Communication system 1 chapter 1 ppt

Communication System
A B
Engineering System
Genetic System
Social System
History and fact of communication

What is a communications system?
• Communications Systems: Systems designed
to transmit and receive information
Info
Source
Info
Sink
Comm.
System

Communication is the transfer of information from one place to
another.
This should be done
- as efficiently as possible
- with as much fidelity/reliability as possible
- as securely as possible
Communication System: Components/subsystems act together to
accomplish information transfer/exchange.
Overview

Block Diagram
Receiver
Rx
received
message
to
sink
)
(
~ t
m
Transmitter
Tx s(t)
transmitted
signal
Channel
r(t)
received
signal
m(t)
message
from
source
Info
Source
Info
Sink
n(t)
noise
Output
Transducer
Input
Transducer

Information
The communication system exists to convey a message.
Message comes from information source
Information forms - audio, video, text or data
Transmitter:
 Processes input signal to produce a transmitted signal that
suited the characteristic of transmission channel.
 E.g. modulation, coding, mixing, translate
 Other functions performed - Amplification, filtering, antenna
 Message converted to into electrical signals by transducers
 E.g. speech waves are converted to voltage variation by a microphone

Channel (transmission media):
a medium that bridges the distance from source to destination.
Eg: Atmosphere (free space), coaxial cable, fiber optics,
waveguide
signals undergoes degradation from noise , interference and
distortion.

Design Factors
• Bandwidth
– Higher bandwidth gives higher data rate
• Transmission impairments
– Attenuation
• Interference
– Issue especially in case of unguided medium
• Number of receivers
– Unicast (one sender, one receiver)
– Multicast (multiple receivers can introduce more errors)

Noise in Analog Communication Systems
• Noise is unwanted signal that affects wanted signal
• Noise is random signal that exists in communication systems
• Noise can be Internal or External to the system
• Internal: It is due to random movement of electrons in electronic
circuits and Major sources are resistors, diodes, transistors etc.
• External: Man- made and natural resources and we have no
control on it. Major sources are Motors, generators, atmospheric
sources.
• Signal to noise level ratio (SNR):

Receiver:
To recover the message signal contained in the
received signal from the output of the channel, and
convert it to a form suitable for the output
transducer.
E.g. mixing, demodulation, decoding
Other functions performed: Amplification, filtering.
Transducer converts the electrical signal at its input
into a form desired by the system used.

Digital Communication System
 Analog signals can be transmitted directly via carrier modulation over the
communication channel and demodulated accordingly at the receiver. We call
such a communication system an analog communication system.
 an analog source output may be converted into a digital form and the message
can be transmitted via digital modulation and demodulated as a digital signal at
the receiver.
 In some applications, the information to be transmitted is inherently digital;
e.g., in the form of English text, computer data, etc. In such cases, the
information source that generates the-data is called a discrete (digital) source.

Digital Communication System
Figure . Basic elements of a digital communication system

parts of Digital Communication System
1. Source encoder: The source encoder converts the message signal
into a sequence of information bits. The information bit rate depends
on the nature of the message signal (e.g., speech, audio, and video)
and the application requirements.
2. Channel encoder: The channel encoder adds redundancy to the
information bits obtained from the source encoder, in order to
facilitate error recovery after transmission over the channel.
3. Modulator: The modulator maps the coded bits at the output of the
channel encoder to a transmitted signal to be sent over the channel.
The channel encoder and modulator are typically jointly designed,
keeping in mind the anticipated channel conditions, and the result is
termed a coded modulator.

4. Channel: The channel distorts and adds noise, and possibly interference, to the
transmitted signal.
Example: Consider communication between a cellular base station and a mobile
device. The electromagnetic waves emitted by the base station can reach the
mobile’s antennas through multiple paths, including bounces off streets and
building surfaces.
5. Demodulator: The demodulator processes the signal received from the channel
to produce bit estimates to be fed to the channel decoder. It typically performs
a number of signal processing tasks, such as synchronization of phase, frequency
and timing, and compensating for distortions induced by the channel.
6. Channel decoder: The channel decoder processes the imperfect bit
estimates provided by the demodulator, and exploits the controlled
redundancy introduced by the channel encoder to estimate the information
bits.

7. Source decoder: The source decoder processes the estimated information
bits at the output of the channel decoder to obtain an estimate of the
message. The message format may or may not be the same as that of the
original message input to the source encoder: for example, the source
encoder may translate speech to text before encoding into bits, and the
source decoder may output a text message to the end user.

Communication channel
1. Wire-line Channels. The telephone network makes extensive use of wire
lines for voice signal transmission, as well as data and video transmission.
Twisted-pair wire lines and coaxial cable basically guided electromagnetic
channels which provide relatively modest band Widths.
Telephone wire generally used to connect a customer to a central office has a
bandwidth of several hundred kilohertz (KHz). On the other hand coaxial
cable has a usable bandwidth of several megahertz (MHz) the frequency
range of guided electromagnetic channels which includes waveguides and
optical fibers.
Signals transmitted through such channels are distorted in both amplitude
and phase and further corrupted by additive noise
2. Fiber Optic Channels. Optical fibers offer the communications system
designer a channel bandwidth that is several orders of magnitude larger
than coaxial cable channels

The transmitter or modulator in a fiber optic communication system is a light
source, either a light-emitting diode (LED) or a laser. Information is transmitted
by varying (modulating) the intensity of the light source with the message
signal.
The light propagates through the fiber as a light wave and is amplified
periodically (in the case of digital transmission, it is detected and regenerated
by repeaters) along the transmission path to compensate for signal
attenuation.
At the receiver, the light intensity is detected by a photodiode, whose output
is an electric signal that varies in direct proportion to the power of the light
impinging on the photodiode.

