TELECOMMUNICATIONS NOTES

A) COMMUNICATION CHANNELS
COMMUNICATION- refers to sending of a message from one point to another point in an
intelligible form.
COMMUNICATION- refers to sending of a message from one point to another point in
an intelligible form.
COMMUNICATION SYSTEM-The set up used to transmit the information from one point to
another point is called communication system.

A communication system consist of three essential elements-

(i) Transmitter: The transmitter converts the message (or information) signal into a
suitable signal which may be passed on to a suitable medium called
transmission channel.
(ii) Transmission/Communication Channel: Transmission/Communication channel refers to
the medium, the path or even the actual frequency range used to convey information
from a transmitter to a receiver.
Communication channel may be wire-pairs, coaxial cables, a radio and microwave
links and optical fibres
(iii) Receiver: Receiver converts the suitable signal prepared by transmitter into actual
message (or information).

COMMUNICATION CHANNELS

Wire Pairs:

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A transmitter is connected to a receiver by a pair of insulated copper wires. The potential
difference between the two wires is the signal. Wire-pairs are used mainly for very short
distances with low frequencies. If high frequency signals are transmitted along a pair of wires
over an appreciable distance, repeated amplification must be provided at regular intervals. This is
due to the very high attenuation of the signal.
.The disadvantages of wire-pairs include:
 Cross-linking/cross-talk:-is signal in one wire pair is picked up by a neighboring wire
pair. If several wire-pairs are arranged next to one another, they will pick up each other‟s
signals. This effect is known as cross-talk or cross-linking.
Cross-linking/cross-talk gives rise to very poor security as it is easy to „tap‟ a telephone
conversation, i e the signal intended for one subscriber is picked up by another,
unintended, subscriber. It is caused by the transmitted signal on one circuit inducing a
copy of the signal into an adjacent circuit

 High attenuation — Energy is lost as heat in the resistance of the wires and also as
electromagnetic radiation since the wires act as aerials. The wires themselves act as
aerials and the changing currents radiate electromagnetic waves, further weakening the
signal. This means that the signals need to be amplified at regular intervals.

 Low bandwidth — the rate of transmission of information is limited. The bandwidth of a

pair of wires is only about 500 kHz. Consequently, as a means of carrying a large amount
of information, it is extremely limited.

 Noise —Noise is not just unwanted sound, but any unwanted random signal that is being
transmitted, i e unwanted signals (interference) are easily picked up during a
transmission. When the signal is amplified the noise is also amplified.

Coaxial Cable:

Coaxial consists of two wire conductors. Any electrical signal is transmitted along a central inner
conductor that is covered by an insulator. The second conductor is in the form of thin wire braid

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that completely surrounds the insulator. The braid is covered by a protective layer insulation;-
plastic covering.

 Function of copper braid:

 Acts as „return‟ for the signal and is earthed.
 The earthed outer braiding shields the inner core from noise/interference & cross-
talk. Consequently the signals in the coaxial cables suffer far less from distortion
than wire-pairs and provide better security.
Coaxial cable is used for connecting an aerial to a television.

 Coaxial cables have a larger bandwidth than copper wires, increasing the rate of
transmission of information. The bandwidth of coaxial cable is about 50MHz, so the
cable is capable of carrying much more information than a wire-pair.
 They do not radiate electromagnetic waves to the same degree, which reduces
attenuation.
 Security is slightly greater because they are somewhat more difficult to tap into.

Optic Fibers:
 Optic fibers are thin flexible glass rods used to carry digital info. in the form of pulses of
infra-red radiation transmitted using total internal reflection. These pulses are provided
by lasers and the light produced has very high frequencies of the order of 108 MHz
 Transmitted with infra-red radiation because it has lower attenuation than for visible light

In theory, a bit or individual light wave could last for only 10-14s. This would allow hundreds of
thousands of individual telephone calls to share the same optic fibre. However, present
technology does not allow control at such high frequencies. The duration of a bit is governed by
how fast the laser providing light to the fibre can be switched on and off. This is, at present, of
the order of GHz but is increasing as technology develops.

