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CELLULARANDMOBILECOMMUNICATIONS Questions&Answers

UNIT-I

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1. Drawthe generalview of telecommunication and explain the function of the each unit?

AnswerThe general view of the cellular s!stem is shown in "ig.1.

"ig.1 A general view of cellular telecommunications s!stem

Antenna

Antenna pattern# antenna gain# antenna tilting# and antenna height $ all affect the cellular s!stem design.

The antenna pattern can %e omnidirectional# directional# or an! shape in %oth the vertical and the hori&onplanes. Antenna gain compensates for the transmitted power. Different antenna patterns and antenna gains

at the cell site and at the mo%ile units would affect the s!stem performance and so must %e considered in

the s!stem design. The antenna patterns seen in cellular s!stems are different from the patterns seen in

free space. If a mo%ile unit travels around a cell site in areas with man! %uildings# the

omnidirectional antenna will not duplicate the omnipattern. In addition# if the front-to-%ac' ratio of a

directional antenna is found to %e () d* in free space# it will %e onl! 1) d* at the cell site. Antenna

tilting can reduce the

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interference to the neigh%oring cells and enhance the wea' spots in the cell. Also# the height of the cell-

site antenna can affect the area and shape of the coverage in the s!stem.

+witching ,uipment

The capacit! of switching euipment in cellular s!stems is not %ased on the num%er of switch ports %ut on

the capacit! of the processor associated with the switches. In a %ig cellular s!stem# this processor should

%e large. Also# %ecause cellular s!stems are unli'e other s!stems# it is important to consider when the

switching euipment would reach the maximum capacit!. The service life of the switching euipment is

not determined %! the life c!cle of the euipment %ut %! how long it ta'es to reach its full capacit!. If the

switching euipment is designed in modules# or as distri%uted switches# more modules can %e added to

increase the capacit! of the euipment. "or decentrali&ed s!stems# digital switches ma! %e more suita%le.

The future trend seems to %e the utili&ation of s!stem handoff. This means that switching euipment can

lin' to other switching euipment so that a call can %e carried from one s!stem to another s!stem without

the call %eing dropped.

Data in's

The data lin's are shown in "ig 1. Although the! are not directl! affected %! the cellular s!stem# the! are

important in the s!stem. ,ach data lin' can carr! multiple channel data /1) '%ps data transmitted per

channel0 from the cell site to the T+2. This fast-speed data transmission cannot %e passed through a

regular telephone line. Therefore# data %an' devices are needed. The! can %e multiplexed# man!-data

channels passing through a wide%and T-carrier wire line or going through a microwave radio lin' where

the freuenc! is much higher than 34)5&. easing T1-carrier wire lines through telephone companies

can %e costl!. Although the use of microwaves ma! %e a long-term mone! saver# the availa%ilit! of the

microwave lin' has to %e considered.

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(. ,xplain a%out NT 7 NTT +!stems.

Answer

NTT Nippon Telegraph and Telephone 8orporation /NTT0 developed an 3))-5& land mo%ile

telephone s!stem and put it into service in the To'!o area in 19:9. The general s!stem operation is

similar to the A;+ s!stem. It accesses approximatel!

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CELLULARANDMOBILECOMMUNICATIONS Questions&

Answers

NT Nordic +!stem This s!stem was %uilt mostl! %! +candinavian countries /Denmar'# Norwa!#

+weden# and "inland0 in cooperation with +audi Ara%ia and +pain and is called the NT networ'. It is

currentl! a 2>A 3)) s!stem is used to convert the

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"ig.6 A t!pical fading signal received while the mo%ile unit is moving.

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4. hat are the limitations of conventional mo%ile s!stems and how are the! overcome%!

cellular mo%ile s!stems?

Answer

imitations of conventional mo%ile telephone s!stems 2ne of man! reasons for developing a cellular

mo%ile telephone s!stem and deplo!ing it in man! cities is the operational limitations of conventional

mo%ile telephone s!stems limited service capa%ilit!# poor service performance# and inefficient freuenc!

spectrum utili&ation.

1. imited service capa%ilit! A conventional mo%ile telephone s!stem is usuall! designed %! selecting

one or more channels from a specific freuenc! allocation for use in autonomous geographic &ones# as

shown in "ig.4. The communications coverage area of each &one is normall! planned to %e as large as

possi%le# which means that the transmitted power should %e as high as the federal specification allows.

