Study Material
UNIT-I
Introduction to Communication
Major Contents:
Introduction to communication system, What is meant by modulation, Need for modulation, Concept of frequency translation, Electromagnetic Spectrum, Frequency and wavelength in EM spectrum, Fundamental definitions of Gain, decibel and attenuation.
INTRODUCTION TO COMMUNICATION SYSTEM:
Communication means sending, receiving and processing of information between two or more devices. A collection of elements (devices) which works together to establish a communication between the sender and receiver is called a communication system. Some examples of communication system include radio broadcasting, television broadcasting, radio telegraphy, mobile communication, computer communication etc. Two or more people communicating with each other by using sound signals is also known as the communication system.
Block Diagram of Communication system:
The basic components of a communication system are information source, input transducer, transmitter, communication channel,
receiver, output transducer, and destination.
Fig 1.1.1 Block diagram of Communication System
Information Source: As we know that the communication system establishes the communication
bridge between the sender (transmitter) and receiver. To establish this communication bridge between the sender
and receiver, first, we need information to send.
This information originates in the
information source. The information generated
by the source may be in the
form of sound (human speech), picture (image source), words (plain text in some particular language such as English,
French, German etc.) For example, if
you are talking with your friend on a phone, you are considered as the information source who generates
information in the form of sound. For beginners to analog communication, it’s important to understand the
difference between message and information.
The message is the part of a
communication which involves sending information from source to destination. Information is a meaningful
data that the receiver consumes.
Input Transducer: If you want to talk (communicate) with your friend who is sitting beside you, then you can directly talk with him by using voice signals (sound signals). But if the same friend is farther away from you, then you can’t directly communicate with him by using voice signals (sound signals) because sound signals cannot travel larger distances. So in order to overcome this problem and transmit information to larger distances, first we need to convert this sound signal into another form of signal (electrical signal or light signal) which travel larger distances.
The device which is used to convert this sound signal into another form of signal is called transducer. A transducer is a device which converts one form of energy or signal into another form of energy or signal. The transducer is present at the input side and output side of the communication system. The transducer that is present at the input side of the communication system is called input transducer. Generally, the input transducer converts the non-electrical signal (sound signal or light signal) into an electrical signal. The best example of an input transducer is the microphone which is placed between the information source and the transmitter section. A microphone is a device which converts your voice signals (sound signals) into electrical signals.
Transmitter: The transmitter is a device which converts the signal produced by the source into a form that is suitable for transmission over a given channel or medium. Transmitters use a technique called modulation to convert the electrical signal into a form that is suitable for transmission over a given channel or medium. Modulation is the main function of a transmitter. When we send the signal to larger distances, it undergoes various circumstances which make the signal weak. In order to send the signals to larger distances, without the effect of any external interferences or noise addition and without getting faded away, it has to undergo a process called modulation. Modulation increases the strength of a signal without changing the parameters of the original signal. Thus the resulted signal overcomes the various effects which make it to becomeweak.
Communication Channel: The
communication channel is a medium through which the signal travels.
(or)
The communication channel is a wired or wireless medium through which the signal (information) travels from source (transmitter) to destination (receiver).
(or)
The communication channel is a wired or wireless medium that is used to send the signal from the source (transmitter) to the destination (receiver).
(or)
The communication channel is a wired or wireless medium that connects the transmitter and receiver for sending the signal.
Communication channels are divided into two categories: wired and wireless. Some examples of wired channels include co-axial cables, fiber optic cables, and twisted pair telephone lines. Examples of wireless channels are air, water, and vacuum. Althoughchannel provides a way for communication, it has one drawback. The communication channel reduces the signal strength (attenuates the signal) that carries the information. This reduction in signal strength is mainly caused by the addition of external noise, physical surroundings, and travel distance. Thus the signal received by the receiver is very weak. To compensate this signal loss, amplifiers (the device that amplifies the signal strength) are used at both the transmitter and the receiver side.
Noise:
Noise is an unwanted signal that enters the communication system via the communication channel and interferes with
the transmitted signal. The noise signal (unwanted
signal) degrades the transmitted signal (signal
containing information).
