-generationsystems are digital

    Encryption all second generation systemsprovide encryption to prevent eavesdropping

    Error detection and correction second-generationdigital traffic allows for detection and

    correction, giving clear voice reception Channel access second-generation systems allow

    channels to be dynamically shared by a numberof users

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    2G Cellular Systems

    Deployed in mid 1990s

    2G cellular systems all use digital voice codingand digital modulation

    Can provide advanced call capabilities and a bettersystem capacity

    More users per unit bandwidth

    Designed before the widespread of the Internet Voice services and limited data services (SMS,

    FAX)

    Data rate: on the order 10 kbps

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    2G cellular Systems

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    2G: GSM

    Global Systems for Mobile Communications(GSM)

    Based on TDMA

    Channel bandwidth: 200kHz

    Traffic channels (slots) per RF channel: 8

    Maximum cell radius (R): 35 km

    Frequency: 900/1800MHz

    Maximum vehicle speed (Vm): 250 km/hr

    Maximum coding delay: approx. 20

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    Steps in Design of TDMA

    Timeslot

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    GSM Frame Format

    Transmission bit rate = 156.25/0.577 = 270.833 kbps

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    Time Slot Fields Trail bits: allow synchronization of transmissions

    from mobile units Encrypted bits: encrypted data (ciphertext bits) Training sequence

    A known bit pattern used to estimate the multi-path radio channel

    Stealing bit: block contains data or stolen forcontrol

    Guard bits used to avoid overlapping with other bursts

    Speech information The actual information data

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    GSM Signal Processing

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    GSM Network Architecture

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    Mobile Station

    Mobile station communicates across Um interface(air interface) with base station transceiver insame cell as mobile unit

    Mobile equipment (ME) physical terminal, suchas a telephone or PCS

    ME includes radio transceiver, digital signalprocessors and subscriber identity module (SIM)

    GSM subscriber units are generic until SIM isinserted SIMs roam, not necessarily the subscriber devices

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    Base Station Subsystem (BSS)

    BSS consists of a base station controller andone or more base transceiver stations(BTS)

    Each BTS defines a single cellIncludes radio antenna, radio transceiver and

    a link to a base station controller (BSC)

    BSC reserves radio frequencies, manageshandoff of mobile unit from one cell toanother within BSS, and controls paging

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    Network Subsystem (NS)

    NS provides link between cellular network andpublic switched telecommunications networks

    Controls handoffs between cells in different BSSs

    Authenticates users and validates accounts

    Enables worldwide roaming of mobile users

    The Mobile Switching Center (MSC) is the centralelement of the NS

    The MSC controls four databases

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    The MSC Databases Home location register (HLR) database stores

    information about each subscriber that belongsto it

    Visitor location register (VLR) database

    maintains information about subscriberscurrently physically in the region

    Authentication center database (AuC) usedfor authentication activities, holds encryption

    keys Equipment identity register database (EIR) keeps track of the type of equipment that existsat the mobile station

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    2.5G

    GPRS: General Packet Radio ServiceBitrates from 9.05 to 171.2 kbit/s

    Multiple Time slots allocated to user Link adaptations.

    EDGE: Enhanced data rates for GSMevolution

    Data rates up to 384 kbit/s Two modulation schemes (GMSK, 8PSK)

    Link adaptations

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    Cellular CDMA

    Frequency diversity resolve multi-paths bymeans of the RAKE receiver

    Multipath resistance chipping codes used forCDMA exhibit low cross-correlation and lowautocorrelation

    Privacy privacy is inherent since spread

    spectrum is obtained by use of noise-like signals Graceful degradation system only gradually

    degrades as more users access the system

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    Cellular CDMA

    Self-jamming non-orthogonal codescreate interference between users

    Near-far problem weak users jammed bystrong users

    Soft handoff smooth handoff from onecell to the next

    more complex than hard handoff Frequency reuse of 1

    No frequency planning needed (N=1)