The user who starts a call in one &one has to reinitiate the call when moving into a new &one %ecause the

call will %e dropped. This is an undesira%le radio telephone s!stem since there is no guarantee that a call

can %e completed without a handoff capa%ilit!. The handoff is a process of automaticall! changing

freuencies as the mo%ile unit moves into a different freuenc! &one so that the conversation can %e

continued in a new freuenc! &one without redialing. Another disadvantage of the conventional s!stem is

that the num%er of active users is limited to the num%er of channels assigned to a particular freuenc!

&one.

"ig.4 8onventional o%ile +!stem

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Answers

;oor +ervice ;erformance In the past# a total of 66 channels were all allocated to three mo%ile

telephone s!stems o%ile Telephone +ervice /T+0# Improved o%ile Telephone +ervice /IT+0 =

s!stems# and Improved o%ile Telephone +ervice /IT+0 C s!stems. T+ operates around

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Answers

$. ,xplain a%out %asic cellular s!stem.

Answer

*asic 8ellular +!stem A %asic cellular s!stem consists of three parts a mo%ile unit# a cell site# and a

mo%ile telephone switching office /T+20# as "ig.$ shows# with connections to lin' the three su%

s!stems.

1. o%ile units A mo%ile telephone unit contains a control unit# a transceiver# and an antenna

s!stem.

(. 8ell site The cell site provides interface %etween the T+2 and the mo%ile units. it has a control

unit# radio ca%inets# antennas# a power plant# and data terminals.

6. T+2 The switching office# the central coordinating element for all cell sites# contains the cellular

processor and cellular switch. It interfaces with telephone compan! &one offices# controls call processing#

and handles %illing activities.

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"ig.$ *asic cellular s!stem

:. ,xplain the direct wave path# line of sight path# out of sight path# and o%structivepath?

Answer

A direct wave path is a path clear from the terrain contour. The line-of-sight path is a path clear from

%uildings. In the mo%ile radio environment# we do not alwa!s have a line-of-sight condition. hen a line-

of-sight condition occurs# the average received signal at the mo%ile unit at a 1-mi intercept is higher#

although the

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3. hat are the different t!pes of noises in cellular fre-uenc!ranges ,xplain in detail?

Answer

Noise level in cellular freuenc!%and The thermal noise 'T* at a temperature T of (9) C /1:G80 and

a %andwidth * of 6) '5& is -1(9 d*m# where ' is *olt&mannHs constant. Assume that the received

frontend noise is 9 d*# and then the noise level is -1() d*m.

There are two 'inds of man-made noise# the ignition noise generated %! the vehicles and the noise

generated %! 3))-5& emissions.

The ignition noise In the past# 3)) 5& was not widel! used. Therefore# the man-made noise at 3))

5& is merel! generated %! the vehicle ignition noise. The automotive noise introduced at 3)) 5& witha %andwidth of 6) '5& can %e deduced from "ig.:.

"ig.: Noise in 8ellularNetwor's

The 3))-5& emission noise As a result of the cellular mo%ile s!stems operating in all the maBor cities

in the United +tates and the spurious energ! generated outside each channel %andwidth# the earl! noise

data measurements are no longer valid. The 3))-5&-emission noise can %e measured at an idle channel

/a forward voice channel0 in the 3$9- to 39

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Amplifier noise A mo%ile radio signal received %! a receiving antenna# either at the cell site or at the

mo%ile unit# will %e amplified %! an amplifier. e would li'e to understand %ow the signal is affected %!

the amplifier noise. Assume that the amplifier %aa an availa%le power gain g and the availa%le noise

power at the output is N. The input signal-to-noise /+@N0 ratio is ;s@Ni# the output signal-to-noise ratio is

;o@No# and the internal amplifier noise is Na. Then the output ;o@No %ecomes

The noise figure " is defined as

here# the maximum possi%le +@N ratio is

measured when the load is an open circuit. The noise figure of the amplifier is

The term 'T* is the thermal noise. The noise figure is a reference measurement %etween a minimum

noise level due to thermal noise and the noise level generated %! %oth the external and internal noise of

an amplifier.

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Answers

9. ,xplain a%out the mar'eting of hexagonal cells?

Answer

1ar'eting Image of 5exagonal-shaped cells The hexagonal-shaped communication cells are artificial

and that such a shape cannot %e generated in the real world. ,ngineers draw hexagonal-shaped cell on a

la!out to simplif! the planning and design of a cellular s!stem %ecause it approaches a circular shape that

is the ideal power coverage area. The circular shapes have overlapped areas which ma'e the drawing

unclear. The hexagonal-shaped cells fit the planned area nicel!# as shown in "ig.3 with no gap and no

overlap %etween the hexagonal cells. The ideal cell shapes as well as the real cell shapes are also shown in

"ig.3.

