lecture 2_cellular [compatibility mode]

7/27/2019 Lecture 2_Cellular [Compatibility Mode]

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Lecture 2:

The Cellular Concept System DesignFundamentals

ng L Khoa

Class 2


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OutlineOutline

Cellular Concept

Handoff

Cochannel Interference

Trunking and Grade of Service

Call procedure

Faculty of Electronics & Telecommunications


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Cellular ConceptCellular Concept

The limited capacity of the first mobile radio-telephone services was related

to the spectrum usednot much sharing and a lot of bandwidth dedicated to

a single call.

good coverage interference: impossible to reuse the same frequency

The cellular concept addressed many of the shortcomings of the first mobile


Frequency reuse

Wasted spectrum allocated to a single user

In 1968, Bell Labs proposed the cellular telephony concept to the FCC

It was approved and then the work began! FCC allocated spectrum (took away TV UHF channels 70-83) in the 825-

845 MHz and 870-890 MHz bands

AT&T put up a developmental system in Chicago


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Cellular Network ArchitectureCellular Network Architecture

Mobile

Switchin

Public


Center networkand Internet

Mobile

Switching

Center

Wired network


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Cellular ConceptCellular Concept

developed by Bell Labs 1960s-70s

areas divided into cells

a system approach, no major technological changes

a few hundred meters in some cities, 10s km at country side


each gets portion of total number of channels

neighboring cells assigned different groups of channels,

interference minimized

hexagon geometry cell shape


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Frequency reuse conceptFrequency reuse concept



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Frequency ReuseFrequency Reuse

Adjacent cells assigned different frequencies to avoid

interference or crosstalk

Objective is to reuse frequency in nearby cells 10 to 50 frequencies assigned to each cell

transmission power controlled to limit power at that frequency


the issue is to determine how many cells must intervene between

two cells using the same frequency


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Frequency ReuseFrequency Reuse

each cell allocated a group k channels

a cluster has N cells with unique and disjoint channel

groups, N typically 4, 7, 12

total number of duplex channels S = kN

Cluster repeated M times in a system


C = MkN = MS

Smaller cells higher M higher C

+ Channel reuse higher capacity

+ Lower power requirements for mobiles Additional base stations required

More frequent handoffs

Greater chance of hot spots


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Effect of cluster size NEffect of cluster size N

channels unique in same cluster, repeated over clusters

keep cell size same

large N : weaker interference, but lower capacity small N: higher capacity, more interference need to maintain

certain S/I level


requency reuse actor: N each cell within a cluster assigned 1/N of the total available

channels

In most of the current networks, frequency reuse factor is 1.


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Design of cluster sizeDesign of cluster size

In order to connect without gaps between adjacent cells (to

Tessellate)

N = i2

+ ij + j2

where i and j are non-negative integers Example i = 2, j = 1

N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7


Next page example move i cells along any chain or hexagon.

then turn 60 degrees counterclockwise and move j cells.

Example 3.1 in page 61


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ExampleExample

N=19

(i=3, j=2)



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Channel Assignment StrategiesChannel Assignment Strategies

Fixed Channel Assignments

Each cell is allocated a predetermined set of voice channels.

If all the channels in that cell are occupied, the call is blocked, andthe subscriber does not receive service.

Variation includes a borrowing strategy: a cell is allowed to

borrow channels from a neighboring cell if all its own channels are


occupied. This is supervised by the Mobile Switch Center: Connects cells to

wide area network; Manages call setup; Handles mobility


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Channel Assignment StrategiesChannel Assignment Strategies

Dynamic Channel Assignments

Voice channels are not allocated to different cells permanently.

Each time a call request is made, the serving base station requests

a channel from the MSC.

The switch then allocates a channel to the requested call based on

a decision algorithm taking into account different factors:


frequency re-use of candidate channel and cost factors. Dynamic channel assignment is more complex (real time), but

reduces likelihood of blocking


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Handover/handoffHandover/handoff

Reasons for handover

Moving out of range

Load balancing Cell, BSC (base station controller), MSC (mobile switching

center)


Handover scenarios Intra-cell handover (e.g., change frequency due to

narrowband interference)

Inter-cell, intra-BSC handover (e.g., movement across cells) Inter-BSC, intra-MSC handover (e.g., movement across

BSC)

Inter MSC handover (e.g., movement across MSC)


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Four Types of HandoffFour Types of Handoff

MS MS MS MS

12 3 4


MSC MSC

BSC BSCBSC

BTS BTS BTSBTS


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HandoffsHandoffs

important task in any cellular radio system

must be performed successfully, infrequently, and imperceptible

to users. identify a new base station

channel allocation in new base station


high priority than initiation request (block new calls rather thandrop existing calls)


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HandoffHandoff

=handoff threshold -

Minimum acceptable

signal to maintain the call

too small:

