cellular coomunications

8/12/2019 Cellular Coomunications

1/46

1

Examples of

Mobile Radio Systems Used ineveryday life:

Remote controllers for home

entertainment Cordless telephones

Hand-held walkie-talkies

Pagers/beepers Cellular telephones


2/46

2

... Examples of Mobile Radio Systems

MobileDescribes a radio terminalattached to a high speed mobile platform(e.g., A cellular phone in a fast movingvehicle).

PortableDescribes a radio terminal thatcan be hand-held and used by someone atwalking speed (e.g., cordless telephone).

SubscriberMobile or portable user. Base stationsLink mobiles through a

backbone network.


3/46

3

Types of Mobile Radio

Transmission Systems

SimplexCommunication ispossible only in one direction,(e.g., paging systems).

Half DuplexTwo waycommunication, but uses thesame radio channel for both

transmission and reception.User can only transmit orreceive information.


4/46

4

...Types of Mobile Radio

Transmission Systems

Full DuplexSimultaneoustwo-way radio transmission and receptionbetween subscriber and base station.

Two simultaneous but separate channels(FDD) or

Adjacent timeslots on a single radio

channel (TDD)


5/46

5

Wireless Telephone Systems

Full duplex communication Few hundred meters

FixedPort

(Base

Station)

wirelesslinkPublic

SwitchedTelephoneNetwork

(PSTN) cordlesshandset


6/46

6

Paging Systems:

Wide Area System

The paging control center dispatches pagesreceived from the PSTN throughout severalcities at the same time.


7/467

Paging systems are communication

systems that send brief messages toa subscriber...

Numeric messages Alpha-numeric message

Voice message

News headlines Stock quotes

Faxes


8/468

Paging Systems

Coverage Area 2 to 5 km

Within individual buildings

Worldwide coverage


9/469


10/4610

Cellular

SystemBase stations(towers)provide radioaccessbetweenmobile usersand MSC.

Mobile SwitchingCenter PSTN


11/4611

FVCRVCFCCRCC

Base Station - Mobile Network

Forward Voice ChannelReverseVoice ChannelForward Control ChannelReverse Control Channel


12/4612

Functions of Cellular System

Provides wireless connectionto the PSTN for any user locationwithin the radio range of the system.

High capacity is achieved:

by limiting the coverage of eachbase station transmitter to a smallgeographical area called a cell, and

by reusing the same radio channels inanother base station located somedistance awayFrequency reuse.


13/4613

...Functions of Cellular System

Switching system, calledhandoff, enables call to proceeduninterrupted when theuser moves from one cell to another.

Typical MSC handles100,000 cellular users and5,000 simultaneous conversations

at a time.


14/4614

Roaming

All cellular systems provide a service calledroaming.This allows subscribers to operate in serviceareas other than the one from which service

is subscribed. When a mobile enters a city or geographic

area that is different from its home service

area, it is registered as a roamerin the newservice area.


15/4615

Cellular Concept

RF spectrum is a valuable and scarce commodity RF signals attenuate over distance

Cellular network divides coverage area into cells, eachserved by its own base station transceiver and antenna

Low (er) power transmitters used by BSs; transmission

range determines cell boundary RF spectrum divided into distinct groups of channels

Adjacent cells are (usually) assigned different channelgroups to avoid interference

Cells separated by a sufficiently large distance to avoidmutual interference can be assigned the same channelgroupfrequency reuse among co-channel cells


16/4616

The Cellular Concept

System Design Fundamentals The cellular concept was a major

breakthrough in solving the problem of

spectral congestionand user capacity. Replaces single high power transmitter

(large cell) with many low powertransmitters (small cells), each providing

coverage to only a small portion of theservice area.


17/4617

Frequency Reuse

Each cellular base station is allocated agroup of radio channels.Base stations in adjacent cells are assignedchannel groups which contain differentchannels than neighboring cells.


18/4618

Cells with the same letter, usethe same set of frequencies.

A cell clusteris outlined

in bold, and replicated overthe coverage area.

In this example, thecluster size, N, is equal to 7;and the frequency reuse factor is 1/7,since each cell contains 1/7 of the totalnumber of available channels.

A CB

GF D

GE

DBB

F FE E

CA AGC

D

Cellular Frequency Reuse Concept


19/4619

Choices:

Factors:Choices of Hexagonal Cell

Equal area

No overlap between cells

A1

SS

S

A2 A3


20/46

20

For a given S

A3 > A1A3 > A2

Here, A3 provides maximum coverage areafor a given value of S.

Actual cellular footprint is determined by thecontour of a given transmitting antenna.

By using hexagon geometry, the fewest

number of cells covers a given geographicregion.


21/46

21

Channel Capacity

Let a cellular system have total ofS duplex channels for use.

If S channels are divided into N cells

(in a cluster) into unique and disjoint channelgroups which each has the same number ofchannels, total number of available radiochannels is:

S = KNWhere K is the number of channels / cell.