A simple mechanism which ma'es the cellular s!stem implement- a%le %ased on hexagonal cells will %e

illustrated in later chapters. 2therwise# a statistical approach will %e used in dealing with a real-world

situation. "ortunatel!# the outcomes resulting from these two approaches are ver! close# !et the latter does

not provide a clear ph!sical picture# as shown later. *esides# toda! these hexagonal-shaped cells have

alread! %ecome a widel! promoted s!m%ol for cellular mo%ile s!stems. An anal!sis using hexagonal

cells# if it is desired# can easil! %e adapted %! the reader.

"ig.3 5exagonal 8ells and the real shapes of their coverages

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Answers

1). Differentiatethe generations in the cordlessphones and cellularphones?

Answer

8ellular ;hones In 19adiolinBa in 1990 in "inland on the

2+ standard which also mar'ed the introduction of competition in mo%ile telecoms when >adiolinBa

challenged incum%ent Telecom "inland who ran a 1-J NT networ'.

The first data services appeared on mo%ile phones starting with person-to-person ++ text messaging in"inland in 1996. "irst trial pa!ments using a mo%ile phone to pa! for a 8oca 8ola vending machine were

set in "inland in 1993. The first commercial pa!ment s!stem to mimic %an's and credit cards was

launched in the ;hilippines in 1999 simultaneousl! %! mo%ile operators Jlove and +mart. The first

content sold to mo%ile phones was the ringing tone# first launched in 1993 in "inland. The first full

Internet service on mo%ile phones was i-mode introduced %! NTTHs Do8oo in =apan in 1999. In ())1

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the first commercial launch of 6J /third generation0 was again in =apan %! NTT Do8oo on the

8DA standard.

Until the earl! 199)Hs# most mo%ile phones were too large to %e carried in a =ac'et poc'et# so the! were

t!picall! installed in vehicles as car phones. ith the miniaturi&ation of digital components and

development of more sophisticated %atteries# mo%ile phones have %ecome smaller and lighter.

8ordless phones Jeorge +weigert an amateur radio operator and inventor from 8leveland 2hio# is

largel! recogni&ed as the father of the cordless phone. 5e su%mitted a patent application in 19$$ for a

Mfull duplex wireless communication apparatusH. The U.+. patent and trademar' office awarded him a

patent in =une of 19$9. +weigert a radio operator in orld ar II stationed at the south pacific islands of

Juadalcanal and *ougainville# developed the full duplex-concept for untrained personnel# to improve

%attlefield communications for senior commanders. 5e was also licensed as 3I+ and N98 in the

amateur radio service. 5e also held a first class radio telephone operatorHs permit issued %! the "ederal

8ommunication 8ommission.

In the 193)s a num%er of manufacturers including +on! introduced cordless phones for the consumer

mar'et. The! used a %ase station that was connected to a telephone line and a handset with a microphone#

spea'er# 'e!pad and telescoping antenna. The handset contained a rechargea%le %atter! t!picall! Ni8d.

The %ase unit is powered %! household current via a wall soc'et. The %ase included a charging cradle#

which as generall! a form of tric'le charger# on which the handset rested when not in use. +ome cordless

phones now utili&e two rechargea%le AA or AAA %atteries in place of the more expensive traditional

proprietar! telephone %atteries.

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Answers

11. *riefl!explain the evaluation of the analog and digital cellular mo%ile s!stem?

Answer

8ellular telephone s!stems can %e MAnalog or MDigital. 2lder 8ellular +!stems /A;+# TA8+#

NT0

are analog and newer s!stems /J+# 8DA# ;8+0 are MDigital.

The maBor difference %etween the two s!stems is how the voice signal is transmitted %etween the phone

and %ase station. Analog and Digital refer to this transmission mechanism. It is li'e audio cassettes and

8Ds. Audio cassettes are analog and 8Ds are digital.

In either s!stem# the audio at the microphone alwa!s starts out as a voltage level that varies continuousl!over time. 5igh freuencies cause rapid changes and low freuencies cause slow changes. ith analog

s!stem the audio is directl! modulated on to a carrier. This is ver! much li'e " /not identical0 radio

where the audio signal is translated to the >" signal.

ith digital s!stems# the audio is converted to digiti&ed samples at a%out 3))) samples per second or so.

The digital samples are num%ers that represent the time var!ing voltage level at specific points in time.

These samples are now transmitted as 1s and )s. At the other end the samples are converted %ac' to

voltage levels and smoothed out so that !ou get a%out the same audio signal.

ith analog transmissions# interference />" noise or some other anomal! that affects the transmitted

signal0 gets translated directl! in to the recovered signal and there is no chec' that the received signal is

authentic. The neat thing a%out the digital is that the 1s and )s cannot %e easil! confused or distorted

during transmission plus extra data is t!picall! included in the transmission to help# detect and correct an!

errors.