Insufficient time


before call is lost

More call losses

too large:

Too many handoffs

Burden for MSC


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Styles of HandoffStyles of Handoff

Network Controlled Handoff (NCHO)

in first generation cellular system, each base station constantly monitorssignal strength from mobiles in its cell

based on the measures, MSC decides if handoff necessary mobile plays passive role in process

burden on MSC


present in second generation systems mobile measures received power from surrounding base stations and

report to serving base station

handoff initiated when power received from a neighboring cell exceeds

current value by a certain level or for a certain period of time faster since measurements made by mobiles, MSC dont need monitor

signal strength

Mobile Controlled Handoff


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Types of HandoffTypes of Handoff

Hard handoff -

(break before

make)

FDMA, TDMA

mobile has

radio link with


only one BS atanytime

old BS

connection is

terminatedbefore new BS

connection is

made.


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Soft handoff (make before break)

CDMA systems

mobile has simultaneous radio link with more than one BS at any

time new BS connection is made before old BS connection is broken

mobile unit remains in this state until one base station clearlyredominates


http://www.youtube.com/watch?v=mYdk89hMyss


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Vertical handoff



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Prioritizing handoffPrioritizing handoff

Dropping a call is more annoying than line busy

Guard channel concept

Reserve some channels for handoffs Waste of bandwidth

But can be dynamically predicted


Queuing of handoff requests There is a gap between time for handoff and time to drop.

Better tradeoff between dropping call probability and network

traffic.

Reduce the burden for handoff

Cell dragging

Umbrella cell


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Umbrella CellUmbrella Cell



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Interference and System CapacityInterference and System Capacity

major limiting factor in performance of cellular radio systems

sources of interference:

other mobiles in same cell

a call in progress in a neighboring cell

other base stations operating in the same frequency band

Non-cellular s stem leakin ener into the cellular fre uenc


band effect of interference:

voice channel: cross talk

control channel: missed or blocked calls

two main types:

co-channel interference

adjacent channel interference


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CoCo--Channel InterferenceChannel Interference

cells that use the same set of frequencies are called co-channel

cells.

Interference between the cells is called co-channel interference.

Co-channel reuse ratio: Q = D/R=sqrt(3N)

R: radius of cell


D: distance between nearest co-channel cells

Small Q small cluster size N large capacity

large Q good transmission quality

tradeoff must be made in actual cellular design


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CoCo--channel Reuse Ratiochannel Reuse Ratio


SINR Power

propagation

factor 2-6=

+

=0

1

2i

i

iI

S

I

S

=

=

0

0

log10)()(d

ddBmPdBmP

d

dPP

tr

tr


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Cochannel Interference

SINR

Power: propagation factor 2-6

Sun, nuclear bomb

Approximation

=

+

=0

1

2i

i

iI

S

I

S

=

=

0

0

log10)()(d

ddBmPdBmP

d

dPP

tr

tr

)3()/(0

NRDRSi

===


AMPS example

=4, S/I=18dB, N needs to be larger than 6.49.

Reuse factor 1/N small

Relations: cochannel interference, link quality, reuse factor

Example 3.2

00

1 )(Di i

=


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Worst Case Interference

S/I ~ R-4/[2(D-R)-4 + 2(D+R)-4 + 2D-4]



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Adjacent Channel InterferenceAdjacent Channel Interference

Interference resulting from signals where are adjacent in

frequency to the desired signal.

Due to imperfect receiver filters that allow

nearby frequencies to leak into pass band.

Can be minimized by careful filtering and assignments, and by


keeping frequency separation between channel in a given cell as

large as possible, the adjacent channel interference may be

reduced considerably.

Example 3.3


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Channel Planning and Power ControlChannel Planning and Power Control

Cell planning

Control channel 5%

Voice/data channel

f1/f2 cell planning

Breathing cell


Power Control

Open loop

Close loop

800Hz in CDMA2000, 1500Hz in WCDMA


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TrunkingTrunking

Trunking: the channel is allocated on demand and recycle after usage

Tradeoff between the number of channels and blocking probability

Grade of service Likelihood of a call is blocked or the delay greater than a thresholdduring the busiest time.


Erlang, a Danish Mathematician studied how a large population could beaccommodated by a limited number of servers.

Erlang capacity: the percentage of line/channel occupied over time


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Terms of Trunking TheoryTerms of Trunking Theory



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Trunking TheoryTrunking Theory

Each user generates a traffic intensity of Au Erlangs.

Au=H, where H is the average duration of a call and is the average

number of call requests per unit time for each user

A=UAu, where U no. of users and A is the total offered traffic intensity Ac=UAu/C, where C is the number of channels

Offered traffic >= the traffic carried by the trunked system


First type of trunked system

Blocked calls cleared: no queuing for call requests, no setup time

M/M/m/m: memory-less, Poisson arrival, exponential service, finite

channels.

