Section VII 30 Communicating Information

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Communicating Information 30. Communicating information

Content

30.1 Principles of modulation 30.2 Sidebands and bandwidth

30.3 Transmission of information by digital means

30.4 Different channels of communication

30.5 The mobile-phone network

Learning outcomes

Candidates should be able to:

(a) understand the term modulation and be able to distinguish

between amplitude modulation (AM) and frequency modulation (FM)

(b) recall that a carrier wave, amplitude modulated by a single audio

frequency, is equivalent to the carrier wave frequency together with two sideband

frequencies (c) understand the term bandwidth

(d) demonstrate an awareness of the relative advantages of AM and FM transmissions

(e) recall the advantages of the transmission of data in digital form

(f) understand that the digital transmission of speech or music involves analogue-to-digital conversion (ADC) on transmission and digital-to-analogue conversion (DAC) on

reception


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(g) show an understanding of the effect of the sampling rate and thenumber of bits in each sample on the reproduction of an input signal

(h) appreciate that information may be carried by a number ofdifferent channels, including wire-pairs, coaxial cables, radio and

microwave links and optic fibres

(i) discuss the relative advantages and disadvantages of channelsof communication in terms of available bandwidth, noise, crosslinking,

security, signal attenuation, repeaters and regeneration, cost andconvenience

(j) describe the use of satellites in communication (k) recall the relative merits of both geostationary and polar orbiting

satellites for communicating information

(l) recall the frequencies and wavelengths used in different channels

of communication (m) understand and use signal attenuation expressed in dB and dB perunit length


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(o) understand that in a mobile-phone system, the public switchednetwork (PSTN) is linked to base stations via a cellular exchange

(p) understand the need for an area to be divided into a number ofcells, each cell served by a base station

(q) understand the role of the base station and the cellularexchange during the making of a call from a mobile phone set

(r) recall a simplified block diagram of a mobile phone handset andunderstand the function of each block


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Principles of modulation,sidebands and bandwidth


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Communication

Any system of communication must have transmitter and a receiver

A simple system of communication at a short distance could be one personA speaking to another person B A is the transmitter B is the receiver Communication system is sound waves

For 2 people in different rooms, a system of communication could be

a microphoneA in one room a loudspeaker B in another room communication system is a twin pair of wires the microphone converts the sound waves into electrical signals that is

transmitted along the wires to the loudspeaker where it is converted back intosound waves

Communication of could also be achieved using radio waves the signal from the microphone would be amplified and applied to atransmitting aerial.

the radio waves produced by the aerial would be transmitted and picked up by areceiving aerial.

after amplification the received signals would be passed to a loudspeaker


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Disadvantages

Transmitting aerial Receiving aerial

Microphone Speaker

Amplifier Amplifier

The 3 systems of communication in the earlier slide have 3 seriousdisadvantages only one system could operate in an area because the receiver/receiving aerial

would not be selective i.e. it would pick up all signals or any signal

the aerial required for the transmission of the low frequencies of sound

waves(about 20 Hz to 20 kHz) would be very long (Try calculating it!) the electrical power required for transmission over long distances would be very

large

All problems are solved by a process known as modulation


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Modulation

Modulation is a process whereby a high frequency wave, known as acarrier wave is transmitted. This carrier wave has either the amplitudevariedor the frequency variedso as to carry information

pg 427 Fig 16.2 A/AS level Physics Chris Mee

In amplitude modulation(AM), the carrier wave has constant frequency.The amplitude of the carrier wave is made to vary in synchrony with thedisplacement of the information signal

The rate at which the amplitude of the carrier wave varies is relateddirectly to the frequency of the information signal

Amplitude modulation is not the same as superposition. Superpositioninvolves the addition of displacements, whereas AM is achieved bymultiplication of the displacements

In frequency modulation(FM), the amplitude of the carrier wave remainsconstant.The frequency of the carrier is made to vary in synchrony withthe displacement of the information signal

The use of a carrier wave allows different radio stations in the same area totransmit at the same time, but each radio station has a different transmittingcarrier wave frequency.

The receiver is tuned or adjusted to the frequency of whichever transmitteror radio station is desired i.e. the receiver accepts the signal transmitted onthat particular carrier wave and rejects all other carrier wave frequencies


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Example

A sinusoidal carrier wave has a frequency of 800 kHz and an amplitude of 5.0V. The frequency deviation of the carrier wave is 30 kHz V-1 i.e. for every 1.0V change in displacement of the signal, the frequency of the carrier wavechanges by 30 kHz. The carrier wave is frequency-modulated by a sinusoidalsignal of frequency 10 kHz and amplitude 2.0 V. Describe the modulatedcarrier wave.

Solution Amplitude of information signal = 2.0 V.

This gives a frequency variation of (2 x 30) = 60 kHz

The carrier wave has a constant amplitude of 5.0 V

The carrier frequency changes from 740 kHz to 860 kHz

This change of frequency occurs 10000 times per second


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AM - sidebands and bandwidth If a carrier wave of frequency fC is AM by an information signal having only one

frequency fS, and this waveform is analysed, the AM wave is found to be made up of3 frequencies namely (f

C

+ fS

), fC

, and (fC

fS

)

The frequency spectrum of this modulated wave i.e. the graph showing the variationwith frequency of the amplitudes of each component is as below:

Amplitude

(fC + fS) fC (fCfS) Frequency

The central frequency fC is the frequency of the carrier wave and it has the largestamplitude

The other 2 frequencies, (fC + fS) and (fC fS) are known as the sidebandfrequenciesor just sidebands

The bandwidth is the range of frequencies occupied by the AM waveform. Thisbandwidth is equal to 2fS


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If music is the food of love, then play on!