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    Cellular CDMA

    The RAKE receiver Resolves multi-path components and combine them

    coherently

    A diversity gain with order equals to the number of

    resolved paths is obtained

    Soft Handoff Mobile station temporarily connected to more than one

    base station simultaneously

    Require more radio resources

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    The RAKE Receiver

    Spreading codes with low correlation properties allow the separationof the different radio paths

    The RAKE receiver uses this property and locks on the different paths

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    The RAKE Receiver

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    Soft handoff in CDMA

    When a mobile unit is in soft handover Two codes are needed on the downlink

    Only one code is needed on the uplink

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    Spreading in Cellular CDMA

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    2G: IS-95

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    2G: IS-95

    DownlinkPilot (0) cell detection, channel estimation

    Synchronization (32) identificationinformation

    Paging (1-7) messages to mobiles

    Traffic (8-31, 33-63) 55 traffic channels withdata rate of 9600 bps

    A unique channel for each user

    Uplink Access channels

    Traffic channels

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    ITUs View of 3G Voice quality comparable to the public switched

    telephone network

    144 kbps data rate available to users in high-speedmotor vehicles over large areas

    384 kbps available to pedestrians standing ormoving slowly over small areas

    Support for 2.048 Mbps for office use

    Symmetrical / asymmetrical data transmission

    rates Support for both packet switched and circuit

    switched data services

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    ITUs View of 3G

    An adaptive interface to the Internet to reflectefficiently the common asymmetry betweeninbound and outbound traffic

    More efficient use of the available spectrum ingeneral

    Support for a wide variety of mobile equipment

    Flexibility to allow the introduction of new servicesand technologies

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    Alternative Interfaces

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    CDMA Design Considerations

    Bandwidth limit channel usage to 5 MHz

    Chip rate depends on desired data rate, need forerror control, and bandwidth limitations; 3Mcps or more is reasonable

    Multi-rate advantage is that the system canflexibly support multiple simultaneousapplications from a given user and canefficiently use available capacity by only

    providing the capacity required for each service

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    UMTS

    Wideband CDMA

    Uplink 1920-1980 MHz

    Downlink 2110-2170 MHz Bandwidth 4,4-5 MHz

    HSDPA: High Speed Downlink Packet

    Access Data rates: 1,8, 3,6, 7,2 and 14,4 Mbit/s

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    W-CDMA

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    LTE

    Long Term Evolution (LTE)

    Advanced OFDM for downlink

    Single carrier FDMA for uplink Data rates exceeding 100 Mbps in the

    downlink with full mobility

    Scalable bandwidth (1.25 to 20 MHz)

    Frequency-reuse 1

    Multiple transmit and receive antennas

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    How Cell PhonesWork

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    An Important Technology

    l Cellular telephony is one of the fastest growingtechnologies on the planet.

    l

    l

    Presently, we are starting to see the third generationof the cellular phones coming to the market.

    l

    l New phones allow users to do much more than hold

    phone conversations.

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    Beyond Voice

    l Store contact informationl Make task/to-do listsl Keep track of appointmentsl Calculatorl Send/receive emaill Send/receive picturesl Send/receive video clipsl Get information from the internet

    l Play gamesl Integrate with other devices (PDAs, MP3 Players,

    etc.)

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    Outline for Today

    l Today, we will review the design of cellular system:what are its key components, what it is designedlike, and why.

    l

    l Also, we will look at how cellular networks supportmultiple cell phone users at a time.

    l

    l

    Finally, we will review the important generations ofcellular systems and start looking at the design ofthe first generation of cell phones.

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    The Cellular Concept

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    Basic Concept

    l Cellular system developed to provide mobiletelephony: telephone access anytime,anywhere.

    l

    l First mobile telephone system was developed andinaugurated in the U.S. in 1945 in St. Louis, MO.

    l

    l This was a simplified version of the system usedtoday.

