cellular basic

8/3/2019 Cellular Basic

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Lecture 9: Cellular NetworkLecture 9: Cellular Network

BasicsBasics


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Cellular BasicsCellular Basics

[email protected]


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Terminologies: BS & MSTerminologies: BS & MS

Base station (BS) Access point (AP)

Mobile station (MS) SS (Subscriber station)

MT (mobile terminal) MN (mobile node)

Downlink Forward link

BSMS

Uplink Reverse link

MSBS

downlink

uplink

BS MS


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1G and 2G cellular systems1G and 2G cellular systems

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st generation AMPS Analog Analog FM modulation FDMA

2nd generation

DAMPS(IS-54) U.S. Digital PSK modulation FDM/TDMA

GSM Europe, Asia

Digital PSK modulation FDM/TDMA

IS-95 CDMA U.S. Digital PSK modulation FDM/CDMA


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Abstract model for a cellular systemAbstract model for a cellular system

Components Gateway to the wireline system (backhaul)

Base station (BS)

Database Security

Air interface

Cellular phone


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(1) Gateway to the wireline system(1) Gateway to the wireline system

Backhaul connection MSC (Mobile Switching Center)

Switch that connects mobile telephone network

to wireline telephone network (PST

N) IWF(Interworking Function) Protocol translation

E.g. circuit-switch to packet-switch


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(2) Base station(2) Base station

BS includes 2 components Transmitters/receivers

BTS (base transceiver system)

Controllers for transceivers BSC (base station controller)

Typically, 1 BSC controls several BTS


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(3) Database(3) Database

Track user location and billing information HLR (home location register)

Track current user locations

VLR (visitor location register) Track roaming users

HLR/VLR

Two-tier location database for user locationmanagement

Concept of home


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(4) Security mechanism(4) Security mechanism

Identify a phone IMEI (International Mobile Equipment Identity)

Like your ROC ID

Equipment Identity Register (EIR)

A database One of the MSCs is used as EIR

Check the identity of cell phones Stolen phones, malicious phones

Not conform to the network Conform to the network

Authentication Center (AC or AuC) Actual encryption and verification


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(5) Air interface(5) Air interface

PHY and MAC layers of wirelesscommunications standards


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(6) Cellular phone(6) Cellular phone

MS (Mobile Station)


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Basic Cellular ConceptBasic Cellular Concept

Cell Typically, cells are hexagonal

In practice, it depends on available cell sitesand radio propagation conditions

Spectrum reuse Reuse the same EM spectrum in other

geographical location

Frequency reuse factor


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Frequency ReuseFrequency Reuse

Cluster A group of cells

Frequency reuse factor

(Total # of channels in a cluster) / (Total # ofchannels in a cell)


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TDMA/FDMA Spatial Reuse

TDMA/FDMA Spatial Reuse


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A frequency reuse exampleA frequency reuse example

Example Frequency reuse factor = 7

Cluster size =7

Question What are other possible

frequency reuse patterns?


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ClusterCluster

The hexagon is an ideal choice formacrocellular coverage areas, because itclosely approximates a circle and offers awide range of tessellating reuse clustersizes.

A cluster of size N can be constructed if, N = i2 + ij + j2.

i,j are positive integer

Allowable cluster sizes are N = 1,3,4,7,9,12,


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Determine frequency reuse patternDetermine frequency reuse pattern

Co-channel interference [CCI] one of the major factors that limits cellular

system capacity

CCI arises when the same carrier frequency isused in different cells.

Determine frequency reuse factor Propagation model

Sensitivity to CCI


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Reuse distanceReuse distance

Notations D :Reuse distance

Distance to cell using the same frequency

r : Cell radius

N : Frequency reuse factor

Relationship between D and r D/r=(3N)^0.5

N = i2 + ij + j2

Proof?


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D

L * iL * j

r

In this case: j=2, i=1

Compute D based on

law of cosineNijjirD

ijjiLD

jiLjLiLD

jLiLjLiLD

3)(3/

)(

)5.0(2

)3/2cos())((2)()(

22

2222

222222

222

!!

!

!

! T

rL 3!


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Cell splittingCell splitting

Smaller cells have greater system capacity Better spatial reuse

As traffic load grows, larger cells could

split into smaller cells


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SectorsSectors

Use directional antenna reduces CCI Why? Think about it! 1 base station could apply several

directional antennas to form several

sectors 3-sector cell


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Forward link and reverse linkForward link and reverse link

Forward link Also called downlink BSMS

Reverse link

Also called uplink MSBS

How forward link and reverse link are separated? FDD (more often)

Frequency Division Duplex

TDD Time Division Duplex

Why is it more difficult to engineer a TDD system?


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More about cellularMore about cellular


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Cell size & FRFCell size & FRF

Cell size should be proportional to1/(subscriber density)

Co-channel interference is proportional to

1/D r

1/N^0.5

Path-loss model

Total system capacity is proportional to 1/N

N : Frequency reuse factor


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Example: N=7Example: N=7

Frequency reuse factor N=7 N = i2 + ij + j2

(i,j)=(1,2) or (2,1)

Other commonly used patterns N=3 (1,1)

N=4 (2,0); (0,2)

N=1 is possible CDMA


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Compute total system capacityCompute total system capacity

Example 11-1 Total coverage area = 100 mile2 = 262.4 km2

Total 1000 duplex channels

Cell radius =1

km N=4 or N=7

Whats the total system capacity for N=4and N=7?

r

22 6.22

33rrA !!


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Evolving deploymentEvolving deployment

Early stage Intermediate stage Late stage

Multiple stages of deployment

Deployment evolves with subscriber growth


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Practical deployment issuesPractical deployment issues

Location to setup antenna Antenna towers are expensive

Local people do not like BSs Antenna/BS does not look like antenna/BS

Antenna Omni-directional

Directional antenna


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Wireless QoSWireless QoS Quality of Service (QoS)

Achieving satisfactory wireless QoS is an important designobjective

Quality measures Channel availability (wireless network is available when users need

it) Blocking probability Dropping probability

Coverage: probability of receiving adequate signal level atdifferent locations

Transmission quality: fidelity/quality of received signals BER FER

Application-dependent Voice Data Multimedia


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Wireless QoSWireless QoS

Admission control Blocking

Poor reception quality

Co-channels Frequency reuse factor Cell planning

Frequency planning


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WorstWorst--Case CCI on the Forward ChannelCase CCI on the Forward Channel

Co channel interference [CCI] is one of the primelimitations on system capacity. We use the propagationmodel to calculate CCI.

There are six first-tier, co-channel BSs, two each at(approximate) distances of D-R, D, and R+D and the

worst case (average) Carrier-to-(Co-Channel)Interference [CCI] is

R= cell radius

)()(2

1FFF

F

!0

RDDRD

R

Worst case CCI

on the forward channel


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OverlayOverlay

Dual-mode or dual-frequency phones Overlay different wireless accesstechnologies Different technologies

Same technology operating in different bands Increase system capacity Reduce blocking

Example: GSM 900/1800 TDMA+CDMA


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Overlaid cellsOverlaid cells


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HandoffHandoff

Handoff threshold: typically, -90~-100 dBm(1~10uW)

Need to prevent from ping-pong effect


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RoamingRoaming

MS moves to a new cell The new cell is served by a new MSC

i.e. handoff that results in MSC change

Different types of handoff Same cell (different sectors) Same BSC

Same MSC

Different operators

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