22/46

22

Channel Capacity

If a cluster is replicated M times within thesystem, the total number of duplex channels,C, or the capacity, is

C = MKN = MS.

Cluster size N = 4, 7 or 12


23/46

23

Design of cluster size N

In order to connect without gaps betweenadjacent 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


24/46

24

To Find the Nearest

Co-channel Neighbor ofParticular Cell:

Move i cells along any chain or hexagon.

Then turn 60 degrees counterclockwise andmove j cells.


25/46

25

How to Locate Co-channelCells in a Cellular System

In thisexample,N = 19

(i.e.,i = 3,

j = 2)A

AA

AA

A

A


26/46

26

Handoff Strategies

Handoff- when a mobile moves into a differentcell while a conversation is in progress, theMSC automatically transfers the call to a newchannel belonging to the new base station

Important task in any cellular radio system

Handoffs must be performed successfully,

as infrequently as possible and not visibleto users.


27/46

27

A Handoff Scenarioat Cell Boundary...


28/46

28

(a) Improper Handoff Situation

Received

signallev

el Level at point A

Handoff threshold

Minimum acceptable

signal to maintain the call

Time

Level at point B(call is terminated)

Pn

Pm

A B

BS1 BS2

PnPm =

should not be too large


29/46

29

Figure,a case where a hand off is not madeand the signal drops below the minimumacceptable level to keep the channel active

This dropped call event can happen whenthere is an excessive delay by the mobileswitching center in assigning a hand off.


30/46

30

(b) Proper Handoff Situation

Received

signallev

el

Level at point B

Level at whichhandoff is made

TimeA B

BS1 BS2


31/46

31

Dwell time

Time over which a call may be maintainedwithin a cell, without hand-off.

Each base station constantly monitors thesignal strength of all its reverse voicechannels to determine the relative locationof each mobile user with respect to the basestation tower.


32/46

32

Interference and

System CapacityMajor limiting factor in performance of cellularradio systems - two main types:

Co-channel interference Adjacent channel interference


33/46

33

Co-Channel Interference

Cells that use the same set of frequencies arecalled co-channel cells.

Interference between the cells is calledco-channel interference.


34/46

34

Co-Channel Interference

Signal to interference ratio (SIR) or S/Ifor amobile receiver is given by:

S/I= SIR = S /( Ii)

S = signal power from designated base station

Ii =interference power caused by the ithinterfering co-channel cell base station

i 1io


35/46

35

First Tier of Co-channel Cells

for a Cluster Size of N = 7When the mobile is at the cell boundary(point x), it experiences worstcaseco-channel interference on the forwardchannel.

The marked distances between the mobile

and different co-channel cells are basedon approximations made for easy analysis.


36/46

36

First Tier ofCo-Channel

Cells for aCluster Sizeof N = 7

R- radius of thecell

D-distancebetween

centers of thenearestco-channelcells

A

AA

x

R

A

A

A

D-R

D-RD

D

D+R

D+R


37/46

37

Adjacent Channel Interference


38/46

38

Adjacent Channel Interference

Interference resulting from signals whichare adjacent in frequency to the desiredsignal.

Due to imperfect receiver filters that allownearby frequencies to leak into pass band.

Can be minimized by careful filtering andassignments; and, by keeping frequencyseparation between channels in a given cellas large as possible, the adjacent channelinterference may be reduced considerably.


39/46

39

Capacity in cellular systems

As the demand for wireless serviceincreases ,the number of channels assignedto a cell becomes insufficient to support therequired number of users

Increases the need to provide morechannels

Techniques to expand the capacity of

cellular systems: Cell splitting sectoring


40/46

40

Cell Splitting

Subdivides a congested cell into smaller cells, eachwith its own base station.

Increases the capacity of a cellular system, since itincreases the number of times the channels are

reused.

By defining new cells which have a smaller cells,called microcells, between the existing cells

,capacity increases due to additional number ofchannels per unit area

By decreasing the cell radius R and keeping the cochannel reuse ratio D/R unchanged ,cell splittingincreases the number of channels per unit area


41/46

41

New cells have its own base station

with reduced antenna height andreduced power


42/46

42

Sectoring Achieves capacity improvement by essentially rescaling

the system. Cell radius R is unchanged but the

co-channel ratio D / R is decreased.

Capacity improvement is achieved by reducing the number

of cells in a cluster, and this increases frequency reuse. Channels used in a particular cell are broken down in to

sectored groups and are used only within a particularsector

Replacing a single omni-directional antenna at base station

with several directional antennas, each radiating within aspecified sector.

The technique for decreasing the co channel interferenceand thus increasing system performance by usingdirectional antennas is called sectoring


43/46

43

Each sector uses a subset of the frequencychannels of the original cellS/I increases, hence cluster size can be reducedto increase the capacity


44/46

44

Cellular concepts -summary

Overall goal: maximize the number of userswhile maintaining acceptable grade ofservice

Constraints:

Limited spectrum

Properties of radio medium

Interference

Key technique: frequency reuse


45/46

45


46/46

cellular coomunications

Documents