Receiver:
The receiver is a device that receives the signal (electrical signal) from the channel and converts the signal
(electrical signal) back to its original form (light and sound) which is understandable by humans at the destination. TV set is a good example
of a receiver. TV set receives the signals sent by the TV transmitting stations and converts the signal into a form which
is easily understandable by the humans who are
watching TV.
Output Transducer: The transducer that is present
at the output side of the communication system is called output
transducer. Generally, the output transducer
converts the electrical signal into a non-electrical signal (sound
signal, light signal, or both sound and light signal). The best example
of an output transducer is the loudspeaker which is placed between the
receiver section and the destination. The loudspeaker converts
the electrical signals
into sound signals
which are easily understandable by the humans at
the destination.
Destination: The destination is the final stage in the communication system. Generally, humans at some place are considered as the destination. A destination is a place where humans consume the information. For example, if you are watching TV, you are considered as the destination.
Fig 1.1.2: An electronic communications system
using electronic signals
MODULATION:
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted In general telecommunications, modulation is a process of conveying message signal, for example, a digital bit stream oran analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform transforms a narrow frequency range baseband message signal into a moderate to high frequency range pass band signal, one that can pass through a filter.
A modulator is a device that performsmodulation.
A demodulator
(sometimes detector or demod) is a device that perform demodulation, the inverse ofmodulation.
A modem (from modulator–demodulator) can perform bothoperations
The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel at a different frequency, for example over a limited radio frequency band or a cable TV network channel.
The importance of performing modulation and the major advantages what we can observe after performing modulation are explained below:
NEED FOR MODULATION:
1.
Reduction in the height of antenna
2.
Avoids mixing of signals
3.
Increases the range
of communication
4.
Multiplexing is possible
5.
Improves quality of reception
1. Reduction in the height of antenna
For the transmission of radio signals, the antenna height must be multiple of λ/4, where λ
is the wavelength.
where c : is the velocity of light
λ = c /f
f: is the frequency of the signal to be transmitted.
The minimum antenna height required to transmit a baseband signal of f = 15 kHz is calculated as follows:
H =λ/4= 𝑐
𝑓 𝑋 4
3 𝑋 108
=
15𝑋103𝑋
4
= 5000 mt
The antenna of this height is practically impossible to install. We observe that as we are operating with low frequency signals then the height of the antenna required to transmit those signal is of 5000 meters. The design of this type of antenna is practically impossible so if we increase the frequency of signal then we can reduce the height of antenna but practically our message signal is of low frequency so we use a high frequency carrier and imparts our low frequency message in that carrier which makes transmitting signal as high frequency now
Now, let us consider a modulated signal at f = 1 MHz . The minimum antenna height is given by,
H =λ/4= 𝑐𝑓 𝑋 4
3 𝑋
108
1 𝑋 106𝑋
4
= 75 mts
1. Avoids mixing of signals
If the baseband sound signals are transmitted without using the modulation by more than one transmitter, then all the signals will be in the same frequency range i.e. 0 to 20 kHz. Therefore, all the signals get mixed together and a receiver cannot separate them from each other. Hence, if each baseband sound signal is used to modulate a different carrier then they will occupy different slots in the frequency domain (different channels). Thus, modulation avoids mixing of signals.
2. Increase the Range of Communication
The frequency of baseband signal is low, and the low frequency signals cannot travel long distance when they are transmitted. They get heavily attenuated. The attenuation reduces with increase in frequency of the transmitted signal, and they travel longer distance. The modulation process increases the frequency of the signal to be transmitted. Therefore, it increases the range of communication.
Multiplexing is a process in which two or more signals can be transmitted over the same communication channel simultaneously. This is possible only with modulation. The multiplexing allows the same channel to be used by many signals. Hence, many TV channels can use the same frequency range, without getting mixed with each other or different frequency signals can be transmitted at the same time.