For the broadcast of music, the information signal will contain a wide rangeof frequencies from about 20 Hz to 20 kHz

A typical frequency spectrum for such an AM wave will have multiplesidebands on either side of the carrier wave frequency

In practice, the very high frequencies in music may not be broadcast so asto reduce the bandwidth of the transmitted signal causing some qualityloss of reproduction but for normal broadcasting this loss will be minimalor not noticeable

The frequency spectrum of a FM wave differs from an AM frequencywaveform in that FM has additional sidebands that are multiples of theinformation signal frequency, resulting in a greater bandwidth for the samerange of broadcast frequencies


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Relative advantages of AM and FM transmissions

Range AM radio transmissions on long-wave(LW), medium-wave(MW) and short-

wave(SW) wavebands are broadcast over very large distances so that onetransmitter can serve a large area

FM transmissions have a range of only about 30 km and this range is by line-of-sighthence many transmitters are required to broadcast over a large area

Interference and Quality

Electrical equipment that produces sparks produce em waves that are picked upby an aerial

When a radio is tuned to an AM broadcast, this interference will add to thedisplacement of the AM signal and will appear as noise

Since an FM signal is based on changes in frequency and not displacement, theinterference is not picked up by the aerial and hence does not alter the frequencyof the signal

This means that the quality of the FM reception is generally better than that ofAM since there will be less noise or interference


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cont..

Cost and simplicity FM can be used to serve small local areas but it is simpler and cheaper to

broadcast and receive using AM, as the AM transmitters are much simplerelectronically than those for FM

Bandwidth and Quality The bandwidth of AM broadcasts on the LW and MW wavebands is 9 kHz which

means that the highest frequency that can be broadcast is 4.5 kHz which is quiteadequate for speech but not for music for which distortion can be easily noticed

The bandwidth of an FM broadcast on the very-high-frequency(VHF) wavebandis about 200 kHz, giving a maximum frequency that can be transmitted orbroadcast of about 15 kHz hence offering higher quality

Transmission waveband and transmitters The LW waveband has a range of frequencies from about 30 kHz to 300 kHz

If the bandwidth of each AM broadcast is 9 kHz, then theoretically (300-30)/9 =30 transmitters could broadcast in the same area without causing interferencebetween each other

For FM broadcasting(300-30)/200 = 1 transmitter only can broadcast in the LWband

Hence the number of transmitters that can share the same waveband is muchlarger for AM than FM

For this reason FM is broadcast only at frequencies in excess of 1 MHz


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Summary of AM/FM transmissions

FM transmissions are more expensive than AM and the area covered by oneFM transmitter is much smaller

The difference in the coverage is an advantage where local radio isconcerned but a disadvantage as regards national radio

The bandwidth necessary for FM is greater

The quality of the received FM signal is much better because of theincreased frequency spectrum and it also suffers less noise


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Example

A particular transmitter is broadcasting an AM signal of frequency 200 kHz.The transmitter is broadcasting a programme of music with a maximum

frequency of 4.5 kHz. Determine for this AM signal:(a) the wavelength

(b) the bandwidth

Solution

(a) wavelength = speed of em waves/frequency = 1500 m

(b) bandwidth = 2 x 4.5 = 9.0 kHz


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Digital transmission


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Analogue signals

Any information that has the same variations with time as the information

itself is known as an analogue signal. e.g. the signal produced by amicrophone is analogous to the sound wave incident on the microphone Much of the information that we wish to transmit and communicate is

analogue in nature e.g. speech, music, television pictures etc

When any signal is transmitted over a long distance, it will pick up strayelectromagnetic waves which we call noise

Noise is not just unwanted sound, but any unwanted random signal thatadds to the signal that is being transmitted

Also the power of the transmitted signal and the received signal becomesless i.e. the signal is attenuated

Hence for long distance transmission, the signal has to be amplified atregular intervals

This amplification of an analogue signal also causes the noise to beamplified and so the signal becomes distortedornoisy

pg 432 fig 16.6 Physics Chris Mee


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Digital signals

A digital signal consists of a series of highs and lows with no valuesbetween the highsand lows

The data in the signal is transmitted as a particular sequence of highs andlows or effectively a sequence of1sand 0s

This digital signal also suffers from noise and attenuation However on amplification, the noisy 1s and 0s can be re-shaped or

regenerated to return the signal to the original form

Such amplifiers are known as regenerator amplifiers whose function is tofilteroutany noise pg 432 fig 16.7 Physics Chris Mee

Unlike an analogue signal, a digital signal can be transmitted over a longdistance with regular regenerations without the signal becoming degraded

Modern digital electronic circuits are in general more reliable and cheaper to

produce than analogue circuits Additional advantage of digital systems is that extra information or data

can be added to the transmissions.

These extra data are a code for the receiving system so that the transmittedsignal may be checked and correctedbefore the signal is finally produced


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Binary numbers or digits

A binary number is a number that has the base 2 whereas a decimal numberhas the base 10

A binary number consists of a number ofdigitsorbits(binary digit) Below are 16 binary numbers shown as 4-bit numbers with their equivalents

in decimal. Larger numbers would require digital numbers with more bits Decimal number Binary number

0 0000

1 0001

2 00103 0011

4 0100

5 0101

6 0110

7 0111

8 10009 1001

10 1010

11 1011

12 1100

13 1101

14 1110

15 1111


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cont..

When reading a digital number, the bit on the left-hand side of the number iscalled the most significant digit or MSB. This bit has the largest value

The bit on the right-hand side has the least significant value and is known asthe least significant digit or LSB

When the LSB is 1, this corresponds to decimal number 1

When the second bit shows 1, this corresponds to decimal number 2

When successive bits show 1 they correspond to decimal numbers

4,8,16,32,64,128,256,512,1024 etc Hence the binary number 1101 corresponds to 8+4+0+1 = 13

Conversely decimal number 11 equals 8+0+4+1 which corresponds tobinary number 1011


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Decimal to binary conversion

Method

1 Divide the denary(decimal) number by 2write the whole number resultunderneath and the remainder in a column to the right.

2 Repeat the process until the number is reduced to zero.

3 The binary number is found by reading the remainder column from the

bottom upwards.

