bcs chapter 09

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@ McGraw-Hill Education1

T L SINGAL : Wireless Communications McGraw-Hill Education 2010

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PROPRIETARY MATERIAL. 2010 The McGraw-Hill Companies, Inc. All rights reserved. No part of this PowerPoint slide may be displayed, reproduced ordistributed in any form or by any means, without the prior written permission of the publisher, or used beyond the limited distribution to teachers and educators

permitted by McGraw-Hill for their individual course preparation. If you are a student using this PowerPoint slide, you are using it without permission.

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9

A Basic

Cellular

System


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A Basic Cellular SystemLimitations of Conventional MobileTelephone System

Consideration of the Componentsof a Cellular System

Operation of a Cellular System

Performance Criteria

Planning a Cellular System


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Why Cellular Systems?

Operational limitations of conventionalmobile telephone systems:

High transmitter powerin a largeautonomous geographical service area

Limited service capability: nocontinuation of call between service areas

Poor service performance :low capacity,high blocking probabilityduring busy hours

Inefficient frequency spectrumutilization


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Definition of A Basic Cellular

System

A basic cellular system comprises of many

low power transmitters (LPTs), each

specifically designed to serve only a small area

called a cell.

The same frequency could be reusedin

different cells with sufficient distance, causingnegligible effects of interferencebetween

users of the same channel.


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A Basic Cellular System

connected to PSTN


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Parts of a Basic Cellular System


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Cell-Site Equipment (CSE)

A fixed base station used for wirelesscommunication with mobile subscriber andsignaling/data communication with MTSO

Located at the center or the edge ofcoverage region of a cell

Consists of a number of transreceivers,Tx/Rx antennas mounted on a tall tower, datalinks, and power plant

Transmission data rate on data links varyfrom 10 Kbps to several Mbps


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Mobile Telephone Switching

Office (MTSO)

The central coordinating element for all the

cell sites connected to it

Interfaces with Public Switch Telephone

Network (PSTN)

Controls call processing and handle billing

activitiesComprises of the switch and the processor


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Mobile Subscriber Unit (MSU)Comprises of a single antenna,transreceiver, and microprocessor-based

control circuit

Usesaduplexer or RF switch to separate

the transmit and receive signals

GSM mobile subscriber unit consists of

the Mobile Equipment (ME)an electronic smart card called a Subscriber

Identity Module (SIM), which plugs into the ME


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Operation of a Cellular System

From a users point of view, the

operation can be divided into four parts

and a handoff procedureMobile unit initialization

Mobile originated calls

Network originated calls

Call termination

Handoff procedure


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Mobile Unit Initialization

When a mobile is switched ON, it scans allcontrol channels and selects the strongest one(usually from the nearest cell site).

This self-location scheme is user-independent and repeated periodically toupdate.

Mobile gets registered itself with the cell site

as being active.The MTSO can then track the location ofthe mobile by paging it on control channel.

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Mobile Originated Calls

The mobile subscriber places the callednumber and press the `sendbutton.

A request for service is sent on a

selected control channel.The cell site receives it and sends a requestto MTSO for allocation of resources.

After authentication, MTSO directs cell

site to allocate a voice channel for the call.

MTSO also connects the called subscriber.

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Call Processing in Cellular

System

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Network Originated Calls

MTSO receives a call request from Network.

It sends a message to certain cell sites,based on called mobile number and search

algorithm.Each cell site transmits the page on itscontrol channel.

The called mobile unit recognizes its ownID, and responds to the cell site.

It locks on to assigned voice channel andinitiate user alert tone.

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Call Termination

When the mobile unit terminates the

call, a particular message signal is

transmitted to the cell site.The voice channel is made free.

The mobile unit resumes monitoring

page messages through the strongestcontrol channel.



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Handoff Procedure

During the call, when the mobile unit movesout of the coverage area of its cell site, thereceived signal becomes weak.

The present cell site requests a handoff toMTSO.

The MTSO switches the call to a new voicechannel in a new cell site automatically.

The call continues as long as theconversation is on.The mobile user does not notice the handoffoccurrences.



