Download Message signals - Alfa Tutorials

Basic terms in communication system:
Transducer: A transducer is device which converts one form of energy into another. An
electrical transducer used in communication system is such that it converts some variable such
as pressure, displacement etc into corresponding variations of electrical signals.
2. Signal: A signal means time varying electrical signal obtained from the original signal by using
a transducer. It contains information from the source, with different sources generating
different type of signals. The signals are of two types:
[a] Analog Signal: In an analog signal the amplitude varies continuously with time. The value of
signal at any instant is represented by the amplitude at that instant. Such signals can be easily
generated from the physical waveform of the information by using suitable transducer.
Digital Signal: In a digital signal, the amplitude of the signal is discontinuous. The amplitude of
the digital signal has only two levels i.e. either low or high. Thus, a digital signal is in the form of
pulses usually uniformly spaced in time. When the digital signal is low it is represented by zero
and when it is high it is represented by 1.
Range: the largest distance of destination from the source of signal upto which the signal
received is of sufficient strength is called the range of the signal.
Attenuation; The loss in the strength of the signal as it propagates through the medium is
called attenuation.
Noise: Noise is the form of amplitude variations in the signal. The source of noise is usually due
to atmospheric electricity, fluctuations of electrical power in industries etc. Noise produces
unwanted signals which accompany the original signal.
Amplification: As the signal propagates its strength decreases, thus the signal received at the
destination may not have sufficient strength to be reproduced. To compensate for the loss of
strength of the signal, its amplification has to be done using a device called amplifier.
Modulation: A low frequency signal cannot be transmitted to large distances as it lacks in
energy. It is superimposed on a high frequency in such a manner that some characteristics of
the carrier wave changes in accordance with the signal.
This process of mixing up of signal
and carrier wave is called modulation
Demodulation: The process of separating out original signal from the modulated signal is called
demodulation.
Transmitter: A device which converts signal into electrical signals mixes it with carrier wave and
then transmits into the medium is called transmitter.
Receiver: A device that extracts the original signal from the modulated signal is called receiver.
Repeater: When the distance of the destination from the transmitter is large or when the
transmitting signal is obstructed by the mountains repeaters are installed along the path of the
signal. It picks up the signal amplifies it and again transmits to the receiver.
Communication System:
Communication: this means that transfer of information from one to another and the set which
is used for transferring this information is called communication system.
Communication system consists of three mains parts:
[1] Transmitter
[2] Communication channel
[3] Receiver
For e.g. if in normal conversation, the person who is speaker is the transmitter, air is the
communication channel and the listener is the receiver.
Transmitter: Transmitter transmits the message over the communication channel to the
receiver. When sound is to be
transmitted from one place to
another. There can be three cases
[a] The speaker and the listener are
standing close to each other: in this
case they communicate directly with
sound waves moving from speaker
to listener through air.
[b] They are separated by a distance of
few hundred meters: Because of the
large distance the intensity of sound
signal goes on decreasing and it can’t reach the listener directly through air. Thus we can convert the
sound signal into an electrical signal, which can travel long distances through he air. At the receiver end
the electrical signal is converted back to sound signal. For this we use microphone at the transmitter end
and the loudspeakers at the receiver end. Both these devices are called transducers which converts one
form of energy into another.
[c] They are separated by very large distance say thousands of kms: In this case the direct
connection between the speaker and
listener is not possible through wires
etc. in this case we convert the signal
into electrical signal, amplify it and
radiate it in the space using an
antenna. At the receiving end the pick
up antenna will receive the signal
sends it to amplifier and then using
transducer converts it back to sound signal.
The two problems associated with this mode are:
[1] the audio frequency range from 20-20000Hz can’t be efficiently radiated and they don’t
propagate well in space.
[2] Simultaneous transmission of signals by different transmitters can cause overlapping and
interference.
The first problem
can be removed by
translating the message
signal to radio frequency
range before transmission
using modulation and
demodulate back to audio
range at the receiver end.
The different transmitting
stations are allotted slots in radio frequency range and a single receiver can then tune in to
these transmitters without overlap.
Message signals:
In communication signal implies a time varying electrical signal generated from the original
signal using transducer. If is single valued function of time that conveys the information, the
signal will have its own amplitude, frequency and wavelength.