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The advantages of transmission using optic fibres are indicated below.
 Optic fibres have a wide bandwidth. This gives rise to a large transmission capacity.
 Signal power losses in optic fibres are relatively small. This allows for longer
uninterrupted distances between regenerator amplifiers and reduces the costs of
installation.
 The cost of optic fibre is much less than that of metal wire.
 The diameter and weight of fibre optic cables is much less that that of metal cables. This
implies easier handling and storage.
 Optic fibres have very high security since they do not radiate energy and thus there is
negligible „cross-talk‟ between fibres.
 Optic fibres do not pick up electromagnetic interference. This means they can be used in
electromagnetically „noisy‟ environments, for example alongside electric railway lines. In
fact, optic fibre cables are installed along the routes of the National Grid.
 Optic fibre is ideal for digital transmissions since the light is obtained from lasers that can
be switched on and off very rapidly.

Microwave link
Microwaves are electromagnetic waves in the frequency range 3GHz to 30GHz. They are used
for point-to-point communication since, for use on Earth, the range of transmissions is limited to
line-of-sight.

The transmitting element is placed at the focus of a parabolic mirror. This causes the wave power
to be radiated in a parallel beam. A parabolic reflector, placed in the path of this beam, reflects
and focuses the wave power on to a receiving element. The reflecting parabolic dishes are not the
aerials themselves. They are a means of directing as much power as possible into a parallel beam
and then collecting this power and directing it to the receiving aerial or element. Parabolic dishes
are most useful with short wavelengths where the spread of the waves due to diffraction is less
pronounced.
The bandwidth of a microwave link is of the order of GHz. Consequently, microwave links have
a very large capacity for carrying information. However, for terrestrial use, the range of the
transmissions is limited to line-of-sight. For long-distance transmissions, many repeater stations
are required.
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Radio waves and microwaves
Radio waves and microwaves are part of the electromagnetic spectrum, with frequencies ranging
from about 30 kHz to 300 GHz. Although there is no fixed boundary between radio waves and
microwaves, it is generally considered that microwaves have a frequency of 3 GHz or greater.
For convenience, radio waves are split into several further bands as shown in the Table 23
below. Table 24 radio waves and microwaves classified into surface, sky and space waves and
their wavelengths, frequencies and range.

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Radio and Microwave Link:
e.g. linking a ground station to a satellite

Surface waves travel close to the surface of the earth and diffract around it due to long
wavelengths. They have the advantage that they can diffract sufficiently to keep close to the
Earth‟s surface and can also diffract round objects so that there are no „shadows‟ in which
reception is poor.

Sky waves are short-wave radio waves that do not diffract sufficiently for this type of
transmission, but travel in the atmosphere in straight lines, reflecting back and forth between the
ionosphere (the layer of charged particles in the Earth’s atmosphere) and Earth‟s surface hence
can go a long distance.

Space waves have a higher frequency so they can pass through the ionosphere and transmit in
the line-of-sight. They are used, in communication satellites. They are transmitted to satellites,
which regenerate them and transmit them back to Earth. Space waves are in the VHF, the
UHF and the microwave regions and are of even shorter wavelength than short-wave radio
waves and hence diffract even less.

Disadvantages of using ionospheric reflection:

 Unreliable because ion layers vary in height/density
 Cannot carry info. required as bandwidth too narrow
 Coverage limited and reception poor in hilly areas

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 MODULATIONS
All communication systems require a source and a receiver.

Sound can be transmitted 1 directly through air as in (a) directly

2 through alternating currents induced in a moving-coil microphone
and received by a moving coil speaker as illustrated in (b).
3 through radio waves by simply amplifying the audio signal and
applying it to a suitable aerial as illustrated in (c).

However, radio wave communication causes two fundamental problems:

 only one radio station could operate in the region because the wave from a second
station operating would interfere with the first,
 the aerial required to transmit frequencies in the audio range (20 Hz to 20 kHz) would
be both very long and inefficient (the radio waves would not travel very far unless huge
powers were used).

Both of these problems are solved by the process of modulation.

Modulation is the process in which a low frequency signal (called modulating signal is
superimposed over high frequency signal (called carrier wave) in such a way that amplitude,
frequency of carrier wave changes in accordance with the modulating wave, i e

Modulation is the variation of either the amplitude or the frequency of the carrier wave.