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    System Architecture

    l A base station provides coverage (communicationcapabilities) to users on mobile phones within itscoverage area.

    l

    l Users outside the coverage area receive/transmitsignals with too low amplitude for reliablecommunications.

    l

    l Users within the coverage area transmit and receivesignals from the base station.

    l

    l The base station itself is connected to the wiredtelephone network.

    l

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    First Mobile Telephone System

    One and only onehigh power basestation with which allusers communicate.

    Entire CoverageArea

    Normal

    TelephoneSystem

    Wired connection

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    Problem with Original Design

    l Original mobile telephone system could only supporta handful of users at a timeover an entire city!

    l

    l With only one high power base station, usersphones also needed to be able to transmit at highpowers (to reliably transmit signals to the distantbase station).

    l

    l Car phones were therefore much more feasible thanhandheld phones, e.g., police car phones.

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    Improved Design

    l Over the next few decades, researchers at AT&TBell Labs developed the core ideas for todayscellular systems.

    l

    l Although these core ideas existed since the 60s, itwas not until the 80s that electronic equipmentbecame available to realize a cellular system.

    l

    l In the mid 80s the first generation of cellularsystems was developed and deployed.

    The Core Idea: Cellular

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    Concept

    l The core idea that led to todays system was thecellular concept.

    l The cellular concept: multiple lower-power basestations that service mobile users within theircoverage area and handoffusers to neighboringbase stations as users move. Together basestations tessellate the system coverage area.

    l

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    Cellular Concept

    l Thus, instead of one base station covering an entirecity, the city was broken up into cells, or smallercoverage areas.

    l

    l Each of these smaller coverage areas had its ownlower-power base station.

    l

    l

    User phones in one cell communicate with the basestation in that cell.

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    3 Core Principles

    l Small cells tessellate overall coverage area.

    l

    l Users handoff as they move from one cell to

    another.l

    l Frequency reuse.

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    Tessellation

    l Some group of small regions tessellate a largeregion if they over the large region without anygaps or overlaps.

    l

    l There are only three regular polygons that tessellateany given region.

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    Tessellation (Contd)

    l Three regular polygons that always tessellate:l Equilateral triangle

    l Square

    l Regular Hexagon

    l

    TrianglesSquares

    Hexagons

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    Circular Coverage Areas

    l Original cellular system was developed assumingbase station antennas are omnidirectional, i.e.,they transmit in all directions equally.

    lUsers located outsidesome distance to thebase station receiveweak signals.

    Result: base station ha

    circular coveragearea.

    Weaksign

    al

    Stro

    ngsign

    al

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    Circles Dont Tessellate

    l Thus, ideally base stations have identical, circularcoverage areas.

    l Problem: Circles do not tessellate.

    l

    l The most circular of the regular polygons thattessellate is the hexagon.

    l Thus, early researchers started using hexagons to

    represent the coverage area of a base station,i.e., a cell.

    l

    l

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    Thus the Name Cellular

    l With hexagonal coverage area, a cellular network isdrawn as:

    l

    l

    l

    l

    l

    l Since the network resembles cells from ahoneycomb, the name cellular was used todescribe the resulting mobile telephone network.

    BaseStation

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    Handoffs

    l A crucial component of the cellular concept is thenotion of handoffs.

    l Mobile phone users are by definition mobile, i.e.,they move around while using the phone.

    l Thus, the network should be able to give themcontinuous access as they move.

    l This is not a problem when users move within thesame cell.

    l When they move from one cell to another, ahandoffis needed.

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    A Handoff

    l A user is transmitting and receiving signals from agiven base station, say B1.

    l

    l Assume the user moves from the coverage area ofone base station into the coverage area of asecond base station, B2.

    l

    l

    B1 notices that the signal from this user isdegrading.

    l B2 notices that the signal from this user is improving.

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    A Handoff (Contd)

    l At some point, the users signal is weak enough atB1 and strong enough at B2 for a handoff to occur.

    l Specifically, messages are exchanged between theuser, B

    1, and B

    2so that communication to/from

    the user is transferred from B1 to B2.

    l

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    Frequency Reuse

    l Extensive frequency reuse allows for many users tobe supported at the same time.

    l

    l Total spectrum allocated to the service provider isbroken up into smaller bands.

    l

    l A cell is assigned one of these bands. This means

    all communications (transmissions to and fromusers) in this cell occur over these frequenciesonly.