3. Wireless Electromagnetic Channels. In radio communication systems,
electromagnetic energy is coupled to the propagation medium by an antenna
which serves as the radiator. The physical size and the configuration of
the antenna depend primarily on the frequency of operation. To obtain
efficient radiation of electromagnetic energy, the antenna must be longer than
1/10 of the wavelength. Consequently, a radio station transmitting in the AM
frequency band, say at 1 MHz (corresponding to a wavelength of λ = c/ f =
300 m) requires an antenna of at least 30 meters.

Aside: Why go to higher
frequencies(Modulation)?
Tx l/2
Half-wave dipole antenna
c = f l
c = 3E+08 ms-1
Calculate l for
f = 5 kHz
f = 300 kHz
There are two main reasons for going from baseband to band pass by using modulation
1-to decrease size of antenna and 2- for multiplexing

Figure Frequency range for guided Wire line channels.

Fig. Frequency range for wireless electromagnetic channels.

LOW AND MEDIUM FREQUENCIES
Extremely Low Frequencies - 30 to 300 Hz
Voice Frequencies - 300 to 3000 Hz
Very Low Frequencies - 3 kHz to 30 kHz
Low Frequencies - 30 kHz to 300 kHz
Medium Frequencies - 300 kHz to 3 MHz

HIGH FREQUENCIES
High Frequencies - 3 MHz to 30 MHz
Very High Frequencies - 30 MHz to 300 MHz
Ultra High Frequencies - 300 MHz to 3 GHz (1 GHz and
above = microwaves)
Super High Frequencies - 3 GHz to 30 GHz
Extremely High Frequencies - 30 GHz to 300 GHz

HIGH FREQUENCIES
High Frequencies - 3 MHz to 30 MHz
Very High Frequencies - 30 MHz to 300 MHz
Ultra High Frequencies - 300 MHz to 3 GHz (1 GHz and
above = microwaves)
Super High Frequencies - 3 GHz to 30 GHz
Extremely High Frequencies - 30 GHz to 300 GHz
OPTICAL FREQUENCIES
Infrared - 0.7 to 10 micron ( wavelength )
Visible light - 0.4 to 0.8 micron ( wavelength )
Ultraviolet - Shorter than 0.4 micron ( wavelength )

Frequency Spectrum &Bandwidth
Bandwidth of the information signal equals to the
difference between the highest and lowest frequency
contained in the signal.
Similarly, bandwidth of communication channel is the
difference between the highest and lowest frequency
that the channel allow to pass through it

What is modulation?
a process of changing one or more properties of
the analog carrier in proportion to the information
signal.
One of the characteristics of the carrier signal is
changed according to the variations of the
modulating signal.
AM – amplitude, A
FM – frequency , ω
PM - phase , θ
Modulation

Power gain(Signal level gain)
 In Engineering Problems, we have known the term signal gain /
mechanical advantage;
 Examples are chain pulley block, cantilever, gear, amplifier,
transformer.
 Voltage amplifier: Av= Vo/Vi.
 Transistors current gain:  = ic/ib,
Power gain
 It is the ratio of output power over input power.
Ap = Po/Pi.
In amplifiers, the apparent power gain may be more than one.
The signal power is amplified. DC electric power is transformed
into signal power.

Power and voltage gain in communication
In communication, due to known characteristic impedance
of the channel, the power and voltage gains become
should be stated clearly.
It is designated in terms of decibels, dB.
Power gain in dB = 10 log (Po/Pi) dB.
 Voltage gain in dB = 20 log (Vo/Vi) dB.
 Here if power gain in range is between 0&1, voltage gain
is also between 0&1 range in absolute value
However in db, If power gain is negative , voltage gain is
also negative

Power and voltage gain in communication
power ratio Po/Pi = 10,000 = 40 dB
Voltage ratio Vo/Vi = 100 = 40 dB. See that Po/Pi =
(Vo/Vi)2
 (Po/Pi) dB = 2(Vo/Vi)dB
Examples:
1. A 64 dB gain means 106.4 = 2.5212x106 watts
(considering the input power is 1W.
2. An attenuation by 0.01 means Po/Pi =0.01 and
in db it is 10 log(0.01) = -20 dB
It is convenient to express signals with some reference such as
1mW power or,
1 V voltage level.
 This permits input- and output- signals to be expressed in terms of
relative dB.
 When referenced to 1mW, it is written dBm
 When referenced to 1 V, it is written as dBV

Communication system 1 chapter 1 ppt

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Communication system 1 chapter 1 ppt