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Need for modulation:
1. Practicability of antenna
In the audio frequency range, for efficient radiation and reception, the transmitting and receiving
antennas must have sizes comparable to the wavelength of the frequency of the signal used. It is
calculated using the relation c = fλ. The wavelength is 75 meters at 1MHz in the broadcast band,
but at 1 KHz, the wavelength turns out to be 300km. A practical antenna for this value of
wavelength is unimaginable and impossible.

2. Modulation for ease of radiation

For efficient radiation of electromagnetic waves, the antenna dimension required is of the order
of to . It is possible to construct practical antennas only by increasing the frequency of the
baseband signal.

3. Modulation for multiplexing

The process of combining several signals for simultaneous transmission on a single channel is
called multiplexing. In order to use a channel to transmit the different baseband signals
(information) at the same time, it becomes necessary to translate different signals so as to make them
occupy different frequency slots or bands so that they do not interfere. This is accomplished by using
carrier signals of different frequencies.

4. Narrow banding:
Suppose that we want to transmit audio signal ranging from 50 - 104 Hz using suitable antenna. The
ratio of highest to lowest frequency is 200. Therefore an antenna suitable for use at one end of the
frequency range would be entirely too short or too long for the other end. Suppose that the audio
spectrum is translated so that it occupies the range from 50+106 to 104+106 Hz. Then the ratio of
highest to lowest frequency becomes 1.01. Thus the process of frequency translation is useful to
change wideband signals to narrow band signals.
At lower frequencies, the effects of flicker noise and burst noise are severe.

In modulation a high frequency wave known as the carrier wave has either its amplitude or
frequency altered by the information signal in order to carry the information. Diagram A shows a
carrier wave, while B shows an information signal that is superimposed on carrier wave.
Diagrams C and D show the resultant amplitude modulated and frequency modulated waves
respectively.
A

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 B

 The use of a carrier wave allows different radio stations in the same area/region to
transmit simultaneously.
 Each station transmits on a different carrier frequency and consequently the carrier waves
do not interfere with one another. This is because any one receiver is tuned to the
frequency of a particular carrier wave. The receiver then responds to/recognizes the
information signal.

Advantages of Modulation over Direct:

 Shorter aerial required
 Longer transmission range
 Less attenuation
 Allows more than one station in a region
 Less distortion
Amplitude Modulation (AM)
 For amplitude modulation (AM), the amplitude of the carrier wave is made to vary in
synchrony/in accordance with the displacement of the information signal
 The frequency of the carrier wave does not vary
 The amplitude of the signal must be less than half of the amplitude of the carrier wave
 The variation in the amplitude of the carrier wave is a measure of the displacement of the
information signal
 The rate at which the carrier amplitude varies is equal to the frequency of the information
signal

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 Sidebands and bandwidth
Diagram A below shows the waveform resulting from the amplitude modulation of a high
frequency carrier wave by a signal that consists of a single audio frequency fa. Analysis of the
diagram shows that, an amplitude modulated wave consists of three components:
i. Original carrier wave of frequency 𝑓𝑐 and amplitude 𝐴𝑐
ii. A wave of frequency 𝑓𝑐 − 𝑓𝑎 and amplitude
iii. A wave of frequency 𝑓𝑐 + 𝑓𝑎 and amplitude

A carrier wave contains only one frequency, the carrier wave frequency fc. When the carrier
wave is modulated in amplitude by a single frequency fa, then the carrier wave is found to
contain two more frequencies, known as sideband frequencies, one at a frequency ( fc − fa) and
the other at ( fc + fa). Diagram B shows these frequencies.

When music or speech is transmitted, the carrier is modulated by a range of frequencies which
change with time. Each frequency fa present in the signal gives rise to an extra pair of
frequencies in the modulated wave. The result is a band of frequencies, called the upper and
lower sidebands stretching above and below the carrier frequency fc by the value of the highest
modulating frequency.

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 C

Diagram C shows the frequency spectrum for a carrier wave of frequency 1MHz modulated with
frequencies between 0 and fa = 15 kHz = 0.015MHz. The highest frequency present in the
spectrum is ( fc + fa) = 1.015 MHz and the lowest frequency is ( fc − fa) = 0.985MHz.
The maximum and minimum values are important, as these must not overlap the sidebands from
any other radio station.

The value of fa needed depends on the quality required in the signal. High-quality music only
needs frequencies up to 15 kHz, even though the ear can hear frequencies up to 20 kHz. Speech
only needs frequencies up to 3.4 kHz for people to understand one another.