The signals can be two types
[a] Analog Signal: it is continuous function of time, with amplitude being continuous. Such
signals arise when the sound or light is converted into electrical signal. Microphone can be used
as transducer for sound and photocell for light waves. The simplest form of analog signal is
f(t) = A sin t
The signals associated with speech are superposition of several such sin waves of varying amplitudes
and frequencies. The range over which the frequency of signal vary is called bandwidth. The bandwidth
of audio signals is 20 Hz to 20KHz. Base band is used to designate the band of frequencies representing
the original signal as delivered by source of information.
[b] Digital Signal: A digital signal is a discontinuous signal having just two voltages low voltage
represented by 0 and high voltage by 1. Either of 0 or 1 is called bit and group of bits is called
byte. A byte made of two bits can have only four combinations. [N= 2 x, where is the number of
bits joined to form a byte.]
Some important digital systems are fax, mobile phones, GPS etc.
Antenna:
An antenna is vital component of any communication system; it is employed both at the
transmitting and the receiving end. At the transmitter end it radiates electromagnetic waves
into free space. While at the receiving end it picks up the transmitted signal. An antenna is
basically a length of conductor and acts as conversion device. The first conversion takes place at
the transmitter here electrical energy is converted into electromagnetic waves. The second
conversion occurs at the receiving end where electromagnetic waves are transformed into
electrical signal that is applied to the input of the receiver. he length of the antenna should be
such that it acts as resonant circuit at the frequency of operation. In most cases the length
chosen is λ/2 where λ is frequency of the RF signal applied.
The two types of antenna are dipole antenna is omni directional and is employed in
transmission of radio waves. A dish antenna is directional antenna and in this type of antenna the basic
active component is dipole placed at the focus of the spherical dish. The dish collects the
electromagnetic energy and then focuses it all on the active element where from the resultant electrical
signal is carried to the input of the antenna. For transmitting signal employing this type of antenna the
signal is fed to the active element where from it is transmitted in the form of parallel beam. Such
antennas are usually employed in radar and satellite communication.
Bandwidth of signals:
In communication system the message signal can be voice, music picture or any kind of data.
Each of these signals have different range of frequencies. The type of communication signal
depends upon the band of frequencies which is considered essential for the communication
purposes.
For speech signals the frequency range of 300Hz to 3100Hz is considered adequate.
Therefore speech signals requires a bandwidth of 2800kHz for commercial telephonic
communication. For music approximate bandwidth of 20kHz is required because of the high
frequencies produced by the musical instruments.
Video signal transmission required about 4.2MHz of bandwidth. A TV signal usually
consisting of voice and video is usually allocated 6MHz of bandwidth.
Bandwidth of transmission medium:
similar to message signals, different types of transmission media offer different bandwidths.
The commonly used media are wire, free space and fiber optical cable. Coaxial cable is widely
used wire medium, which offers bandwidth of approximately 750MHz. such cables are
operated below 18GHz. Communication through free space requires radio waves takes place
over wide range of frequencies from few hundred kHz to few GHz. This range of frequencies is
further subdivided into various services. Optical communication using fibers is performed in the
frequency range of 1THz to 1000THz.
Service
Standard am broadcast
FM broadcast
Television
Cellular mobile radio
Satellite communication
Frequency Band
540-1600kHz
88-108MHz
54-72 MHz
76-88 MHz
174-216MHz
420-890 MHz
896-901MHz
840-935MHz
5.925-6.425GHz
3.7-4.2GHz
VHF
TV
UHF
TV
Mobile to base station
Base station to mobile
Uplink
downlink
Propagation of electromagnetic waves:
In communication system using radio waves, an antenna transmits the signals which travels
through space and reach the receiving antenna at the other end. As the em waves travels
through the medium, the strength of the signal decreases. Thus, the atmosphere of earth plays
an important role in propagation of waves.
Ground wave:
To radiate the signals with high efficiency antennas should have size comparable to the
wavelength λ of the signal [at least λ/4]. At longer wavelengths or low frequencies thus the
antennas have large physical size. In standard am transmission ground based vertical towers
are used for the propagation of the signal. The mode is surface wave propagation as the waves
glides over the surface of the earth. A wave induces a current in the ground over which it
passes and it is attenuated as a result of absorption of energy by earth. This attenuation
increases rapidly with the increase in frequency. The maximum range depends on the
transmitting power and frequency [less than a MHz]
Sky Waves:
In the frequency range from
few MHz upto 30 to 40MHz,
long distance communication
can
be
achieved
by
ionospheric reflection of radio
waves back towards the earth.