Example

52

26 0 So 5210 = 1101002

13 0

6 13 0

1 1

0 1


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Example

Convert 218710 to a binary number

2187

1093 1

546 1

273 0

136 1

68 0

34 0 17 0

8 1

4 0

2 0

1 0

0 1

218710 = 1000100010112


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Binary to decimal conversion

Method

Step 1: Write down the values of the columns8 4 2 1

Step 2: Write the binary number underneath

8 4 2 1

e.g. 1 0 0 1

Step 3: Evaluate the values of the columns

8 x 1 = 8

4 x 0 = 0

2 x 0 = 0

1 x 1 = 1

Step 4: Add up the values

8 + 1 = 9


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Example

Convert 1011001012 to a denary number

Step 1: Write down the column values by starting with a 1 on the right-handside then just keep doubling as necessary

28 27 26 25 24 23 22 21 20

256 128 64 32 16 8 4 2 1

Step 2: Enter the binary number under the column headings

256 128 64 32 16 8 4 2 1

1 0 1 1 0 0 1 0 1

Step 3: Add up all the column values where the binary digit is 1

256 + 64 + 32 + 4 + 1 = 357

So, 1011001012 = 35710 or just 357 since denary can be assumed in this case


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The transmission of a signal

In an analogue signal such as speech or music, the generated voltage variescontinuously and for digital transmission this analogue signal must be

converted into a digital signal before transmission This is achieved by using an analogue-to-digital converter(ADC) In a ADC, the analogue signal is sampled at regular intervals of time, at

what is known as the sampling frequency or sampling rate The value of the sampled voltage measured at each sampling time is

converted into a digital(binary) number that represents the voltage value andis then transmitted

pg 435 fig 16.8 Physics Chris Mee

Example Assuming that a 4-bit number is being used i.e. 24 = 16 levels, then the

number representing a signal that is sampled as 5.0 V would be 0101

When sampling, the number representing the sample would be the wholenumber below the actual value of the sampled voltage If the signal were to be sampled as 11.4 V, then the 4-bit number would be

1011.

A sampled signal of 11.8 V would also be 1011


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The reception of a signal

At the receiver, the transmitted signal that is received has to be converted back intoan analogue signal

This is achieved by using a digital-to-analogue converter(DAC) In a DAC, the digital signal is converted into an analogue signal

The recovered signal and the faithfulness of the reproduction of the initial analoguesignal, can be improved by using more voltage levels and also sampling at a higherfrequency

The number of bits in each digital number limits the number of voltage levels

In the last example 4 bits were used i.e. 16 levels In practice, 8 or more bits would be used for sampling. A 8-bit number would give

28 = 256 voltage levels.The number of bits needs to be as high as possible The choice of sampling also determines the amount of information that can be

transmitted Around 1900, Nyquist showed that in order to recover an analogue signal of

frequency f, then the signal must be sampled at a frequency greater than 2f The greater the sampling frequency, the more faithful is the reproduction of the

original signal For example forgood quality music, the higher audible frequencies must be present

i.e. frequencies up to 20 kHz, hence for compact discs(CDs) the sampling frequencyis 44.1 kHz

Forspeech example in a telephone system, the sampling frequency is 8 kHz as the

highest frequency transmitted is limited to 3.4 kHz, otherwise it would prove costly


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Channels of communication


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Transfer of information

A signal whether analogue or digital is transmitted and received usingdifferent channels of communication

Also between conversions from analogue to digital signals or vice-versa, thesignal has to be transferred from one place to another

This may be achieved in various ways using different channels ofcommunication including Wire-pairs Coaxial cables Radio links Microwave links Optic fibres


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Picture of wire-pair


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Wire-pairs

It is the simplest link between a transmitter and receiver of information

In the early days of electrical communication using Morse code, thetransmitter and receiver were connected directly to one another by means of2 copper wires(or 1 copper wire and earthreturn) known as a wire-pair

In modern communication systems, wire-pairs are used mainly for short-distance communication at relatively low frequencies e.g. linkingtelephones to the nearest exchange or linking a door bell in a house to theswitch outside the door


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Wire-pairs cont

High-frequency electrical signals lose their energy over short distances inwire-pairs i.e. the signals have high attenuation

partly due to the heating caused by the electrical resistance of the wiresand

partly due to the emission of radiation(radio waves) since the wires actsas aerials

Hence a signal going any distance in a wire-pair must be amplified atregular intervals

Since the wire-pairs act as aerials, they also pick up any electromagneticwaves and unwanted signals as noise which causes deterioration of thesignal

Again, since wire-pairs are close to each other, they pick up each otherssignal

This effect is known as cross-talk or cross-linking which means that wire-pairs give rise to poor security since the signals can be tapped easily

The bandwidth of a wire-pair is only 500 kHz and as a result wire-pairs arelimited as to the amount of information that they can carry


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Summary of wire-pairs

Are used mainly for short distance communications

Cause high attenuation of signals Easily pick up noise

Suffer from cross-talk

Are of low security

Have limited bandwidth


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Picture of coaxial cable


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Coaxial cables

A coaxial cable consists of 2 wire conductors which are insulated an inner metal conductor covered by an insulator

a second conductor(in the form of thin wire braid) that covers the first insulator

another protective layer of insulation covering the braided conductor

The braided 2nd conductor acts as the return for the signal and is earthed

The earthed outer braiding shields the inner conductor from externalinterference

Hence, coaxial cables are far less noisy than wire-pairs and provide bettersecurity

Attenuation is also reduced, hence repeater amplifiers can be further aparton coaxial cables

The bandwidth of coaxial cables is about 50 MHz, hence much more

information can be carried along a coaxial cable than along a wire-pair However, coaxial cables are more costly Coaxial cables are used to connect an aerial to a television, astro aerial to the

astro decoder


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Radio waves

The alternating current in a wire acts as an aerial Energy is radiated from the aerial in the form of electromagnetic waves

which travel outwards from the aerial with the speed of light Electromagnetic waves in the range of 30 kHz 3 GHz are generally

referred to as radio waves The first radio waves used for communication were of very low frequency

and hence very long wavelength. These radio waves were switched on andoff so that communication was by Morse code

Development of AM enabled voice communication

Further developments including FM and the use of different carrier wavesenabled higher quality communication and also more radio stations tooperate in the same area