The range of frequencies from the min to max in modulated carrier wave is called its
bandwidth.
Bandwidth = (𝑓𝑐 + ) − (𝑓𝑐 − 𝑓𝑎) = 2𝑓𝑎

Wavelength of carrier wave =

Size of the bandwidth is significant in that it determines the number of stations that can be
transmitted in an individual location or region.

Number of stations =

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Increased sound quality requires an increase in the maximum frequency fa of the signal that
modulates the carrier wave, and so the bandwidth needed increases. Increase in bandwidth
decreases the number of available stations in the LW region of the spectrum.

Worked examples
1 Radio stations, which broadcast in the long wave (LW) region of the electromagnetic
spectrum, use a carrier frequency between 140 kHz and 280 kHz. The sidebands are within
4.5 kHz on either side of the carrier frequency.

(a) State the bandwidth of each radio station in the LW region of the spectrum
(b) Calculate the maximum number of radio stations which can transmit in the LW region.

(a) The bandwidth of an individual station is twice the width of an individual sideband:

Bandwidth = 2 × 4.5 = 9.0 kHz

(b) The LW region is divided into regions of width 9.0 kHz. Hence:

Number of stations =

=
= 15.5 = 15stations

2 Fig 2.1 shows the frequency spectrum of the signal from a radio transmitter. A carrier and two
sideband frequencies are present.

Fig 2.1

2 a Give the name of the type of modulation that produces two sideband frequencies.
2 a Amplitude modulation
b (i) State the carrier frequency and
(ii) the frequency of the signal used to modulate the carrier wave.
b(i) 40 kHz b(ii) 5kHz

c Calculate the bandwidth of the transmitted signal.

c bandwidth = 2fa = 2(40-35)kHz = 10kHz

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3 a Calculate the number of separate AM radio stations of bandwidth 9 kHz that are possible in
the frequency spectrum available for AM between 530 and 1700 kHz.

4a Number of stations = = = 130

b Suggest why FM stations of bandwidth 200 kHz are not used for this range of frequencies.
b Very few FM stations are possible (only five).
Number of stations = = =5

5 State and explain if the greater bandwidth available on FM an advantage or a disadvantage.

5 It is an advantage if the quality of the signal is required because greater bandwidth
means greater signal frequency. It is a disadvantage if more stations are needed and are
not in line of sight with the transmitter because increasing bandwidth reduces number of
transmitting stations. In this case then AM is better.

6 FM is used largely in towns and AM in rural settings. Suggest why.

6 AM covers a larger area than FM. FM can reach all the inhabitants of a town but the
expense of aerials and transmitters means this is not possible in rural areas.

Calculation For Carrier frequency fc and Signal frequency fa

7. The diagram below shows an AM carrier modulated by a pure tone.

(i) Calculate the carrier frequency and the pure tone frequency.
(ii) Sketch the frequency spectrum of the AM signal.

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 SOLUTION
7. (i) Carrier frequency: 1 cycle lasts 10μs
fc = = 100kHz
Signal frequency : 1 cycle from peak to peak = 150μs-50μs = 100μs.
fa = = 10kHz
(ii) Sketch the frequency spectrum of the AM signal.

Frequency Modulation (FM)

 For frequency modulation (FM), the frequency of the carrier wave is made to vary in
synchrony with the displacement of the information signal
 The amplitude of the carrier wave does not vary

 The change in frequency of the carrier wave is a measure of the displacement of the
information signal
 The rate at which the carrier wave frequency is made to vary is equal to the frequency of
the information signal

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Comparison of AM and FM
Amplitude Modulation (AM)

ADVANTAGES DISADVANTAGES
 Smaller bandwidth so more stations  Requires a high power transmitter
available in frequency range
 Greater area covered by one transmitter  More electrical noise and interference
 Cheaper radio sets

Frequency Modulation (FM)

PROS CONS
 Less electrical noise and interference  Shorter range
 Greater bandwidth produces better  More complex circuitry
quality sound  More expensive

Advantages of AM and FM

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Advantages of AM and FM transmissions.

 The quality of signals received, in terms of interference; of AM reception is

generally poorer than that of FM. AM modulation is affected by noise and distortion
due to interference by electromagnetic waves from lightning, motorbike and car engines
switching on/off electricity.