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    Frequency Reuse (Contd)

    l Neighboring cells are assigned a different frequencyband.

    l

    l This ensures that nearby transmissions do not

    interfere with each other.l

    l The same frequency band is reused in another cellthat is far away. This large distance limits theinterference caused by this co-frequency cell.

    l

    l More on frequency reuse a bit later.

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    Example of Frequency Reuse

    Cells using the same frequencies

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    Multiple Access in Cellular

    Networks

    Multiple Transmitters, One

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    Receiver

    l In many wireless systems, multiple transmittersattempt to communicate with the same receiver.

    l

    l For example, in cellular systems. Cell phones usersin a local area typically communicate with thesame cell tower.

    l

    l

    How is the limited spectrum shared between theselocal transmitters?

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    Multiple Access Method

    l In such cases, system adopts a multiple accesspolicy.

    l Three widely-used policies:

    l Frequency Division Multiple Access (FDMA)

    l Time Division Multiple Access (TDMA)

    l Code Division Multiple Access (CDMA)

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    FDMAl

    In FDMA, we assume that a base station canreceive radio signals in a given band of spectrum,i.e., a range of continuous frequency values.

    l The band of frequency is broken up into smaller

    bands, i.e., subbands.l Each transmitter (user) transmits to the base station

    using radio waves in its own subband.

    FrequencySubbands

    Cell Phone User 1Cell Phone User 2::

    Cell Phone User NTime

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    FDMA (Contd)

    l A subband is also a range of continuousfrequencies, e.g., 824 MHz to 824.1 MHz. Thewidth of this subband is 0.1 MHz = 100 KHz.

    l

    l

    l

    l When a users is assigned a subband, it transmits tothe base station using a sine wave with thecenter frequency in that band, e.g., 824.05 MHz.

    l

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    FDMA (Contd)

    l When the base station is tuned to the frequency of adesired user, it receives no portion of the signaltransmitted by another in-cell user (using adifferent frequency).

    l

    l This way, the multiple local transmitters within a celldo not interfere with each other.

    l

    l

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    TDMA

    l In pure TDMA, base station does not split up itsallotted frequency band into smaller frequencysubbands.

    l

    l Rather it communicates with the users one-at-a-time, i.e., round robin access.

    FrequencyBands

    Time

    User1

    User2

    User3

    UserN

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    TDMA (Contd)

    l Time is broken up into time slots, i.e., small, equal-length intervals.

    l

    l Assume there are some n users in the cell.l

    l Base station groups n consecutive slots into aframe.

    l

    l Each user is assigned one slot per frame. This slotassignment stays fixed as long as the user

    communicates with the base station (e.g., lengthof the phone conversation).

    TDMA (C d)

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    TDMA (Contd)

    l Example of TDMA time slots for n = 10.l

    l

    l

    l

    l

    l

    l In each time slot, the assigned user transmits aradio wave using a sine wave at the centerfrequency of the frequency band assigned to thebase station.

    TimeSlot

    User1

    User2

    User10

    User1

    User10

    User1

    Frame

    H b id FDMA/TDMA

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    Hybrid FDMA/TDMA

    l The TDMA used by real cellular systems (likeAT&Ts) is actually a combination ofFDMA/TDMA.

    l

    l Base station breaks up its total frequency band intosmaller subbands.

    l

    l Base station also divides time into slots and frames.

    l

    l Each user is now assigned a frequency and a timeslot in the frame.

    l

    l

    H b id FDMA/TDMA (C td)

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    Hybrid FDMA/TDMA (Contd)

    Time

    User1

    User2

    User10

    Us

    er11

    Us

    er12

    Us

    er20

    U

    ser31

    U

    ser32

    U

    ser40

    User21

    User22

    User30

    Assume a base station divides its frequency band into4 subbands and time into 10 slots per frame.