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Radio waves cont

The intensity of the radio waves will always be reduced or attenuated as thedistance from the transmitter increases but the degree of attenuation depends

on the frequency of the waves For simple radio communications, AM broadcasts on the medium

wave(MW) and long wave(LW) are relatively cheap and technically lesscomplex and are transmitted as surface wavesas they provide coverage overlarge areas

The choice of aerial for transmission determines whether the radio waves

are emitted in all directions as in broadcasting(omnidirectional) or in onedirection as in point to point communication(unidirectional)

Similarly for the receiving of radio signals, the choice of aerial isdetermined by whether the signal from all directions or only one direction isto be received

Aerials with dish reflectors enable the radio waves to be transmitted as a

parallel beam As the frequency of the carrier wave increases, the bandwidth also increases

The wavelength of the radio waves determines the length of the aerial

For mobile phones, the aerial must be for the sake of convenience short andhence the wavelength must also be relatively short


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Summary data on radio waves

Name of radio wave Frequency range Distance

Space wave > 30 MHz line-of-sight betweentransmitter and receiver

plus satellite communication

Sky wave 3 MHz30 MHz worldwide, as a result ofmultiple reflections from theionosphere and the ground orsea

Surface wave < 3 MHz up to 1000 km


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Radio frequency bands

Communication type Frequency range Wavelength in air Frequency band

LW radio 30 kHz300 kHz 10 km1 km low frequencies LFMW radio 300 kHz3 MHz 1 km100 m medium frequencies MF

SW radio 3 MHz30 MHz 100 m10 m high frequencies HF

FM radio 30 MHz300 MHz 10 m1 m very high frequencies VHF

TV broadcasting & 300 MHz3 GHz 1 m10 cm ultra high frequencies UHF

mobile phones

Microwave links 3 GHz30 GHz 10 cm1 cm super high frequencies SHF

Satellite links 30 GHz300 GHz 1 cm1 mm extra high frequency EHF


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Picture of microwave link


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Microwaves

Microwaves are also electromagnetic waves and are in the range of 3 GHzto 30 GHz and are generally used for point to point communications as the

range of transmissions is limited to line of sight Reflecting parabolic dishes are used so that the transmission is in the form

of a parallel beam and so that as much wave power as possible can befocussed onto the receiving aerial

The reflecting parabolic dishes at the transmitter or the receiver are not theaerials . The aerial is found at the focus of the transmitting or reflecting dish

The bandwidth of a microwave link is of the order of 1 GHz which meansthat the microwave beam has a large capacity for transmitting information


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Picture of optic fibre cable or optical fibre


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Optic fibres

An optic fibre consists of a fine strand of very pure glass(thinner than hair)surrounded by a protective covering

Pulses of light or infra-red radiation carrying digital data travel along thefibre as a result oftotal internal reflection

The radiation pulses are provided by lasers and have very high frequenciesof the order of 108 MHz(i.e.1014 Hz)

In theory a single pulse need only last for 10-14 seconds; however laserscannot be controlled at such high frequencies and the duration of a single

pulse or bit is governed by the frequency at which the laser can be switchedon and off

With present technology the frequency is about 800 MHz, but technology isalways improving

Such high frequencies mean a large bandwidth so many different phonecalls can share the same optic fibre


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Summary of advantages of optic fibres

Large bandwidth giving rise to large transmission capacity

Much lower cost than metal wires

Diameter and weight of cable is much less than metal cable, hence easierhandling and storage

Much less signal attenuation, so far fewer regenerator amplifiers arerequired, reducing the cost of installation

Do not pick up electromagnetic interference so very high security andnegligible cross-talk

Can be laid alongside existing routes such as electric railway lines andpower lines


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Signal attenuation

Loss of power when a signal passes along a wire or optic fibre is referred toas attenuation

The amount of attenuation increases as the distance that the signal travelsincreases i.e. attenuation is proportional to distance

Power is lost in various ways; In the case of an electrical signal in a metal wire signal power is lost as heating

in the wire

In optic fibres light power is lost as a result ofabsorption in impurities in the

glass and also scattering due to imperfections In a beam of electromagnetic waves travelling through air, power is lost as a

result ofabsorption and scattering

In order that a signal is detected, the power of the signal must be aminimum number of times greater than the noise power. This ratio isknown as the signal-to-noise ratio(S/N ratio). This ratio can be very large

or very small Signals are amplified and the output of the amplifier is a certain number oftimes greater than the input. The amplifier gain could be 100,000

When a microwave signal is sent from Earth to a satellite, the signal powermay be reduced by a factor of 1019

In order to condense the scale of such variations and to make the numbers

more manageable, the power levels are compared on a logarithmic scale


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Bel and decibels

For easy comparison and convenience, the logarithmic comparison of theS/N ratio is given as a ratio in a unit known as the bel which has the symbol

B Hence number of bels = lg(P2/P1) where P2 and P1 are 2 powers that are

being compared

Since the bel is a large unit, the ratio is usually expressed as a decibel where1 bel = 10 decibelsor10 dB

Hence number of decibels (dB) = 10 lg(P2/P1)

If P2 > P1the dB number is positive If P2 < P1the dB number is negative


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Example

A signal having a power of 2.4 W is amplified in a 2-stage amplifier. The firststage has again of 18 dB and the second stage provides a further amplification

of 25 dB. Calculate:a) the total gain of the 2-stage amplifier

b) the power of the output signal from the amplifier

Solution

a) For the 1st

stage the input is V1 and the output is V giving a gain G1 = V/V1For the 2nd stage the input is V and the output is V0 giving a gain G2 = V0/V

Total gain = V0/V1 = (G2V)/(V/G1) = G1 x G2

= gain of 1st stage x gain of 2nd stage

Gain, when expressed in dB is a logarithm, therefore when 2 gains aremultiplied, then the gains in dB must be added together

lg(total gain) = lg(gain of 1st stage x gain of 2nd stage)= lg(gain of 1st stage) + lg(gain of 2nd stage)

= 18 + 25 = 43 dB

b) Gain in dB = 10 lg(P2/P1)

43 = 10 lg(P2/(2.4 x 10-6)) and taking antilogs, P2= 0.048 W


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Logarithm fundamentals

Base 10 The log of a number to the base 10 is the power to which the number 10

must be raised in order to give that number e.g.100 = 102, so the logarithm to the base ten of 100 i.e. log 100, is 2.00

2.00 = 100.301 so the logarithm to the base ten of 2.00 i.e. log 2.00, is 0.301

101.699 = 50 and so log 50.0 = 1.699

Multiplication

When 2 numbers are multiplied, if the numbers are expressed as numbers tothe base 10, then the powers of the base ten are addede.g.75 x 3.00 = 101.875 x 100.477 = 102.352, so 102.352 = 225

Division When 2 numbers are divided, if the numbers are expressed as numbers to

the base 10, then the powers of the base ten are subtractede.g.