 On the long wave (LW) and medium wave (MW) wavebands, the bandwidth on an AM
radio station is 9kHz. This means that the maximum audio frequency that can be
broadcast is 4.5 kHz. This frequency is well below the highest frequency audible to
the human ear (about 15 kHz) and therefore such broadcasts lack higher
frequencies and thus quality.

 On the very-high frequency (VHF) waveband, the bandwidth of an FM radio station is

about 200 kHz and the maximum audio frequency broadcast is 15 kHz. Thus, the quality
of music received on AM is poorer than that of FM but in this case, on the basis of
bandwidth.

 The LW waveband occupies a region of the electromagnetic spectrum from 30 kHz to

300 kHz. The number of separate AM radio stations that could share this waveband is,
theoretically, 270 / 9 = 30. However, the number of separate FM stations would be only
270 / 200 = 1. So, more AM radio stations than FM radio stations can share any
waveband. For this reason, FM is used only at frequencies in excess of 1MHz.

 The AM transmissions on the LW, MW and SW (short-wave) wavebands are

propagated very large distances so that broadcasts can be made to a very large area
from only one transmitter. FM transmissions have a range of only about 30km by line-
of-sight. To broadcast to a large area, many FM transmitters are required. It is,
therefore, much cheaper and simpler to broadcast by AM than by FM.
 AM transmitters and receivers are electronically simpler and cheaper and they also
occupy a much smaller bandwidth than those of FM.

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1

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2 The graph shows an amplitude-modulated radio wave carrying a signal.

a Determine the radio frequency of the carrier wave. [2]

b Calculate the frequency of the signal. [2]
c Draw the frequency spectrum of the modulated radio wave [3]

B) ANALOGU AND DIGITAL SIGNALS

 Noise: random, unwanted signal that adds to and distorts a transmitted signal
 Attenuation: progressive power loss in the signal as it travels along the transmission
path
Analogue quantity is one that can have any value, for example the height of a person.
 Analogue signal: a signal that has same variation (with time) as the data and is
continuously variable
 Digital signal: consists of a series of „highs‟ and „lows‟ and has no intermediate values.

Much of the information that is transmitted and communicated is analogue in nature.

If the analogue signal is transmitted over long distance, it will pick up noise. The power of the
signal becomes less, ie the signal is attenuated. For long distance transmission the signal has to
be amplified at regular intervals. The problem is that the noise will be amplified together with

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the analogue signal. The signal will become noisy or distorted. This effect is shown in diagram
below.
There is little improvement possible for the analogue signal; amplification will not remove the
noise.

However, regeneration will remove the noise from a digital signal. The signal is „cleaned‟ of the
noise and returned to its initial shape.

If the information to be transmitted is in digital form, then it too suffers from attenuation and
the addition of noise.
 The amplifiers which are used with digital signals are required only to produce a high
voltage or a low voltage and are not required to amplify small fluctuations in
amplitude.
 Since noise typically consists of such small fluctuations, the amplification of a digital
signal does not also amplify the noise. These amplifiers are called regenerator
amplifiers and are able to reproduce the original digital signal and ‘filter out’ the
noise.
 Thus a digital signal can be transmitted through very long distances with regular
regenerations without becoming increasingly noisy as happens to analogue signals.
This is illustrated in below.

A Schmitt trigger is commonly used to do this.

Advantages of Digital Signals:

 Signal can be regenerated and noise can be eliminated
 Extra data can be added to check for errors
 Multiplexing: digital signals from a large number of different sources can be made to
share the same path

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  Digital circuits are more reliable & cheaper to produce
 Data can be encrypted for security

Other advantages of using digital signals are:

o digital signals are compatible with modern technology and can be stored and processed
more easily, for example in a computer or on a compact disc (CD);
o digital electronic systems are, in general, more reliable and easier to design and build;
o digital signals build in safeguards so that if there is an error in reception it is noticed and
parts of the signal can be sent again.

Analogue-to-Digital Conversion
 In digital transmission, the analogue signal is converted to digital using an analogue-to-
digital converter (ADC). The process is called digitization.
 When received, it is converted back to analogue using a digital-to-analogue converter
(DAC). Binary numbers 0 and 1 are used in analogue to digital conversion ADC and
digital to analogue conversion DAC.