    User1

    User2

    User10

    Us

    er11

    Us

    er12

    Us

    er20

    U

    ser31

    U

    ser32

    U

    ser40

    User21

    User22

    User30

    Frame

    Frequency Subband 1

    Frequency Subband 2

    Frequency Subband 3

    Frequency Subband 4

    CDMA

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    CDMA

    l CDMA is a more complicated scheme.l

    l Here all users communicate to the receiver at thesame time and using the same set of frequencies.

    l This means they may interfere with each other.l The system is designed to control this interference.l A desired users signal is deciphered using a unique

    code assigned to the user.

    l

    l There are two types of CDMA methods.

    CDMA Method 1: FrequencyH i

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    Hopping

    l First CDMA technique is called frequency hopping.l

    l In this method each user is assigned a frequencyhopping pattern, i.e., a fixed sequence of

    frequency values.l

    l Time is divided into slots.l

    l

    In the first time slot, a given user transmit to thebase station using the first frequency in itsfrequency hopping sequence.

    F H i (C td)

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    Frequency Hopping (Contd)

    l In the next time interval, it transmits using thesecond frequency value in its frequency hopsequence, and so on.

    l

    l This way, the transmit frequency keeps changing intime.

    l

    l We will look at frequency hopping in greater detail inan exercise (in a bit).

    Second Type of CDMA: DirectS

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    Sequence

    l This is a more complicated version of CDMA.

    l

    l Basically, each in-cell user transmits its message tothe base station using the same frequency, at thesame time. Here signals from different usersinterfere with each other.

    l

    l But the user distinguishes its message by using aspecial, unique code. This code serves as aspecial language that only the transmitter andreceiver understand. Others cannot decipher thislanguage.

    Di t S CDMA

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    Direct Sequence CDMA

    l Because of the complexity of this second type ofCDMA, we will not describe it in detail.

    l

    l Rather we will give an intuitive understanding of it.l

    l Specifically, think of this access scheme like a groupof conversations going on in a cocktail party.

    l

    l

    C kt il P t A l

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    Cocktail Party Analogy

    l In this cocktail party, people talk to each other at thesame time and thus interfere with other.

    l

    l To keep this interference in control, we require that

    all partiers must talk at the same volume level; noone partier shouts above anybody else.

    l

    l Also, to make sure that each speaking partier is

    heard correctly by his/her intended listener (andnobody else can listen in), we require eachspeaker to use a different language tocommunicate in.

    C kt il P t (C td)

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    Cocktail Party (Contd)

    l The caveat in this analogy is that if you speak in onelanguage, it is assumed that only your desiredlistener can understand this language.

    l

    l Thus, if you were at this party and only understoodone language, say English, then all non-Englishconversations would sound like gibberish to you.

    l The only signal you would understand is English,coming from your intender speaker (transmitter).

    l Similar methodology is used by Direct SequenceCDMA transmitters/receivers.

    l

    Exercise on FrequencyH i CDMA

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    Hopping CDMA

    l Assume you are the receiver (base station) in afrequency hopping cellular system.

    l

    l There are a total of 10 users in your cell.

    l

    l They are each assigned their own unique frequencyhopping pattern.

    E ercise Description (Contd)

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    Exercise Description (Contd)

    Recall:l A user will use its frequency hopping pattern to

    transmit messages to the base station.

    l In the first time slot, the user will transmit usingthe first frequency value in the frequencyhopping sequence.

    l In the second time slot, the user will use thesecond frequency value in the hopping

    sequence, and so on.

    Exercise Description (Contd)

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    Exercise Description (Contd)

    l Assume that the base station (you) can receivesignals in the range of 824 MHz to 825 MHz.

    l This means that you have 1 MHz of frequency

    available for use to communicate with local users.l

    l The network designers decided to divide the total 1MHz = 1000 KHz of frequency assigned to you

    into 100 KHz subbands, i.e., into 10 subbands.ll Additionally, the designers have divided time into 1

    millisecond (1 millisecond = 0.001 second) timeslots.

