Analog Transmission
01204325 Data Communications and
Computer Networks
Withawat Tangtrongpairoj, D.Eng.
Based on lecture materials from Data Communications and Networking, 6th ed.,
Behrouz A. Forouzan, McGraw Hill, 2021.
Revised 2023-19-07
Outline
•Digital to Analog Conversion
◦Amplitude Shift Keying (ASK)
◦Frequency Shift Keying (FSK)
◦Phase Shift Keying (PSK)
◦Quadrature Amplitude Modulation (QAM)
•Analog to Analog Conversion
◦Amplitude Modulation (AM)
◦Frequency Modulation (FM)
◦Phase Modulation (PM)
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Analog Transmission
•Digital-to-analog conversion refers to the process of
converting digital data into a bandpass analog signal.
•Analog-to-analog conversion, on the other hand, involves
converting a low-pass analog signal into a bandpass analog
signal.
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Digital to Analog Conversion
•Digital-to-analog conversion modifies an analog signal
characteristic using digital data.
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Carrier Signals
• Carrier signal is a high-frequency signal acting as a base for
information signal
•Also known as Carrier Frequency
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Sine Waves: Revisited
•Simplest form of periodic signal
signal strength
period
T = 1/f
peak
amplitude
time
•General form: 𝑦(𝑡) = 𝐴×sin(2𝜋𝑓𝑡 + 𝜙)
phase / phase shift
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Digital to Analog Conversion
•A sine wave is characterized by amplitude, frequency, and phase.
•By altering one characteristic of a simple electric signal, we can use it to
represent digital data.
•Three mechanisms for modulating digital data into an analog signal are:
◦ Amplitude shift keying (ASK)
◦ Frequency shift keying (FSK)
◦ Phase shift keying (PSK)
•Quadrature amplitude modulation (QAM) combines changes in both
amplitude and phase.
•QAM is the most efficient mechanism commonly used today.
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Types of Digital to Analog Conversion
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Data Element VS Signal Element
•The concept of data element versus signal element was introduced
in Digital Transmission Section.
•A data element was defined as the smallest unit of information
exchanged, known as a bit.
•A signal element was defined as the smallest constant unit of a
signal.
•In this chapter, we will continue to use these terms, but the nature
of the signal element in analog transmission is slightly different.
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Data Rate VS Signal Rate
•Similar to digital transmission, we can define the data rate (bit rate)
and the signal rate (baud rate).
•The relationship between the data rate and signal rate remains the
same.
1
𝑆 = 𝑁×
𝑟
•where N is the data rate (bps) and r is the number of data elements
carried in one signal element.
•The value of r in analog transmission is r = log2L, where L is the
number of different signal elements.
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Example
•An analog signal has a bit rate of 8000 bps and a baud rate of
1000 baud. How many data elements are carried by each
signal element? How many signal elements do we need?
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Bandwidth and Carrier Signal
•Bandwidth
◦ Required bandwidth for analog transmission of digital data is typically
proportional to the signal rate, except for FSK.
•Carrier Signal
◦ Analog transmission involves a high-frequency carrier signal as a base
for the information signal.
◦ The receiving device is tuned to the expected frequency of the carrier
signal.
◦ Digital information modifies the carrier signal through modulation (shift
keying).
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Amplitude Shift Keying (ASK)
•While multiple levels of signal elements are possible, ASK
commonly employs two levels.
•Binary amplitude shift keying (OOK) is a form of ASK that
uses two levels.
•Binary ASK (BASK)
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Amplitude Shift Keying (ASK) Implementation
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Frequency Shift Keying (FSK)
•Frequency shift keying (FSK) involves changing the frequency
of the carrier signal to convey data.
•Peak amplitude and phase remain unchanged across all
signal elements in FSK.
•Binary FSK (BFSK)
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Frequency Shift Keying (FSK) Implementation
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Multilevel FSK
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Phase Shift Keying (PSK)
•Phase shift keying (PSK) uses variations in the phase of the carrier
signal to represent different signal elements.
•PSK maintains constant peak amplitude and frequency while
altering the phase.
•Currently, PSK is more prevalent than ASK or FSK in digital-toanalog modulation.
•However, the dominant method of digital-to-analog modulation is
quadrature amplitude modulation (QAM), which combines ASK and
PSK.
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Phase Shift Keying (PSK)
•Binary PSK (BPSK)
◦Two signal elements, one with a phase of 0°, and the other with a
phase of 180°
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Phase Shift Keying (PSK)
•PSK is less affected by noise compared to amplitude shift
keying (ASK) because noise has a greater impact on
amplitude than phase.
•PSK outperforms frequency shift keying (FSK) as it doesn't
require two carrier signals.
•However, distinguishing between different phases in PSK
requires more advanced hardware.
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BPSK Implementation
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QPSK Implementation
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Constellation Diagram
Visualization by Aj.art
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Three constellation diagrams
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Quadrature Amplitude Modulation (QAM)
•PSK is limited by equipment's ability to detect small phase
differences, restricting its bit rate.
•To overcome this limitation, ASK and PSK are combined,
leading to quadrature amplitude modulation (QAM).
•QAM utilizes two carriers, in-phase and quadrature, with
different amplitude levels.
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Quadrature Amplitude Modulation (QAM)
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Analog to Analog Conversion
•Analog information is represented by analog signals.
•Modulation is required for bandpass mediums or limited bandpass
channels, like in radio.
•Radio stations have assigned narrow bandwidths, and their low-pass
signals need to be shifted for listening to different stations.
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Amplitude Modulation (AM)
•The frequency and phase of the carrier remain the same; only
the amplitude changes to follow variations in the information.
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AM band Allocation
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Frequency Modulation (FM)
•Modulating a carrier signal involves altering its frequency according
to the changing voltage level (amplitude) of the modulating signal.
•The peak amplitude and phase of the carrier signal remain
unchanged during modulation.
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FM band Allocation
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Phase Modulation (PM)
•PM (Phase Modulation) and FM (Frequency Modulation) are similar,
except for one distinction.
•In FM, the instantaneous change in the carrier frequency relates to
the amplitude of the modulating signal.
•In PM, however, the instantaneous change in the carrier frequency is
proportional to the derivative of the modulating signal's amplitude.
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Phase Modulation (PM)
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Summary
•Digital-to-analog conversion transforms analog signals using ASK,
FSK, PSK, and QAM.
•ASK varies amplitude, FSK changes frequency, and PSK modifies
phase.
•QAM combines ASK and PSK using two carriers with different
amplitudes.
•Analog-to-analog conversion represents analog information with
AM, FM, and PM.
•AM varies amplitude, FM modulates frequency, and PM alters
phase.
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