This mode of propagation is
called sky wave propagation
and is used short wave radio
broadcasters. The ionosphere
consists of charged ions
because of the ionization
caused by the absorption of
ultraviolet and other high
energy radiations. The degree of ionization varies with height. The ionosphere acts reflector for
certain range of frequencies [3 to 30 MHz]. EM waves of frequency higher than 30 MHz
penetrate the ionosphere and escape. The phenomenon of bending of em waves so that they
are diverted towards the earth is similar to total internal reflection in optics.
Name of layer
Exists during
Frequencies most
affected
Troposphere
D[part of stratosphere]
Approximate height
over the surface of
earth
10km
65-75km
Day and night
Day Only
E[part of stratosphere]
100km
Day only
F1[part of mesosphere]
170-190km
Day time merges with
F2 at night
300km at night
250-400km during day
time
Day and night
VHF [upto several GHz]
Reflects LF, absorbs MF
and HF to some degree
Helps surface waves,
reflects HF
Partially absorbs HF
waves yet allowing
them to reach F2
Efficiently reflects HF
waves particularly at
night.
F2[Thermosphere]
Space Waves:
Another mode of radio propagation is space waves. A space wave travels in the straight line
from the transmitting antenna to the receiving antenna. Space waves are used for line of sight
communication as well as satellite communication. At frequencies above 40MHz,
communication is essentially limited to the line of sight paths. At these frequencies the
antennas are much smaller in height and can be placed at height of many wavelengths above
the surface of the earth. Because of the line of sight propagation, direct waves gets blocked at
some point by the curvature of earth. If the signal is to be received beyond horizon the
receiving antenna must be high enough to intercept the line of sight waves.
If the transmitting antennas is at a height hT , then distance to the horizon is [2RhT]1/2,
where R is the radius of the earth. Thus, if h R is the height of the receiving antenna then
maxima distance of transmission between transmitting and receiving antenna is
DM= 2RhT  2Rh R
Modulation and its necessity:
The purpose of communication system is to transmit information or message signals. Message
signals are also called baseband signals, which essentially designate the band of frequencies
representing the original signal, as delivered by the source of information. No signal in general
is a single frequency sine wave, but it spreads over a range of frequencies called bandwidth.
The two factors which prevent transmission through large distances are [a] size of antenna and
[b] power radiated by antenna.
Size of antenna: for transmitting a signal, the size of antenna should be comparable to the
wavelength of the signal so that the antenna properly senses the time variation of the signal.
For em waves of frequency 20kHz, the wavelength is 15km, such long antennas are not
possible. Hence, direct transmission of such baseband signals is not possible. Therefore, there is
need of translating the information contained in our original low frequency baseband signal
into high or radio frequencies before transmission.
Effective power radiated by the antenna:
Theoretical study of radiation from linear antenna shows that the power is inversely
proportional to the square of the wavelength. This implies that for same antenna length the
power radiated increases with decreasing λ. Hence effective power transmitted by long
baseband signal would be small. For good transmission we need high powers and thus high
frequency or low wavelength transmissions.
Mixing up of signals from different Transmitters:
Another reason for modulation is the practical reason. If many people are talking at the same
time or many transmitters are transmitting baseband information signals simultaneously. All
the signals will get missed up and there is no way to distinguish them. Thus we communicate
the singal at high frequencies and allotting a band of frequency to each message signal so that
they don’t get mixed up. Thus there is need for changing low frequency baseband signal to high
frequency before transmission in such a way that the high frequency signal continues to posses
the information contained in low frequency baseband signal. We achieve this with the help of
high frequency carrier wave and the process is called modulation. A sinosoidal carrier wave is
generally represented as
C[t] = Ac sin[ωct + φ]
Where Ac is the amplitude of the carrier wave and ωc is the frequency of the carrier wave and φ
is the initial phase of the carrier. During modulation any of these three[ A, ω c or φ] is controlled
by the message signal. This results in three types of modulation [a] amplitude modulation [b]
frequency modulation and [c] phase modulation.