    Exercise Description (Contd)

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    Exercise Description (Contd)

    l In the handout, you will see a sequence of bits fordifferent frequency and time value.

    l

    l These sequences represent the messages that thebase station determines from the received radiowaves (after demodulation) at the differentfrequency and time values.

    l

    l

    Exercise Description (Contd)

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    Exercise Description (Contd)

    l In each handout, a desired users frequencyhopping pattern is given.

    l

    l Please use this hopping pattern, to determine the bitsequence of the desired user.

    l

    Exercise Description (Contd)

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    Exercise Description (Contd)

    l Now, assume that each user is sending a textmessage to the base station.

    l

    l We wish to determine this message.l

    l To do so, break up the bit sequence into sequenceof bytes.

    l

    l Recall, 1 byte = 8 bits.l

    l

    Exercise Description (Contd)

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    Exercise Description (Contd)

    l Computers use a standard method to convert letterswe use to write text messages, i.e., the letters ofthe alphabet, into bits (sequences of 0s and 1s).

    l

    l This standard method is called ASCII coding.l

    l In the handout, we show a part of the ASCIIcodebook.

    Exercise Description (Contd)

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    Exercise Description (Cont d)

    l The codebook can be used to determine the textmessage sent by the user.

    l

    l For each byte, we lookup the byte sequence in the

    codebook (chart) to determine the letter that itcorresponds to.

    l

    l String the letters together to get the text message.

    Important Parameter inExercise

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    Exercise

    l In the system described in the exercise, a usertransmits 3 bytes in 6ms, where 1ms = 0.001seconds.

    l

    l There are 8 bits in a byte; so the user transmits 24bits in 6ms.

    l

    l This means the user has a data rate of 24 bits/6ms

    = 4000 bits/sec.l

    l

    Final Points onFDMA/TDMA/CDMA

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    FDMA/TDMA/CDMA

    l When users are in the middle of a phone call, thesystem uses FDMA/TDMA/CDMA to give themaccess.

    l

    l But there are only so many frequencies, time-slots,or codes available to share between users in acell.

    l

    l

    If we divide the frequency into too many bands, oruse too many time slots, or too many codes, thequality of speech heard by the end user will beunsatisfactory.

    Channels

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    Channels

    l Channel is a general term which refers to afrequency in an FDMA system, atimeslot/frequency combination in TDMA,

    or a code in CDMA.l

    l This way, a base station has a fixed numberof channels and can support only that many

    simultaneous users.

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    Random Access: Another

    Important Multiple Access Method

    Motivating Random AccessChannels

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    Channels

    l As mentioned earlier, FDMA/TDMA/CDMA are usedwhen users are engaged in a phone call.

    l

    l Before being assigned a frequency, timeslot, orcode (i.e., a channel), a user has to ask the basestation if it has a channel leftover to assign thisuser.

    l

    l In other words, the user has to have some otherway of communicating with the base station.

    Motivating Random Access

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    Motivating Random Access

    l Of all the frequencies available at a base station, aprescribed portion of them are set aside for thispurpose.

    l

    l These frequencies are called control channels, asopposed to the rest of the frequencies in cell,which are called voice channels.

    l

    l

    A user will transmit a signal to the base station on acontrol channel basically saying, Im here and Idlike to talk to you.

    Random Access: Failure

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    Random Access: Failure

    l There maybe other users who do this at the sametime using the same frequency.

    l

    l If they do, the signals will interfere with each other

    and the base station will not receive anything.l

    l This indicates a failure (aka collision), when thishappens, each user will backoff for some random

    amount of time and try again. Since they backofffor a random amount of time, chances are theywont retry at the same time.