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Answers

1(. ?rite notes on Digital 8ellular +!stem.

Answer

Digital 8ellular +!stem Digital cellular s!stems are the cellular s!stems that use the digital

communication techniues li'e in modulation# transmission format and demodulation and so on. The

characteristics of these s!stems are

1. These offer an effective data transmission compared to the conventional analog cellular s!stems. These

s!stems emplo! the pac'et switched communication techniue which is faster than the circuit switching

techniue.

(. These s!stems emplo! powerful error detection and 8orrection techniues# which can countr! the

de%ilitating effect of noise# fading and interference on the signal.

6. These s!stems also provide the securit! on transmitting data through encr!ption and decr!ption

techniues authentication.

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Answers

16. ,xplain the Trun'ing ,fficienc!.

Answer

Trun'ing ,fficienc! To explore the trun'ing efficienc! degradation inherent in licensing two or more

carriers rather than one# compare the trun'ing efficienc! %etween one cellular s!stem per mar'et

operating $$$ channels and two cellular s!stems per mar'et each operating 666 channels. Assume that all

freuenc! channels are evenl! divided into seven su%areas called cells. In each cell# the %loc'ing

pro%a%ilit! of ).)( is assumed. Also the average calling time is assumed to %e 1.:$ min.

ith N1E$$$@: E 94 and *E ).)( to o%tain the offered load A1 E36.1 and with N(E666@:E

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O

The trun'ing efficienc! factor can %e calculated as

"or a %loc'ing pro%a%ilit! of ( percent# "igure 16 shows# %! comparing one carrier per mar'et with more

than one carrier per mar'et situations with different %loc'ing ;ro%a%ilit! conditions. The degradation of

trun'ing efficienc! decreases as the %loc'ing pro%a%ilit! increases. As the num%er of carriers per mar'et

increases the degradation increases. 5owever# when a high percentage of %loc'ing pro%a%ilit!# sa! more

than () percent# occurs# the performance of one carrier per mar'et is alread! so poor that further

degradation %ecomes insignificant# as "ig.9 shows.

"or a ( percent %loc'ing pro%a%ilit!# the trun'ing efficienc! of one carrier per mar'et does show a greater

advantage when compared to other scenarios.

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The factor ro/t0 follows a >a!leigh distri%ution# assuming that onl! reflected waves from local

surroundings are the ones received /a normal situation for the mo%ile radio environment0. Therefore# the

term >a!leigh fading is often used.

"ig.1) A mo%ile radio signal fading representation.

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Answers

14. ,xplain a%out planning of cellular s!stem in detail.

Answer

5ow to start planning Assume that the construction permit for a cellular s!stem in a particular mar'et

area is granted# the planning stage %ecomes critical. A great deal of mone! can %e spent and !et poor

service ma! %e provided if we do not 'now how to create a good plan. "irst# we have to determine two

elements regulations and the mar'et situation.

>egulations The federal regulations administered %! the "88 are the same throughout the United

+tates. The state regulations ma! %e different from state to state# and each cit! and town ma! have its own

%uilding codes and &oning laws. *ecome familiar with the rules and regulations. +ometimes waivers need

to %e applied for ahead of time. *e sure that the plan is wor'a%le.

1ar'et situation There are three tas's to %e handled %! the mar'etingdepartment.

1. ;rediction of gross income e have to determine the population# average income# %usiness t!pes# and

%usiness &ones so that the gross income can %e predicted.

(. Understanding competitors e also need to 'now the competitorHs situation# coverage# s!stem

performance# and num%er of customers. An! s!stem should provide a uniue and outstanding service to

overcome the competition.

6. Decision of geographic coverage hat general area should ultimatel! %e covered? hat near-term

service can %e provided in a limited area? These uestions should %e answered and the decisions passed

on to the engineering department.

The engineerHsrole The engineers follow the mar'et decisions %!

1. Initiating a cellular mo%ile service in a given area %! creating a plan that uses a minimum num%er of

cell sites to cover the whole area. It is eas! for mar'eting to reuest %ut hard for the engineers to fulfill.

(. 8hec'ing the areas that mar'eting indicated were important revenue areas. The num%er of radios

/num%er of voice channels0 reuired to handle the traffic load at the %us! hours should %e

determined.

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6. +tud!ing the interference pro%lems# such as co-channel and adBacent channel interference# and the inter

modulation products gene rated at the cell sites# and finding wa!s to reduce them.

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