A binary number is a number that has the base 2 and the digits in binary numbers are 1
and 0. Each digit in a binary number is called a bit. In the table four-bit numbers are
shown.
 The binary bit 1 represents a „high‟ voltage and 0 represents a „low‟ hence a digital signal
is made of a series of high and low voltages
 The bit on the left-hand side of a binary number is the most significant bit (MSB) and
has the highest value and the bit on the right-hand side has the least value and is known
as the least significant bit (LSB)

BITS & BYTES

Bit is the measurement unit of data
8 bits = 1 byte; 1024 bits = 1kbits and 1024 x1024 bits = 1Mbits
 The number of bits per sample limits the number of possible voltage levels (with 4 bits
there are 24 = 16 levels; with 8 bits, there are 28 = 256 levels)

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 SAMPLING
Changing an analogue signal into a digital signal involves sampling. In analogue-to-digital
conversion (ADC), sampling is the measurement of the analogue signal at regular time intervals.
 To convert an analogue signal into digital, its voltage value is measured at regular
intervals (sampling)
 These instantaneous voltage values (samples) are converted into binary numbers
representing their value

The sampling process is illustrated in diagram (a) while b) the voltage amplitudes in decimal
numbers at the respective times in µs. In c) 4-bit binary numbers are produced. This process of
converting an analogue quantity into a sequence of numbers is called quantisation. The resulting
number sequence is called the digitised signal.
d) gives the recovered digital signal.

(a)
12

10

8
signal/V

0
0 200 400 600 800 1000 1200 1400
time/µs

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SUMMARY

Sampling Rate:-The sampling rate (SR) is the rate at which amplitude values are
digitized from the original waveform.
Sampling frequency , is the number of samples per second.
A higher sampling frequency results in more information being gathered from the
analogue signal and quality of signal increases/improves.
A larger number of bits increases the quality of signal reproduced.

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For instance, a 12 bit sampler with output of 12 bits of data, means that there are 212 possible
digital signal values that each sample can be converted to.

Improving reproduction of input signal:

o increase number of bits in digital number at each sampling so that step height is reduced
o increase sampling frequency so width of step reduced
 When transmitting the digital signal, a parallel to serial converter can be used to take all
the bits and transmit them one after another down a single line rather than having e.g. 8
cables for an 8bit number (cheaper)
 When received, a serial to parallel converter can convert the signal back to the original
form

Advantages of digital sampling

 Digital signals can be much easier to process and to save than analogue
counterparts. Digital filters, where the filtering of the signal is done by computation, can
be more sophisticated and much more accurate than analogue techniques.

 Once converted, the digital signal can be easily saved in memory or to disk for
later, easy retrieval. Analogue recording methods, such as tape and vinyl disc for audio
signals, have many associated problems, not least the introduction of noise and signal
loss.

 Digital signals are much less prone to corruption by noise. All that is needed to
interpret a digital signal is to distinguish the 0 from 1, so noise can be introduced with
little or no eventual harmful effects. This is illustrated below:

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The same amount of noise added to the original analogue signal could well overwhelm it –
removal of this noise would be difficult.

 Digital signals can be encoded (made unreadable by anyone other than the
intended recipient). A digital signal can be made unreadable by anyone other than the
intended recipient, who will have the required decoding software to unlock the encoding.
There is no equivalent way to do this for an analogue signal.

 A single data link can easily be used by many different data sources, with no risk of
them getting confused. By encoding each data stream differently the receiving end can
distinguish between them. Thus it is possible to send many thousands of telephone calls
down an optical fibre, and to extract and separate them at the other end.

Disadvantage of digital sampling- a disadvantage of digital sampling is that a digital

signal will often require a greater bandwidth than its analogue equivalent

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MULTIPLEXING
Data transmission rates/the bit rate in a system can be increased if the transmission path is
shared by a number of different data streams (for example, many telephone calls down one
optical fibre). This is achieved by using a technique called multiplexing. There are essentially
two types of multiplexing, ‘frequency-division multiplexing’ (FDM) and ‘time-division
multiplexing’ (TDM).
Only at time-division multiplexing will be discussed. Frequency multiplexing is not required for
this option.