    Random Access: Success

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    Random Access: Success

    l

    l If only one user transmits, then the base station willreceive the users signals and respond to it bysaying, Okay you can talk to me, tune into this

    other channel and tell me what you want.

    l

    l The user will then tune this channel and be able toexclusively transmit and receive signals to thebase station.

    l

    Random Access: Success(Contd)

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    (Cont d)

    l This new channel assigned to the user is also acontrol channel.

    l

    l Using this channel the user can then send a signalthat says for example I want to make a phone tothis phone number.

    l

    l To which the base station will respond by assigningthe user a voice channel, if there are someavailable.

    l

    Random Access Summary

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    Random Access Summary

    l This type of competing access method is calledrandom access.

    l

    l There are different rules followed by usersparticipating in random access.

    l

    l We will return to this notion when looking at wi-fisystems.

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    Standards: Rules for aCellular Network

    The Inner Workings

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    The Inner Workings

    l Government agencies (FCC) give licenses tocompanies (service providers) to provide cellularaccess in a particular geographic region.

    l

    l These licenses allow the service provider to setupcellular towers in that region which can transmitover a prescribed band of frequencies.

    Standards

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    Standards

    l The service providers must use one of the approvedcellular standards for developing the cellularnetwork in that region.

    l

    l These standards are mutually agreed upon rulesadopted by the industry on how the cell phonesystem operates.

    l

    l

    These standards described the air interface, i.e.,how cell phones and base stations mustcommunicate with each other.

    More on Standards

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    More on Standards

    l These mutually agreed upon standards change overtime, as technology progresses.

    l

    l The first cellular systems deployed in the U.S.adhered to a standard called Analog MobilePhone System (AMPS). This system existed inthe mid 80s to early 90s.

    l

    l The first cellular network used analog technology.Specifically, speech was converted to an FMsignal and transmitted back and forth from userphones.

    l

    l We describe this system in detail a bit later.

    Second Generation of Cellular

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    Second Generation of Cellular

    l The second generation (2G) of cellular networkswere deployed in the early 90s.

    l

    l 2G cellular phones used digital technology and

    provided enhanced services (e.g., messaging,caller-id, etc.).l

    l In the U.S., there were two 2G standards that

    service providers could choose between.

    Second Generation (Contd)

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    Second Generation (Cont d)

    l The two standards used in U.S. are different fromthe 2G system used in Europe (called GSM) andthe system used in Japan.

    l

    First U.S. standard is called Interim Standard136 (IS-136) and is based on TDMA (time-division multiple access).

    l Second is called IS-95 and is based on CDMA(code-division multiple access).

    l

    l Most present systems are what is called the 2.5generation (2.5G) of cellular.

    Present Cellular Systems

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    Present Cellular Systems

    l Most present cell systems are 2.5G. They offerenhanced services over second generationsystems (emailing, web-browsing, etc.).

    l

    l Some 2.5G systems (such as AT&Ts) arecompatible with the European system, GlobalSystem Mobile (GSM).

    l

    l Presently, service providers are setting up thirdgeneration (3G) cellular systems.

    Present Systems (Contd)

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    Present Systems (Cont d)

    l 3G offers higher data rates than 2.5G. This allowsusers to send/receive pictures, video clips, etc.

    l

    l This service is starting to become more and more

    available in the U.S.l

    l There are two standards for 3G, Wideband CDMA(WCDMA) and cdma2000. These two standards

    have been adopted world-wide.l

    l Both are based on CDMA principles.

    l

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    AMPS: A Model for Learning

    about Cellular Networks

    Complete Cellular Network

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    Complete Cellular Network

    A group of local base stations are connected (bywires) to a mobile switching center (MSC). MSC isconnected to the rest of the world (normal telephonesystem).

    MSC MS

    C

    MSC

    MSC

    Public(Wired)

    Telephone

    Network

    Mobile Switching Centers

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    Mobile Switching Centers

    l Mobile switching centers control and coordinate thecellular network.

    l They serve as intermediary between base stationsthat may be handing off users between eachother.

    l Base stations communicate with each via the MSC.

    l MSC keep track of cell phone user subscription.l MSC connects to the wired phone network (rest of

    the world).

    l

    The AMPS System

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    The AMPS System

    l AMPS uses FDMA: a service provider is givenlicense to 832 frequencies to use across ageographic region, say a city.

    l

    l

    Service provider chops up the city into cells.l

    l Each cell is roughly 10 square miles.l

    l Each cell has a base station that consists of a towerand a small building containing radio equipment.