Erland B formula GOS

K

Ablocking C

k

kC

AC

==

=0

!

!

)Pr(


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Erlang B ExampleErlang B Example



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Erlang BErlang B



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Blocked Calls DelayedBlocked Calls Delayed

Blocking calls are delayed until channels are available, queuing

Erlang C

=

+

=>

1

0 !)1(!

)0Pr(C

k

k

CAC

C

kACA

Adelay


Exponential service distributions

Pr[delay>t]=Pr[delay>0]Pr[delay>t|delay>0]

=Pr[delay>0]exp(-(C-A)t/H)

The average delay D

D=Pr[delay>0]H/(C-A)


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Erlang CErlang C



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ExamplesExamples

Example 3.4

Example 3.5

Example 3.6 Example 3.7



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Approaches to Increasing Capacity

Frequency borrowing

frequencies are taken from adjacent cells by congested cells

Cell splitting cells in areas of high usage can be split into smaller cells

Cell sectoring


cells are divided into a number of wedge-shaped sectors, each with

their own set of channels

Microcells

antennas move to buildings, hills, and lamp posts


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Cell Splitting

subdivide a congested cell into smaller cells

each with its own base station,

reduction in antenna and

transmitter power

more cells more clusters


achieves capacity

improvement by essentially

rescaling the system.

Cell Splitting fromradius R to R/2

Table 3.4 example


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Cell Splitting ExampleCell Splitting Example Power reduction is 16 times and 12 dB for half cell radius for

propagation factor is 4.

antenna downtilting

Umbrella cell

Example 3.8



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SectoringSectoring

In basic form, antennas are omnidirectional

Replacing a single omni-directional antenna at base station with

several directional antennas, each radiating within a specified sector.

achieves capacity improvement by essentially rescaling the system.

less co-channel interference, number of cells in a cluster can be

reduced

EE 542/452 Spring 2008

Larger frequency reuse factor, larger capacity

S t i E lS t i E l


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Sectoring ExamplesSectoring Examples

Only two cochannel cell

S/I improvement 7.2dB

Capacity 12/7 First type handoff


Urban area not good

Example 3.9

R tR t


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RepeaterRepeater Extend coverage range

Directional antenna or distributed antenna systems



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Micro Cell Zone

Superior to sectoring, any base station channel may be assigned

to any zone by the base station

Same channel

No handoff

Only the active zone



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Micro Cell Zone Concept

Large control base station is replaced by several lower powered

transmitters on the edge of the cell.

The mobile retains the same channel and the base station simply

switches the channel to a different zone site and the mobilemoves from zone to zone.


which mobile is traveling, base station radiation is localized andinterference is reduced.

ExampleExample


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ExampleExample

Dz/Rz=7, but D/R is 3.

2.33 times

capacity gain


B i f O tli f C ll l P


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Brief Outline of Cellular Process

Telephone call to mobile user

Step 1 The incoming telephone call to Mobile X is received at the MSC.

Step 2 The MSC dispatches the request to all base stations in the cellularsystem.

Step 3 The base stations broadcast the Mobile Identification Number(MIN), telephone number of Mobile X, as a paging message over the FCCthroughout the cellular system.


monitors and responds by identifying itself over the reverse control channel.

Step 5 The base station relays the acknowledgement sent by the mobileand informs the MSC of the handshake.

Step 6 The MSC instructs the base station to move the call to an issuedvoice channel within in the cell.

Step 7 The base station signals the mobile to change frequencies to anunused forward and reverse voice channel pair.

At the same time, another data message (alert) is transmitted over the forwardvoice channel to instruct the mobile to ring.

T l h C ll Pl d b M bil


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Telephone Call Placed by Mobile

Step 1 When a mobile originates a call, it sends the base

station its telephone number (MIN), electronic serial number

(ESN), and telephone number of called party. It also transmits a

station class mark (SCM) which indicates what the maximumpower level is for the particular user.

Step 2 The cell base station receives the data and sends it to


the MSC.

Step 3 The MSC validates the request, makes connection to

the called party through the PSTN and validates the base station

and mobile user to move to an unused forward and reverse

channel pair to allow the conversation to begin.

R i


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Roaming

All cellular systems provide a service called roaming. This

allows subscribers to operate in service areas other than the one

from which service is subscribed.

When a mobile enters a city or geographic area that is differentfrom its home service area, it is registered as a roamer in the

new service area.


Registration MSC polls for unregistered mobiles

Mobiles respond with MINs

MSC queries mobiles home for billing info Calls

MSC controls call and bills mobiles home

HomeworkHomework


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HomeworkHomework

3.1

3.5

3.7

3.9


.

3.15

3.27

3.28

3.29

lecture 2_cellular [compatibility mode]

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