Time-division multiplexing (TDM)

The idea behind TDM is to share the time between multiple data streams by allocating a defined time
slot (‘time slice’) to each.

The principle is illustrated in the diagram below.

 Each data stream is converted into „packets‟ of data. Each packet contains information
which identifies the data stream.
 The multiplexer connects to each data stream in turn, spends the allotted time with it,
then switches to the next one.
 The de-multiplexer remains in sync, and so connects the received data to the correct data
stream.
 The rest of the receiver electronics recombines the received packets from each data
stream, so that the original data stream is restored.

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For example, to see how multiplexing works in practice, consider a situation in which sampling
is done at the rate of 16 kHz, that is one sample every 62.5μs. If each sample takes 8μs to convert
to an 8-bit word, then there is a “vacant” time slot of length of 54.5μs before the next word is
transmitted. In theory, this slot can be filled by words from 6 different channels (54.5 ÷8).

The merits of TDM are that:-

 it is a relatively cheap way of increasing data traffic rates,
 the resulting transmitted composite signal does not require any more bandwidth than any
one of the original data streams.
 the original data streams can be properly pieced back together.

But clearly the data rate for each stream will be reduced in proportion to the number of streams
being multiplexed. Note that synchronization information must be sent along with the data so
that transmitter and receiver remain in sync, and thus that TDM finds significant use in the public
switched telephone network (PSTN).
.

C) COMMUNICATION SATELLITES
The basic principle of satellite communication is illustrated in the diagram below

1 A transmitting station T directs an uplink carrier wave of frequency fup of about fup = 6GHz
towards the satellite.

2 The satellite receives this signal, amplifies it and changes the carrier frequency to a lower value
fdown before directing it towards a receiver R back on Earth. The downlink carrier wave would
have a frequency fdown = 4GHz (the 6/4 GHz band).

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 Alternative carrier frequencies are the 14/11GHz band and the 30/20GHz band.

The two carrier frequencies are different to prevent the satellite’s high power transmitted
signal swamping its reception of the very low power signal that it receives. There is no
interference of the actual information being carried by the waves because this is stored as a
modulation of the carrier waves.

The transmitter T could transmit more or less directly to the receiver R without the use of a
satellite. It could only do so on the SW or MW wavebands.

However, in modern communication systems, this is not done for three reasons.
i. Long-distance communication on these wavebands is unreliable. Sky waves rely on
ionospheric reflection. These layers of ions vary in height and density according to the
time of day. In hilly areas, surface waves give rise to regions of poor reception where
there are „shadows‟.
ii. The wavebands are already filled by existing broadcasts.
iii. The available bandwidths are too narrow to carry the required amount of information.

A GEOSTATIONARY SATELLITE

Diagram below illustrates a satellite in a geostationary orbit.

 Geostationary satellites orbit the Earth above the Equator with a period of 24 hours at a
distance of 3.6 × 104km above the Earth‟s surface.
 It orbits in the same direction as the direction of rotation of the Earth, with the same
angular velocity as the Earth.
 So to an observer on the Earth, the satellite will always appear to be above a fixed
position on the Equator

The frequencies of the carrier waves used to communicate with the satellite are in the range 0.1-
10 GHz. The up-link frequency is different to the down link frequency since if they were the
same, the two signals could interfere with each other giving unwanted feedback.

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POLAR ORBITING SATELLITES
Another type of communication satellite is the polar orbiting satellite. A polar satellite is not
geostationary and as such, its orbital height above the surface of Earth is much less than that of a
geostationary satellite.

Polar orbiting satellite is a satellite that follows a Polar Orbit.

Polar orbit passes above or nearly above North and South Poles each time in each of its
revolutions.

Satellites may orbit the Earth in polar orbits, as illustrated in diagram below

 Polar orbits are relatively low with a period of rotation of the order of 90 minutes. Such
satellites will, as a result of the rotation of the Earth, at some time each day orbit above
every point on the Earth‟s surface. For a satellite having a period of 90 minutes, each
orbit crosses the Equator 23° to the west of the previous orbit.

 It is not possible to have continuous communication links with one such satellite because,
from Earth, the satellite appears to move rapidly across the sky and, for part of the time,
is below the horizon. Polar orbiting satellites are used, as well as for communications, for
monitoring the state of the Earth‟s surface, weather forecasting, spying etc.