    The AMPS System (Contd)

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    The AMPS System (Cont d)

    l AMPS uses frequency duplexing, i.e., each cellphone uses one frequency to transmit on andanother frequency to receive on.

    l

    l Total 832 channels are divided into half.l

    l One half is used on the uplink, i.e., used by cellphones to transmit to the base station.

    l

    l The other half is used on the downlink, i.e., used bythe base to transmit to cell phone users.l

    l

    Voice and Control Channels

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    o ce a d Co t o C a e s

    l Of the 832/2 = 416 channels, 21 of them used ascontrol channels.

    l

    l This means that there are 416-12=395 voice

    channels.l

    l Now, these voice channels are divided up amongthe cells based on the frequency reuse.

    AMPS: Voice Channels

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    VoiceChannels

    ControlChannels

    ControlChannels

    Frequency Reuse in AMPS

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    q y

    l In frequency reuse, a group of local cells usedifferent frequencies to transmit/receive signals intheir cell.

    l

    l

    This group of local cells is referred to as a cluster.l

    l

    Clustersize of 7

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    l Assume a clustersize of 7. This means that the total395 voice channels are divided into groups ofseven.

    l

    l

    Thus, each cell has about 56 voice channels. Thisis the most number of users that can besupported in a cell, i.e., roughly 10 square miles innormal environments.

    l

    l This may/may not be sufficient based on thedistribution of users.

    Clustersize of 7 (Contd)

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    ( )

    l To see what a system with clustersize of 7 looks like,color a cell with color 1.

    l

    l This cell (if drawn as a hexagon) has 6 neighbors.

    Color each of the seven neighbors using adifferent color (also different from each other).

    l

    l Now repeat this rule to get the overall reuse

    pattern.

    Clustersize of 7, Reuse Pattern

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    ,

    What if we had a smallercluster?

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    l Now consider a system with a cluster of 4.l

    l Then the number of voice channels per cell is 395/4,which is roughly 98.

    l

    l Thus, in theory, we can hold more users per cell ifthis were true.

    l

    l But there is a problem with a clustersize.

    Problem with SmallerClustersize

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    Interfering cells are closer by when clustersize is smaller

    Problem with SmallerClustersize (Contd)

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    ( )

    l If interfering cells are closer, then the totalinterference power will be larger.

    l

    l With higher interference power, the quality of thespeech signal will deteriorate.

    l

    l To reduce the interference power, we can make thecells larger.

    l

    l With larger cell, the number of users covered perunit area reduces. So, the gain (total number ofusers supported) of a smaller clustersize is not ashigh as we think.

    Directional Antenna

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    l One way to get more capacity (number of users)while maintaining cell size is to use directionalantenna.

    l

    l Assume antenna which radiates not in alldirections(360 degrees) but rather in 120 degrees only.

    l

    Directional Antenna at BaseStation

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    With 120 degree antenna, we draw the cells as:l

    l

    l

    l

    l

    Directional Antenna (Contd)

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    ( )

    l Because these directional antenna only receivesignals in particular direction, the amount ofinterference power they receive assuming aclustersize of 7 is reduced by 1/3.

    l

    l With less interference power, the speech quality ismuch better than it needs to be.

    l

    l So we can reduce the clustersize (increaseinterference power) and still have good speechquality.

    Directional Antenna

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    l Trials show that in systems with 120 degreeantenna, the clustersize can be as small as 3.

    l

    l This allows more users to be supported, while

    keeping cell size fixed.l

    l Because of the benefits offered by 120 degreeantenna, these are most readily used by base

    station towers.

    120 Degree Antenna Towers

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    Next Time

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    l Next time, we will continue discussing the AMPSsystem.

    l

    l We will also look at how digital cellular systems

    differ from AMPS and look at whats inside a cellphone and what a base station looks like.