75 3 = 101.875 100.477 = 101.398, so 101.398 = 25Gain The gain of an amplifier is usually expressed in dB i.e.

Gain in dB = 10 lg(Pout/Pin) in series, hence the combined gain is foundeither by multiplying together the 2 actual gains or by adding together thegains when expressed in dB


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Signal attenuation

In a transmission line the amount of attenuation is dependent on the lengthof the line L

Hence attenuation per unit length = (1/L) 10 lg(P2/P1) Attenuation per unit length is measured in dB km-1

Channel Attenuation per unit length(dB km-1)

Optic fibre 13

Coaxial cable 3 at 10 MHz

Coaxial cable 40 at 3 GHz

(hence attenuation for coaxial cables depends on frequency)


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Example

The signal input to an optic fibre is 7.0 mW. The average noise power in the fibre is 5.5x 10-19 W and the signal-to-noise ratio must not fall below 24 dB. The fibre has an

attenuation of 1.8 dB km-1

. Calculate:a) the minimum effective signal power on the cable

b) the maximum uninterrupted length of the optic fibre through which the signal canbe transmitted

Solution

a) number of decibels(dB) = 10 lg(P2/P1)24 = 10 lg(Pmin/(5.5 x 10-19))

102.4 = Pmin/(5.5 x 10-19)

therefore min effective signal power is = 1.38 x 10-16 W

b) total attenuation of signal to reach minimum = 10 lg(Pinput/Pmin)= 10 lg((7.0 x 10-3)/(1.38 x 10-16))

= 137 dB

maximum uninterrupted length = total attenuation/attenuation per unitlength

= 137/1.8

= 76 km


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Communication satellites

Although long distance communication using radio waves is possible on theMW waveband(as surface waves) and the SW waveband(as sky waves), for

modern communication systems, there are 3 main disadvantages: 1) Long distance communication using sky waves is unreliable as it depends on

reflection from layers of ions in the upper atmosphere which vary inheight and density.

This gives rise to variable quality of signal. Surface waves are also unreliable because there is poor reception in hilly areas

2) The wavebands available on MW and SW are already crowded 3) The bandwidths that are available are narrow and completely unable to

carry large amounts of information

Satellite communication enables more wavebands to be made available andat much higher frequencies, thus giving rise to a much greater data carryingcapacity

i i i l f lli i i


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Basic principle of satellite communications

Pg 445 fig 16.13 Physics by Chris Mee

Procedure

1) A carrier wave of frequency fup is sent from a transmitter T on Earth to asatellite

2) The satellite receives the signal greatly attenuated 3) The signal is amplified 4) The carrier frequency is changed to a lower frequency fdown 5) The carrier wave of fdown is then directed back to a receiver R on Earth

The uplink and downlink carrier frequencies fup and fdown respectivelyare different so that the very low power signals received from Earth are notswamped by(i.e. can be distinguished from) the high power signal that istransmitted back to Earth

Typical values of uplink/downlink are 6/4 GHz(6/4 GHz band), 14/11 GHzand 30/20 GHz

The communication satellite may be in geostationary orbit which meansthat the satellite is above the equator, orbits the Earth with a period of 24hoursat a height of 3.6 x 104 km above theEarths surfaceand in the samedirection as the rotation of the Earth which is from west to east

From the viewpoint of a person on Earth, the satellite remains above thesame point on the Earths surface


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cont..

The transmitting aerial and the receiving aerial on Earth both have largeparabolic reflectors

For geostationary satellites, these aerials can be in fixed positions and hencethe satellite does not need to be tracked

This also means that a geostationary satellite can have a permanent link witha transmitting ground station hence maintaining communications with any

point on the Earths surface that can receive the signal from the satellite

A number of satellites with overlapping areas allows for long distance

communications removing the need for long distance submarine cables International television broadcasting is possible allowing forlive events in

one country to be viewed by another

All i ll i h i lli


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All is not well with geostationary satellites

There are problems with geostationary satellites

Geostationary satellites are in equatorial orbits which means that

communication in polar regions may not be possible because a satellite willnot be in line-of-sight

The height of the orbit may also pose a problem as between the transmissionand receipt of the signal, the wave must travel at least twice the distance

between the satellite and Earth i.e. 7.2 x 104 km for which the time to travelis 0.24 seconds

This delay may be increased where several satellites were involved andwould not be acceptable for telephone conversation

To avoid these problems, geostationary satellites may be used in conjunctionwith optic fibres

P l lli


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Polar satellites

Polar satellites are satellites that have low orbits and pass over the poles The orbital period is about 100 minutes

Since the Earth rotates below these satellites, then in any period of 24 hours,the satellite will pass over every region of the Earths surface

Continuous communications with a single polar satellite is not possible,however, information may be transmitted to the satellite while it is overhead

This data can be stored and transmitted back to Earth when the satellite isabove the appropriate area

Continuous communications is possible using a number of polar satellitesin orbits which are inclined to one another so that at least one satellite isalways above the transmitter and receiver, but in this case the aerials musttrack the satellites in their orbits

The advantage of using polar satellites is that their orbital height is only ofthe order of 105 m(a few hundred km) and hence delays in telephone

conversations are not noticed Since polar satellites pass over the whole of the Earth in any 24 hour period,

they are used for remote sensing e.g. military espionage, geologicalprospecting, weather forecasting etc