4. Improves Quality of Reception
With frequency modulation (FM) and the digital communication techniques such as PCM, the effect of noise is reduced to a great extent. And because of the high frequency carrier, we can protect information from various disturbances and we will get information without any loss. This improves quality of reception.
In addition with respect to above mentioned necessaries there are plenty of advantages we may observe by performing the concept of modulation. The major part of considering modulation is to perform noise free transmission.
Frequency Translation
The process of transferring a signal form one part of the frequency axis to the other is called Frequency translation. It occurs frequently in a Wireless communication system, that is, Frequency translation is used to transfer the pass band signal to base band signal. The Decimation is the most efficient method for the frequency translation.
For Example, consider a modulated signal S1(t) with a centered spectrum of frequency f1 and a factor of cos2π(f2–f1)t. Where, f2 is the centered spectrum of frequency of band pass filter. Figure 1 depicts the process of obtaining upward frequency translation.
|
Fig: 1.4.1 Upward Frequency Translation
The signal x(t) in Figure 1, can be obtained by the multiplication of signal S1(t) with cos 2π(f2–f1)t. Then the resultant signal x(t) is passed through the Band pass filter (BPF) which is at a centered spectrum of frequency f2. The band pass filter output results as an upward frequency translated signal s2(t).
Fig: 1.4.2 Output Response of Band Pass Filter
The signal S1(t) represented in Figure 2, with a centered spectrum of frequency f1 and output of band pass filter signal results in an upward translated frequency signal s2(t).Similarly, the downward frequency translation is estimated by multiplying the modulated signal with cos2π(f2–f1)t and then filtering out the high frequency component using the lower frequency component.
Electromagnetic Spectrum
The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.
Fig: 1.5.1 Electromagnetic Spectrum
Frequency and Wavelength: Frequency:
- It consists of two voltage polarity reversals, current reversals, or electromagnetic field oscillations.
- Frequency is measured in cycles per second (cps). The unit of frequency is the hertz (Hz).
Wavelength
is the distance occupied by one cycle of a wave and is usually expressed in meters.
Wavelength is also the distance traveled
by an electromagnetic wave during the time of one cycle.
The
wavelength of a signal
is represented by the Greek letter lambda
(λ).
With the help of speed of light and wavelength we can measure frequency.
Fig: 1.5.2.1Representation of Frequency & Wavelength
Wavelength (λ) = speed of light ÷ frequency
Speed of light = 3 × 108 meters/second Therefore:λ = 3 ×108/ f
Frequency Ranges from 30 Hz to 300 GHz
The electromagnetic spectrum is divided into segments:
|
Extremely Low Frequencies (ELF) |
30–300 Hz. |
|
Voice Frequencies (VF) |
300–3000 Hz. |
|
Very Low Frequencies (VLF) |
Include the higher
end of the human
hearing range up to about 20 kHz. |
|
Low Frequencies (LF) |
30–300 kHz. |
|
Medium Frequencies (MF) |
300–3000 kHz AM radio 535–1605 kHz. |
|
High Frequencies (HF) (short waves;
VOA, BBC broadcasts; government and military two-way communication; amateur radio, CB. |
3–30 MHz |
|
Very High Frequencies (VHF)FM radio broadcasting (88–108 MHz), television
channels 2–13. |
30–300 MHz |
|
Ultra High
Frequencies (UHF) TV channels
14–67, cellular phones,
military communication. |
300–3000 MHz |
|
Microwaves
and Super High Frequencies(SHF) Satellite communication, radar, wireless LANs,
microwave ovens |
1–30 GHz |
|
Extremely High Frequencies (EHF) Satellite communication, computer data, radar |
30–300 GHz |
Gain
Gain is defined as the difference in power between the amplifier output signal and the input signal. Gain can be calculated by subtracting the input from the output levels when both are expressed in dB (Decibel).
Power Gain:
Originally, the decibel (abbreviated dB) was created to measure the intensity of sound. In communication systems, it is used to express the power gain of an amplifier or the power
+attenuation (loss) of a transmission medium. The equation to convert a power
ratio into decibels is: GdB = 10log10
(POUT/ PIN)
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