Communication system

1. Bipolar Junction Transistor as a Switch
(Mohammed Belal Abdulazim)
(Electrical Dep. Communication Sec., MTI university/faculty of engineering, MOHAMED.89922@eng.mti.edu.eg)
Introduction:
As we are living in an era of communication wherein, we can easily transfer any form of information
(video, audio, and other data) in the form of electrical signals to any other device or destined area.
Although it is common in our perceptual experience that sending or receiving signals or data is simple, but
it involves quite complex procedures, possibilities, and involved scenarios within the communication
systems. So, in the scope of communication systems, modulation plays hold crucial responsibility in the
communication system to encode information digitally in the analog world. It is very important to
modulate the signals before sending them to the receiver section for larger distance transfer, accurate data
transfer, and low-noise data reception. To be clear, let us dive into the detailed concept of knowing what is
modulation, different types in it, and what are the types of modulation techniques used in communication
systems.
I. What is Modulation?
Modulation is a process of changing the characteristics of the wave to be transmitted by superimposing the message
signal on the high-frequency signal. In this process video, voice and other data signals modify high-frequency signals
– also known as the carrier wave. This carrier wave can be DC or AC or pulse chain depending on the application
used. Usually, a high-frequency sine wave is used as a carrier wave signal.
These modulation techniques are classified into two major types: analog and digital or pulse modulation. Prior to
discussing further, the different types of modulation techniques, let us understand the importance of modulation.
II. Why Modulation is Used in Communication?
 In the modulation technique, the message signal frequency is raised to a range so that it is more useful
for transmission. The following points describe modulation’s importance in the communication system.
 In signal transmission, the signals from various sources are transmitted through a common channel
simultaneously by using multiplexers. If these signals are transmitted simultaneously with a certain
bandwidth, they cause interference. To overcome this, speech signals are modulated to various carrier
frequencies in order for the receiver to tune them to the desired bandwidth of his own choice within the
range of transmission.
 Another technical reason is antenna size; the antenna size is inversely proportional to the frequency of
the radiated signal. The order of the antenna aperture size is at least one tenth of the wavelength of the
signal. Its size is not practicable if the signal is 5 kHz; therefore, raising frequency by modulating
process will certainly reduce the height of the antenna.
 Modulation is important to transfer the signals over large distances since it is not possible to send low-
frequency signals for longer distances.
 Similarly, modulation is also important to allocate more channels for users and to increase noise
immunity.

2. Modulation in Communication system
III. the types of signals in the modulation process.
To start to know about detailed information of modulation techniques, let us know regarding the types of signals in
the modulation process.
 Modulating Signal:
This signal is also termed as a message signal. It holds the data that must be transmitted and so this is termed as a
message signal. It is considered as the baseband signal where it undergoes a modulation process to get broadcast or
communicated. Because of this, it is the modulating signal.
 Carrier Signal:
This is the high range of frequency signal which is with specific amplitude, frequency, and phase levels, but it does
not hold any data. So, it is termed as carrier signal as it is an empty one. This is simply utilized to transmit the
message to the receiver section after the process of modulation.
 Modulated Signal:
The consequential signal that is obtained after the procedure of modulation is called a modulated signal. This is the
product of both the carrier and modulating signals.
IV. Different Types of Modulation
There are two types of modulation: analog and digital modulation, In both the techniques, the baseband information
is converted to Radio Frequency signals, but in analog modulation, these RF communication signals are a continuous
range of values, whereas in digital modulation these are prearranged discrete states.
Fig (1) illustrates different types of techniques in both analog and digital Modulation.
Fig (1)

3. Modulation in Communication system
V. Analog Modulation
In this modulation, a continuously varying sine wave is used as a carrier wave that modulates the message signal
or data signal. The Sinusoidal wave’s general function is shown in the figure below, in which, three parameters
can be altered to get modulation – they are mainly amplitude, frequency, and phase, so the types of analog
modulation are:
 Amplitude modulation (AM)
 Frequency modulation (FM)
 Phase modulation (PM)
amplitude modulation (AM)
The amplitude of the carrier wave varies in proportion to the message signal, and the other factors like
frequency and phase remain constant. The modulated signal is shown in the below figure, and its spectrum
consists of a lower frequency band, upper-frequency band, and carrier frequency components. This type of
modulation requires greater bandwidth and more power. Filtering is very difficult in this modulation.
Frequency modulation (FM)
varies the frequency of the carrier in proportion to the message or data signal while maintaining other parameters
constant. The advantage of FM over AM is the greater suppression of noise at the expense of bandwidth in FM.
It is used in applications like radio, radar, telemetry seismic prospecting, and so on. The efficiency and
bandwidth depend on the modulation index and maximum modulating frequency.
phase modulation (PM)
The carrier phase varies in accordance with the data signal. In this type of modulation, when the phase is
changed it also affects the frequency, so this modulation also comes under frequency modulation.
Fig (2) illustrates how modulated signal looks in the three techniques.

4. Modulation in Communication system
VI. Digital Modulation
The analog signals were used for long distance communication. This analog form of communication has many
problems in transmission such as interference, security problems etc. The basic digital modulation techniques are
ASK, PSK, FSK and MSK as explained below. In order to make communication safer and more reliable digital
signals are used.
Amplitude Shift Keying (ASK)
In this method signal level is represented by variations in the amplitude of the signal. In ASK
only the amplitude is varied keeping phase and frequency constant.
Frequency Shift Keying (FSK)
In this method signal level is also represented by variations in the frequency of the signal. In FSK only the frequency
is varied keeping amplitude and phase constant. We then use different frequency levels to show the signal levels 0
and 1.
Phase Shift Keying (PSK)
The variation in phase of the signal in this method represents the signal level. In PSK only the frequency is varied
keeping amplitude and frequency constant. Also, we use different phases to show the signal levels 0 and 1.
The basic and simple form of PSK is Binary Phase Shift Keying (BPSK). In this modulation two different phases
represent the two signals. Another method in PSK is Quadrature PSK (QPSK). In this method four different phases
then transmit two bits of information.
Minimum Shift Keying (MSK)
For converting digital signals to analog signals, we use this technique. There is a delay in Q component in this
method. Then the delay is equal to half of the symbol period. In MSK encoding is in alternating bits which are
between the quadrature components. Also, it is the most efficient and easy technique amongst all the above
techniques. Its implementation is easy compared to PSK, FSK and ASK.
 Basically, the position of even and odd bits for a given bit stream are important along with their bit positions
frequency table.
 Also, this method can manage the variation of bits from 1 to 0 very easily and efficiently. Hence, they are
used widely compared to other techniques.