The Global Positioning System(GPS or satnav) uses the signals from anumber of satellites that are not in geostationary orbits


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The mobile phone system


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Inside the Cell Phone

Components:

Circuit board

Antenna/aerial

Liquid Crystal Display

Keyboard

MicrophoneSpeaker

Battery

One of the most intricate devices used daily


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Circuit Board

Analog-to-Digital, Digital-to-Analog converters

Digital Signal Processor

(DSP)Radio Frequency (RF)Control

RF AmplifiersPower Control

ROM and Flash memory

Microprocessor

Front Back

Circuit Board: Flash memory,


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Circuit Board: Flash memory,

Microprocessor

Flash Memory

Microprocessor


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LCDDisplay

Keypad

Cell-phone speaker, microphone and

battery backup

The public switched telephone network(PSTN)


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p p ( )

In the early days of telephones, every caller was connected directly to allother callers. This was possible because there were few phones and all werelocal, for example all in one building

As the number of phones increased, the telephone exchangewas introduced When a call was to be made, the caller would contact the exchange

The operator at the exchange would make the electrical connections necessaryfor the call to be made

If the person to be called was not connected to the local exchange of the caller,the operator at the local exchange would contact the other persons localexchange via a trunk exchange

Trunk exchanges were connected by trunk lines

In modern systems, the telephone operator has been replaced by electronicrelays that carry out the switching operations

International exchanges called gateways have been introduced so thattelephone communications may be worldwide

The public switched telephone network(PSTN) uses the principle ofexchanges. It is a "switched" network because a connection is madebetween caller and receiver before any communication begins


PSTN => local exchange trunk exchange gateway trunkexchange local exchange

PSTN t


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PSTN cont..

The telephone network can be visualised as a "hub and spokes" arrangement.

When the call is made, if it is local to the exchange, then the signal is carried along a

spoke to the local exchange (i.e. the hub) then routed down another spoke to thereceiver.

If the call is a "trunk" call (i.e. not local to the exchange) then the local exchangeroutes the call to the trunk exchange for onward delivery to the exchange local to thereceiver.

For international calls, the local exchange will route to an international gateway

Local exchange trunk exchangeinternational gateway=PSTN

In this PSTN system,

The caller is connected to the PSTN through a local exchange

Each caller has a fixed line, either a wire-pair or an optic fibre that links thesubscriber(the phone user) to the local exchange

The fixed line means that the user has limited mobilitywhile making a call More recently, mobile phone systems were developed that did not require a

permanent line to the local exchange Cellular system developed to provide mobile telephony: telephone access

anytime,anywhere

First mobile telephone system was developed and inaugurated in the U.S. in 1945 inSt. Louis, MO.

This was a simplified version of the system used today.


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First Mobile Telephone System

One and only one

high power base

station with which allusers communicate.

Entire Coverage

Area

Normal

Telephone

System

Wired connection


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Problem with Original Design

Original mobile telephone system could only support a handful of

users at a timeover an entire city!With only one high power base station, users phones also needed to be

able to transmit at high powers (to reliably transmit signals to the

distant base station).

Car phones were therefore much more feasible than handheld phones,

e.g. police car phones.

I d D i


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Improved Design

Over the next few decades, researchers at AT&T Bell Labs, USAdeveloped the core ideas fortodays cellular systems.

Although these core ideas existed since the 60s, it was not until the

80s that electronic equipment became available to realize a cellular

system.

In the mid 80s the first generation of cellular systems was developed

and deployed.

The mobile phone system and network


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The mobile phone system and network


A mobile or cellular(cell) phoneorhandset is a portable device which has a

small transmitter and receiver. It is a combination of radio and telephone When a call is made, the transmitter provides a radio link between the caller

and a base station The base station is linked to a cellular exchange through a cable The cellular exchangeprovides access to the PSTN

A base station provides coverage (communication capabilities) to users onmobile phones within its coverage area.

Users outside the coverage area receive/transmit signals with too lowamplitude for reliable communications.

Users within the coverage area transmit and receive signals from the basestation.

The cellular exchange is connected to the wired telephone network (PSTN). Mobile phone handset radio link base station cellular exchange

PSTN


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cont..

When a mobile phone is linked to a base station, this is achieved using aparticular carrier-wave frequency

The range of carrier-wave frequencies is limited to a number that is far lessthan the number of mobile phones

This means that each mobile phone does not have its own carrier

frequency, and that the same frequency must be shared with manyother phones at the same time

This is made possible by having a network of base stations



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Communication type Frequency range Wavelength in air Frequency band

LW radio 30 kHz300 kHz 10 km1 km low frequencies LFMW radio 300 kHz3 MHz 1 km100 m medium frequencies MF

SW radio 3 MHz30 MHz 100 m10 m high frequencies HF

FM radio 30 MHz300 MHz 10 m1 m very high frequencies VHF

TV broadcasting & 300 MHz3 GHz 1 m10 cm ultra high frequencies UHF mobile phones

Microwave links 3 GHz30 GHz 10 cm1 cm super high frequencies SHF

Satellite links 30 GHz300 GHz 1 cm1 mm extra high frequency EHF

The mobile phone architecture


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The mobile phone architecture

The carrier wave between the handset and the base station is in the UHFbandwhich has limited terrestrial range

This means that the aerial on or in the handset is conveniently short, butmore importantly the base station transmitter aerials have a limited rangeand operate on low power

The country to be covered by the mobile phone network is divided intocells,each cell having its own base station

The base station has an omnidirectional antenna and the transmitted radio

waves are powered so as to have a range approximately equal to the radiusof the cell (a few km)

The base station is usually near the centre of the cell so that the transmittersending out radio waves in all directions covers the whole of the cellwithout overlappingsignificantly into neighbouring cells

In order that interference between phone calls does not occur near the

boundary between 2 cells where the signals from the base stations overlap,the band of carrier wave frequencies allocated to neighbouring basestations is different


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The Core Idea or Cellular Concept

Countries to be covered by the mobile network are divided into cells ,each having its own base station

Thus, instead of one base station covering an entire city, the city wasbroken up into cells, or smaller coverage areas.

