Aswan University
Faculty of Engineering
Electrical Engineering Dep.
Communication & Electronics Dep.
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Report on:
SSB & QAM
Subject:
Analog communication
Presented by :
Ahmed Hussein Mohamed
Presented to :
Eng :- Rehab
Supervised by:Dr. Samia Heshmat
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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 .
What is modulation?
Modulation is a process of changing the characteristics of the wave to be transmitted by
superimposing the message signal on the highfrequency 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.
How modulation works
Information can be added to the carrier by varying its amplitude, frequency, phase, polarization
Modulation is usually applied to electromagnetic signals: radio waves, lasers/optics and computer
networks. Modulation can even be applied to a direct current -- which can be treated as a degenerate
carrier wave with a fixed amplitude and frequency of 0 Hz -- mainly by turning it on and off, as in
Morse code telegraphy or a digital current loop interface. The special case of no carrier -- a response
message indicating an attached device is no
longer connected to a remote system -- is called baseband modulation.
Modulation can also be applied to a low-frequency alternating current -- 50 60 Hz -- as with power
line networking.
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What are the types of modulation?
There are many common modulation methods:
•
Amplitude modulation (AM): The height (i.e., the strength or intensity) of the signal carrier is
varied to represent the data being added to the signal.
•
Frequency modulation (FM): The frequency of the carrier waveform is varied to reflect the
frequency of the data.
•
Phase modulation (PM): The phase of the carrier waveform is varied to reflect changes in the
frequency of the data. In PM, the frequency is unchanged while the phase is changed relative to
the base carrier frequency. It is similar to FM.
•
Polarization modulation: The angle of rotation of an optical carrier signal is varied to reflect
transmitted data.
•
Pulse-code modulation: An analog signal is sampled to derive a data stream that is used to
modulate a digital carrier signal.
•
Quadrature amplitude modulation (QAM): Uses two AM carriers to encode two or more bits
in a single transmission.
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single sideband modulation (SSB Modulation) :
Single sideband, SSB modulation is basically a derivative of amplitude modulation,
AM. By removing some of the components of the ordinary AM signal it is possible to
significantly improve its efficiency.
It is possible to see how an AM signal can be improved by looking at the spectrum of
the signal. When a steady state carrier is modulated with an audio signal, for example
a tone of 1 kHz, then two smaller signals are seen at frequencies 1 kHz above and
below the main carrier.
If the steady state tones are replaced with audio like that encountered with speech of
music, these comprise many different frequencies and an audio spectrum with
frequencies over a band of frequencies is seen. When modulated onto the carrier,
these spectra are seen above and below the carrier.
.
Single sideband modulation
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SSB advantages
Single sideband modulation is often compared to AM, of which it is a derivative. It
has several advantages for two ways radio communication that more than outweigh
the additional complexity required in the SSB receiver and SSB transmitter required
for its reception and transmission.
1- As the carrier is not transmitted, this enables a 50% reduction in transmitter power
level for the same level of information carrying signal. [NB for an AM transmission
using 100% modulation, half of the power is used in the carrier and a total of half
the power in the two sideband - each sideband has a quarter of the power.]
2- As only one sideband is transmitted there is a further reduction in transmitter
power.
3- As only one sideband is transmitted the receiver bandwidth can be reduced by
half. This improves the signal to noise ratio by a factor of two, i.e. 3 dB, because
the narrower bandwidth used will allow through less noise and interference.
Disadvantages of SSB
1- The discovery & generation process of the single-side band signal is complex
2- Signal quality will be affected when the transmitter &receiver of SSB have outstanding
frequency strength.
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quadrature amplitude modulation(QAM)
Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in
phase by 90 degrees (i.e. sine and cosine) are modulated and combined. As a result of
their 90° phase difference they are in quadrature and this gives rise to the name. Often
one signal is called the In-phase or “I” signal, and the other is the quadrature or “Q”
signal.
The resultant overall signal consisting of the combination of both I and Q carriers
contains of both amplitude and phase variations. In view of the fact that both amplitude
and phase variations are present it may also be considered as a mixture of amplitude
and phase modulation.
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QAM advantages and disadvantages
Although QAM appears to increase the efficiency of transmission for radio
communications systems by utilising both amplitude and phase variations, it has a
number of drawbacks. The first is that it is more susceptible to noise because the
states are closer together so that a lower level of noise is needed to move the signal to
a different decision point. Receivers for use with phase or frequency modulation are
both able to use limiting amplifiers that are able to remove any amplitude noise and
thereby improve the noise reliance. This is not the case with QAM.
The second limitation is also associated with the amplitude component of the signal.
When a phase or frequency modulated signal is amplified in a radio transmitter, there
is no need to use linear amplifiers, whereas when using QAM that contains an
amplitude component, linearity must be maintained. Unfortunately linear amplifiers are
less efficient and consume more power, and this makes them less attractive for mobile
applications.
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