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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.
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