Each of these smaller coverage areas had its own lower-power basestation.

User phones in one cell communicate with the base station in that cell.The cellular concept:

multiple lower-power base stations of certain carrier frequency thatservice mobile users within their coverage area and handoffusers toneighbouring base stations as users move. Together base stationstessellate the system coverage area.


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3 Core Principles

Small cells 'tessellate' (a word in mathematics which means to cover aplane surface by repeated use of a single shape, without gaps or

overlapping)

Users 'handoff' as they move from one cell to another.

'Frequency reuse'

T ll i


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Tessellation

Some group of small regions tessellate a large region if they cover thelarge region without any gaps or overlaps.

There are only three regular polygons that tessellate any given region.

Three regular polygons that always tessellate:

Equilateral triangle

Square

Regular Hexagon

TrianglesSquares

Hexagons


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Circular Coverage Areas

Original cellular system was developed assuming base stationantennas are omnidirectional, i.e. they transmit in all

directions equally.

Users located outside

some distance to the

base station receive

weak signals.

Result: base station has

circular coverage

area.


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Circles dont Tessellate!

Thus, ideally base stations have identical, circular coverage areas.

What is the problem? Interference and blind spots

The most circular of the regular polygons that tessellate is the hexagon.

Thus, early researchers started using hexagons to represent the coverage area of

a base station, i.e. a cell.


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Thus the name Cellular

With hexagonal coverage area, a cellular network is drawn as:

Since the network resembles cells from a honeycomb, the name

cellular was used to describe the resulting mobile telephone network.

Base

Station

Handoffs


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Handoffs

A crucial component of the cellular concept is the notion ofhandoffs.

Mobile phone users are by definition mobile, i.e. they move around whileusing the phone.

Thus, the network should be able to give them continuous access as theymove.

This is not a problem when users move within the same cell.

When they move from one cell to another, a handoffis needed.

A user is transmitting and receiving signals from a given base station, say B1.

Assume the user moves from the coverage area of one base station into the

coverage area of a second base station, B2.

B1 notices that the signal from this user is degrading.

B2 notices that the signal from this user is improving.


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A Handoff (contd)

At some point, the users signal is weak enough at B1 and strong enough at B2for a handoff to occur.

Specifically, messages are exchanged between the user B1 and B2 so that

communication to/from the user is transferred from B1 to B2.


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Frequency Reuse

Extensive frequency reuse allows for many users to be supported at the sametime.

Total em spectrum allocated to the service provider is broken up into smallerbands.

A cell is assigned one of these bands. This means all communications

(transmissions to and from users) in this cell occur over these frequencies onlyNeighbouring cells are assigned a different frequency band.

This ensures that nearby transmissions do not interfere with each other.

The same frequency band is reused in another cell that is far away. This largedistance limits the interferencecaused by this cell

In frequency reuse, a group of local cells use different frequencies totransmit/receive signals in their cell.

This group of local cells is referred to as a cluster.


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Clustersize of 7

Assume a clustersize of 7. This means that a total 395 voice channelsare divided into groups of seven.

Thus, each cell has about 56 voice channels. This is the most numberof users that can be supported in a cell, i.e. roughly 10 square miles innormal environments.

This may/may not be sufficient based on the distribution of users.


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Clustersize of 7, Reuse Pattern

How the mobile phone/handset works


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How the mobile phone/handset works

When a mobile phone handset is switched on, it transmits a short signal atregular intervals in order to identify itself

This signal is detected by one or more base stations The base stations transfer the signal to the cellular exchange where a

computer selects the base station with the strongest signal and alsoallocates a carrier-wave frequency for communication between the basestation and the mobile phone

During the time that the handset is switched on or is in actual use, the signal

strength is monitored at the cellular exchange If the caller moves from one cell to another, then the relative strengths of

the signal from base stations changes The call from the handset is re-routed through the base station that

provides the strongest signal

Block diagram of the mobile phone handset


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Block diagram of the mobile phone handset

Aerial/ Switch

Tuning circuit

R.F. amplifier

De-modulator /Oscillator

Serial-to-parallelconverter

DAC

A.F. amplifier

Loudspeaker

Amplifier

Modulator /Oscillator

Parallel-to-serialconverter

ADC

A.F. amplifier

Microphone

Inner workings of the mobile phone handset


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Inner workings of the mobile phone handset

Transmitter

The caller speaks into the microphone

The microphone produces an analogue voltage of the sound wave

This signal is amplified using the audio-frequency (a.f.) amplifier

The analogue voltage is converted into a digital signal using the ADC

The parallel-to-series converter takes the whole of each digital number and emits it as a seriesof bits(binary digits)

The frequency of the oscillator is allocated by the computer at the cellular exchange

This carrier-wave frequency is modulated by the series of bits from the parallel-to-series

The modulated carrier wave is amplified and switched to the aerial where it is transmitted as aradio wave

Receiver

Any signal received at the aerial is switched to a tuning circuit

This circuit selects only the carrier-wave frequency that has been allocated to the handset bythe computer at the cellular exchange

The selected signal is amplified by the radio-frequency (r.f.) amplifier

This signal is then demodulated so that the information signal is in digital form The series-to-parallel converter enables each sampled digital voltage to be separated

These digital numbers are then converted into an analogue waveform in the DAC

The analogue signal is amplified before sound is produced in the loudspeaker

Communications & Interference


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Communications & Interference

The transmission path is very complex, ranging from the simpleline-of-sight transmission to encountering such terrain as

buildings, hills and trees.

Wireless channels are extremely unpredictable.

Abrupt drop, or fading, of signal strength in the land mobilewireless channel is quite common.

The fading feature of the mobile channel depends on the radiowave propagation environment.

Cell phone towers


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Cell phone towers

The box houses the radio transmitters and receivers

that let the tower communicate with phones. The radio

transmitters connect with the antennae on the

tower through thick cables.

Cell phone disposal


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Cell phone disposal

300 million registered cell phones and increasing.By 2015, there will be at least 500 million cell phones in use and another500 million older phones to be disposed.