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MSUs Operating on Same MTSO

MSU1CSEBSCMSCBSCCSEMSU2



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MSUs Operating on Different MTSOs

MSU1CSE1MTSO-APSTN MTSO-BCSE2MSU2



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Performance Criteria

Voice Quality

Grade of Service

Spectral Efficiency

Radio Capacity

Service Quality and SpecialFeatures



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Voice Quality (Circuit Merit Level

and MOS)



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Trunking and Grade of ServiceOffered traffic load, A av= x H

where = average rate of calls, and H =average holding time per successful call

Total offered traffic intensity, A t= Z x A av

ErlangBformula (also known as theblocked calls cleared formula) provides a

conservative estimate of the GOS for aninfinite number of mobile subscribers



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Spectral EfficiencySpectral efficiency defined in terms ofchannels/MHz/km2can be expressed as

1= (Total number of channels)/ [(system

bandwidth) (total coverage area)]Spectral efficiency defined in terms ofErlangs/MHz/km2can be expressed as

2= (Total traffic load)/ [(system bandwidth)

(total coverage area)]Overall system spectral efficiency,

= 1x 2



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Spectral Efficiency of FDMA SystemThe spectral efficiency of FDMA is the ratio

of total bandwidth available for user data

transmission and allocated system bandwidth

in a cluster, that is,

f= (Ndx Bc) / Bt < 1

1= Nd/cell/ (Btx Acell)channels/MHz/km2

2= (t rx Nd/cell) / (Btx Acell)Erlangs/MHz/km2



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Spectral Efficiency of TDMA System



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Radio Capacity and C/I



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Analog AMPS with Digital TDMA Cellular



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Dropped Call RateThe dropped call is defined as the drop of an

on-going call after the call is established but

before it is properly terminated.

If there is a possibility of a call drop due tonon-availability of voice channel, this is

counted as a blocked call, not a dropped call.

If there is a possibility of a call drop due tothe poor signalof assigned voice channel,

this is considered a dropped call



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Dropped Call RateEvaluationDropped call can also happen when the

mobile units are at a standstill and the radio

carrier is changed from a strong setup channel

to a weak voice channel due to the selectivefrequency fading phenomenon.

The dropped call rate and the specified

voice quality are inversely proportional.

If voice quality is not to be maintained,

dropped call rate can be set very low.



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Factors affecting Dropped Call Rate

Provide signal coverage based on theprobability (say 90%) that all the receivedsignals will be above the specified threshold

signal level.Maintain the specified co-channel andadjacent channel interference levels in eachcell during a busy hour i.e. worst case.

The response time for a handoff request hasto be shorter in order to reduce the calldropped rate.



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Service Quality ParametersCoverage:The cellular system shouldserve an area as large as possible. However,with radio coverage, radio system usually tries

to cover 90% of an area in flat terrain and 75%of an area in hilly terrain.

Required grade of servicemeans lessblocking probability (


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Special FeaturesA cellular system would like to provide as

many special features as possible, such as

call forwarding

call waiting

automatic roaming

SMS, etc.

For some special services, the customersmight have to pay extra charges.



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Planning a Cellular System

A good Plan is necessary to ensure agood service to the Customers.

Objectives of the cellular system

planning are

To find the optimal network solution

Scalability in terms of capacity demand

Future technology evolution



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Stages of Cellular Planning

Regulations and Market Situations

Engineers Role

Finding Solutions

Become fully familiar with rulesand regulations administered bythe central and state government

agencies Be sure that the plan is workable



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Market Situations

Prediction of gross incomebased on thepopulation, average income, businesstypes, business zones, etc.

Understanding competitorsincludingtheir area coverage, system performanceand number of customers. Provide aunique and outstanding service.

Decision of geographic coverage,depending upon the type of service to beprovided.



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Engineers Role Create a plan that uses minimum numbers of cell

sites to cover the given area.

Determine the number of voice channels requiredto handle the traffic load at the busy hours

Study the interference problems.

Study blocking probability at each cell site.

Check that area being covered is importantrevenue area.

Plan to absorb new customers depending onservice charges, system performance andseasons of the year.



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Finding Solutions

Use practical design tools

to create a plan that uses minimum numbers of

cell sites.

To determine the number of voice channelsrequired to handle the traffic load at the busy hours

Find way to reduce interference problems

Try to minimize blocking problems

Try to develop new technologies to utilize fullythe limited rf spectrum assigned

Request for a large spectrum, if necessary



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Network Planning Process



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Nature and Scope of Planning

Areas

Radio network planning

Transmission network planning Signalling network planning

External connectivity network

planning



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Timing and Information Flow



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Summary

Essential components of a basic cellular

system are CSE, MTSO and MSU.

Performance determining parametersinclude voice quality, grade of service,

spectrum efficiency, radio capacity,

service quality, and special features.

A good system plan is necessary to provide

a satisfying service to the customers.



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