D) ATTENUATION
An electrical signal travelling along a metal wire gradually loses power (mostly as
thermal/heat energy in the wire). As an electrical signal passes along a wire, there is a voltage
drop across the resistance of the wire itself. This reduces the voltage of the signal that arrives at
the end of the wire. The energy loss in the wire causes electrical heating in the resistance of the
wire (I2R).

 A light pulse travelling along an optic fibre also loses power (mostly by
absorption due to impurities in the glass and scattering by imperfections in the
fibre).

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  Electromagnetic waves radiating from a transmitter also lose power by absorption
and dispersion. A radio wave spreads out from a transmitter. On its own this spreading
causes a decrease in intensity, but there is also a loss in signal strength because of the
absorption of energy by the medium through which the wave travels.

The reduction in signal power is known as attenuation.

For a signal to be detected adequately, its power must be a minimum number of times greater
than the power associated with noise. The signal-to-noise ratio must be very large;- larger or
equal to 100.
The decrease in signal power from the transmitted value P1 to that received P2 can be very high.

The ratio P2 to P1 is measured using a logarithmic scale rather than by the simple ratio of the
two powers.

The logarithm to base 10 of the ratio P2 to P1 gives the number of bels (B). When multiplied
by 10 we obtain the number of decibels (dB).

Number of bels B = lg[ ]

Number of decibels dB =10lg[

Example
The gain of an amplifier is 45dB. Calculate the output power Pout of the amplifier for an input
power Pin of 2.0μW.
Number of dB =10lg( )
45dB = 10log( )

Pout = W

= 6.3 x 10-2W

A transmission line has an input power P2 and the power at a point distance L along the line is P1
as illustrated in diagram below.

Then, attenuation in the line = 10lg [ ] dB.

Since a transmission line may vary in length, an important feature of a transmission line is its
attenuation per unit length.

Attenuation per unit length = lg[ ]

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Example
The input power to a cable of length 25 km is 500mW. The attenuation per unit length of the
cable is 2dBkm-1. Calculate the output power of the signal from the cable.

Signal Attenuation Summary

 Attenuation is the gradual decrease in power of a signal the further it travels
 Power ratios are expressed in decibels (dB) because the numbers involved are smaller and cover
a wider range
 Attenuation/amplification between two positions can be expressed in dB by:

Number of decibels = 10lg [ ]

 If value is positive, there is an increase in power hence the signal has been amplified
 If value is negative, there is a decrease in power hence the signal has been attenuated
 Attenuation of cables is given as attenuation per unit length and is found by:

Attenuation per unit length (dB km-1) =[ ] = lg[ ]

 Signal must be distinguishable above the level of noise and this can be measured by the signal-
to-noise ratio:

Signal-to-noise ratio = 10lg [ ]

 Repeaters amplify both signal & noise so signal-to-noise ratio remains constant however
regeneration of digital signal removes most noise ∴ high signal-to-noise ratio

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ADVANTAGES AND DISADVANTAGES OF SATELLITE COMMUNICATIONS

GEOSTATIONARY SATELLITE POLAR SATELLITE

ADVANTAGES DISADVANTAGES ADVANTAGES DISADVANTAGES
1 The fact that they 1 Their great height 1 Polar satellites will 1 Difficult to track
keep the same above the Earth‟s be able to down the satellite
position means, surface means that communicate with 2 Not always in the
geostationary more power is all points in both same position
satellites need no needed for the Northern and relative to earth so
tracking system. transmitting to and Southern dishes must be
from the satellites. hemispheres at moved
2 Remains in fixed some time during 3 Cannot be used for
position above 2 Because of the their orbit. continuous comm.
point on equator so curvature of Earth,
no need to move geostationary 2 Polar satellites,
dish satellites are not because of their
able to short orbital height,
3 Easy to track down communicate with are much cheaper
the satellite points further than to put into orbit
about 60°-70° than geostationary
4 Can be used for North or South. satellites
continuous comm.
3 Geostationary 3 At smaller height
5 Continuously satellites, because above the earth and
monitor climatic of their large can detect objects
change orbital height, are of smaller detail
much more costly
to put into orbit 4 Has smaller delay
than polar satellites times

4 Large delay time 5 Satellite passes

over every area on
earth

6 Used for remote

sensing

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