Similarly the significant characterstics of pulse are pulse amplitude, pulse duration or pulse
width and pulse position. Hence different type of pulse modulation are [a] pulse amplitude
modulation [b] pulse duration modulation [c] pulse position modulation.
Amplitude Modulation:
In amplitude modulation, the amplitude of the carrier wave
is varied in accordance with the amplitude of the audio
frequency of the modulating signal, however frequency
remains same as that of the carrier wave.
Let us consider an amplitude modulating signal
M(t) = Am sin ωmt
Where Am is the amplitude of the modulating signal and ωm
is the frequency of the modulating signal. Assume a carrier
wave of amplitude Ac and frequency of carrier wave be ωc
such that wave can be represented as
C(t) = Ac sinωct
In amplitude modulated wave the amplitude of the
modulated wave is Ac + Amsinωmt, as its frequency is same
as that of the carrier wave therefore we represent the
modulated wave as
Cm(t) = (Ac + Amsinωmt) sin ωct


A
Cm(t) = Ac 1  m sin  m t  sin c t
Ac


Where μ = Am/Ac is called the amplitude
modulation index and the wave can thus be
written as
Cm(t) = Acsinωct + Ac sinωmtsinωct
The value of μ is always less than or equal to one
to avoid distortion.
Cm(t) = Acsinωct +
Ac
xcosc  m t  cosc  m t 
2
The above equation for the amplitude
modulated wave consists of carrier wave of frequency ωc plus two additional sinusoidal waves
of frequency ωc-ωm and [ωc + ωm]. these two frequencies are called side bands and their
frequencies are called band frequencies. Band width of the amplitude modulated wave is
difference between upper side band [higher] and lower side band [ lower] frequency.
Band width = [ωc + ωm ] – [ωc – ωm] = 2 ωm
Thus band width is always equal to twice the frequency of the modulating signal. The side band
will have the same amplitude μAc/2 and as μ is less than or equal to one means amplitude is
never more than half the amplitude of the carrier wave.
Production of amplitude modulating wave:
The method fr producing amplitude modulating wave is shown in the block diagram. Let us
consider an amplitude modulating signal
M(t) = Am sin ωmt
Where Am is the amplitude of the modulating signal and ωm is the frequency of the modulating
signal. Assume a carrier wave of amplitude Ac and frequency of carrier wave be ωc such that
wave can be represented as
C(t) = Ac sinωct
When modulating signal is added to the carrier wave then resultant is
X(t) = Am sinωmt + Ac sinωct
Let this signal is passed through a square law device and it produces an output
Y = Bx(t) + C [X(t)]2
Where B and C are constants, Using equation for resultant wave
Y(t) = B [Am sinωmt + Ac sinωct] + C [Am sinωmt + Ac sinωct]2
Y(t) = B [Am sinωmt + Ac sinωct] + C [ Am2 sin 2  m t  Ac2 Sin 2 c t  2 Am Ac sin  m t sin  c t ]
Using formula
Sin2A =
1  cos 2 A
and
2
SinA sinB =
1
[cos(A-B) –Cos (A+B)]
2
We get
Y[t] = B[Am sinωmt + Ac sinωct] +
CA
CAc2
CA m
2
 Ac 
cos 2 m t 
cos 2c t  CAm Ac c   m t  CAm Ac c   m t
2
2
2
This signal is passed through band pass filter which rejects dc and the sinusoids of frequency
ωm, and 2ωm,2ωc and retains other frequencies. The output therefore is Am signal with
frequencies ωc, ωc+ ωm and ωc-ωm.
2
2
m
The modulated signal is never transmitted as such. The modulator is to be followed by
power amplifier which provides necessary power and then the modulated signal is fed to an
antenna of appropriate size.
Detection of amplitude modulated wave:
The transmitted signal gets attenuated in propagating through the channel. The receiving
antenna is therefore to be followed by an amplifier and detector. In addition, to facilitate
further processing, the carrier frequency is usually changed to lower frequency by what is called
an intermediate frequency stage preceding the detection. The detected signal may not be
strong enough and needs to be amplified.
Detection is the process of recovering the modulating signal from the modulated carrier
wave. In order to obtain the original message signal, a simple method is shown in the form of
block diagrams.
The modulated signal of the form is passed through a rectifier to produce the output as
shown in figure. The envelope signal [b] is the message signal. In order to retrieve m[t] the
signal is passed through an envelope detector.