Toxins that accumulate in the environment - arsenic, antimony,beryllium, cadmium, copper, lead, nickel and zinc cause cancer andneurological disorders

Recycling program

Risks


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Risks

Brain tumors?

The brain cancer patients did not report more cellular phoneuse

Side of the head on which the brain cancer occurred and the

side on which the cellular phone was usedno link.

An important Technology beyond voice


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An important Technology beyond voice

Cellular telephony is one of the fastest growing technologies on the planet. Newer phones allow users to do much more than hold phone conversations.

Store contact information

Make task/to-do lists

Keep track of appointments

Calculator Send/receive email

Send/receive pictures

Send/receive video clips

Surf the internet

Play games Integrate with other devices (PDAs, MP3 Players, etc.)


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Multiple Access in Cellular

Networks


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Multiple transmitters, one receiver

In many wireless systems, multiple transmitters attempt tocommunicate with the same receiver.

For example, in cellular systems cell phones users in a local area

typically communicate with the same cell tower.

How is the limited spectrum shared between these local transmitters?

In such cases, system adopts a multiple access policy.

There are three widely-used multiple access methods or policies

Cellular Access Technologies


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g

The 3 most common cell-phone network technologies for transmittinginformation are:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)

All the above allows for multiple access by splitting calls

FDMA


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FDMA

In FDMA, we assume that a base station can receive radio signals in a

given band of spectrum, i.e., a range of continuous frequency values.

The band of frequency is broken up into smaller bands, i.e., sub-bands.

Each transmitter (user) transmits to the base station using radio waves

in its own sub-band.

Frequency

Sub-bands

Cell Phone User 1

Cell Phone User 2

::

Cell Phone User N

Time

91


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TDMA

In pure TDMA, base station does not split up its allotted frequencyband into smaller frequency sub-bands.

Rather it communicates with the users one-at-a-time, i.e. roundrobin

access

Time is broken up into time slots, i.e., small, equal-length intervals

FrequencyBands

Time

User1

User2

User3

UserN


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CDMA

CDMA is a more complicated scheme.Here all users communicate to the receiver at the same time and usingthe same set of frequencies.

This means they may interfere with each other.

The system is designed to control this interference.

A desired users signal is deciphered using a unique code assigned tothe user.


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Cocktail Party Analogy!

In this cocktail party analogy, people talk to each other at the same time andthus interfere with other.

To keep this interference in control, we require that all partiers must talk at thesame volume level; no one partier shouts above anybody else.

Also, to make sure that each speaking partier is heard correctly by his/herintended listener (and nobody else can listen in), we require each speaker to use

a different language to communicate inThe caveat in this analogy is that if you speak in one language, it is assumedthat only your desired listener can understand this language.

Thus, if you were at this party and only understood one language, say English,then all non-English conversations would sound like gibberish to you.

The only signal you would understand is English, coming from your intenderspeaker (transmitter).


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Channels

Channel is a general term which refers to a frequency in an FDMAsystem, a timeslot/frequency combination in TDMA, or a code in

CDMA.

This way, a base station has a fixed number of channels and can

support only that many simultaneous users.

Dual band, Dual mode

T ib d T i d


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Triband, Trimode

What is Dual band?

CDMA digital cellular (800 MHz) or CDMA digital PCS(1900 MHz).

What is Triband?

GSM 900, 1800 and 1900 (MHz)


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Standards: Rules for a

Cellular Network


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The Inner Workings

Government agencies (FCC) give licenses to companies (serviceproviders) to provide cellular access in a particular geographic region.

These licenses allow the service provider to setup cellular towers in

that region which can transmit over a prescribed band of frequencies.


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Standards

The service providers must use one of the approved cellular standards fordeveloping the cellular network in that region.

These standards are mutually agreed upon rules adopted by the industry on how

the cell phone system operates.

These standards described the air interface, i.e., how cell phones and basestations must communicate with each other

These mutually agreed upon standards change over time, as technology

progresses.

The first cellular systems deployed in the U.S. adhered to a standard called

Analog Mobile Phone System (AMPS). This system existed in the mid 80s toearly 90s.

The first cellular network used analog technology. Specifically, speech was

converted to an FM signal and transmitted back and forth from user phones.


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Second Generation of Cellular

The second generation (2G) of cellular networks were deployed in the early90s.

2G cellular phones used digital technology and provided enhanced services(e.g., messaging, caller-id, etc.).

In the U.S., there were two 2G standards that service providers could choosebetween

The two standards used in U.S. are different from the 2G system used in Europe(called GSM) and the system used in Japan.First U.S. standard is called Interim Standard 136 (IS-136) and is based onTDMA (time-division multiple access).

Second is called IS-95 and is based on CDMA (code-division multiple access).

Most present systems are what is called the 2.5 generation (2.5G) of cellular.


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Present Cellular Systems

Most present cell systems are 2.5G. They offer enhanced services over secondgeneration systems (emailing, web-browsing, etc.).

Some 2.5G systems (such as AT&Ts) are compatible with the European system,Global System Mobile (GSM).

Presently, service providers are setting up third generation (3G) cellular systems

3G offers higher data rates than 2.5G. This allows users to send/receive pictures,video clips, etc.

This service is starting to become more and more available in the U.S.

There are two standards for 3G, Wideband CDMA (WCDMA) and cdma2000.These two standards have been adopted world-wide.

Both are based on CDMA principles.


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Complete Cellular Network

A group of local base stations are connected (by wires) to a mobile

switching center (MSC). MSC is connected to the rest of the world

(normal telephone system).

MSC

MSC

MSC

MSC

Public (Wired)

Telephone

Network

bil i hi


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Mobile Switching Centers

Mobile switching centers control and coordinate the cellular network.

They serve as intermediary between base stations that may be handing

off users between each other.

Base stations communicate with each via the MSC.

MSC keep track of cell phone user subscription.

MSC connects to the wired phone network (rest of the world).

Section VII 30 Communicating Information

Documents

Transcript of Section VII 30 Communicating Information