Data Communication Fundamentals - Digital Data, Analog Signals

Module
2
Data
Communication Fundamentals
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Lesson
6
Digital Data, Analog Signals
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Specific Instructional Objective
On completion, the students will be able to:
•
Explain the basic concepts of Digital data to Digital signal conversion
•
Explain different aspects of ASK, FSK, PSK and QAM conversion techniques
•
Explain bandwidth and power requirement
2.6.1 Introduction
Quite often we have to send digital data through analog transmission media such as a telephone network. In such situations it is essential to convert digital data to analog signal. Basic approach is shown in Fig. 2.6.1. This conversion is accomplished with the help of special devices such as modem (modulator-demodulator) that converts digital data to analog signal and vice versa.
Since modulation involves operations on one or more of the three characteristics of the carrier signal, namely amplitude, frequency and phase, three basic encoding or modulation techniques are available for conversion of digital data to analog signals as shown in Fig. 2.6.2. The three techniques, referred to as amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK), are discussed in the following sections of this lesson. There are many situations where ASK and PSK techniques are combined together leading to a modulation technique known as Quardrature Amplitude Aodulation (QAM). In this lesson, these modulation techniques are introduced.
Figure 2.6.1 Conversion of digital data to analog signal
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Figure 2.6.2 Types of digital-to-analog modulation
2.6.2 Amplitude-shift keying (ASK)
In ASK, two binary values are represented by two different amplitudes of the carrier frequency as shown in the Fig. 2.6.3. The unmodulated carrier can be represented by
ec(t) = Ec cos 2πfct
The modulated signal can be written as
s(t) = k emcos 2πfct
s(t) = A1cos 2πfct for 1
s(t) = A2cos 2πfct for 0
Special case: On/Off Keying (OOK), the amplitude A2 = 0
ASK is susceptible to sudden gain changes and OOK is commonly used to transmit digital data over optical fibers.
Frequency Spectrum: If Bm is the overall bandwidth of the binary signal, the bandwidth of the modulated signal is BT = Nb, where Nb is the baud rate. This is depicted in Fig. 2.6.4.
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Fig 2.6.4 Frequency spectrum of the ASK signal
considered as an inefficient modulation technique
2
In this case two binary values are represented
carrier frequency as shown in Fig. 2.6.5. Version 2 CSE IIT, Kharagpur
Figure 2.6.5 Frequency Shift-Keying
In FSK two carrier frequencies f1 and f2 are used to represent 1 and 0 as shown in the above figure.
Here s(t) = A cos 2πfc1t for binary 1
And s(t) = A cos 2πfc2t for binary 0
This method is less susceptible to errors than ASK. It is mainly used in higher frequency radio transmission.
Frequency spectrum: FSK may be considered as a combination of two ASK spectra centered around fc1 and fc2, which requires higher bandwidth. The bandwidth = (fc2 - fc1) + Nb as shown in Fig. 2.6.6.
Figure 2.6.6 Frequency Spectrum of the FSK signal
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In this method, the phase of the carrier signal is shifted by
phase measured relative to the previous bit interval. The binary 0 is represented by sending a signal of the same phase as the preceding one and 1 is represented by sending the signal with an opposite phase to the previous one as shown in Fig. 2.6.7.
Figure 2.6.7 Phase-shift keying
I
to represent 0 or 1.
s(t) = A cos (2πfct) for binary 0
constellation diagram, which provides a graphical representation of the complex envelope of each possible symbol state. The x-axis of a constellation diagram represents the in-phase component of the complex envelope, and the y-axis represents the quadrature component of the complex envelope. The distance between signals on a constellation diagram indicates how different the modulation waveforms are, and how well a receiver can differentiate between all possible symbols in presence of noise.
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the
odulation techniques allow both envelope (amplitud
frequency) of the RF carrier to vary. Because the envelope and phase provide two degrees of freedom, such modulation techniques map baseband data into four or more possible RF carrier signals. Such modulation techniques are known as M-ary modulation. In M-ary modulation scheme, two or more bits are grouped together to form symbols and one of possible signals S1(t), S2(t), …, Sm(t) is transmitted during each symbol period Ts. Normally, the number of possible signals is M = 2n, where n is an integer. Depending on whether the amplitude, phase or frequency is varied, the modulation is referred to as M-ary ASK, M-ary PSK or M-ary FSK, respectively. M-ary modulation technique attractive for use in bandlimited channels, because these techniques achieve better bandwidth efficiency at the expense of power efficiency. For example, an 8-PSK technique requires a bandwidth that is log28 = 3 times smaller than 2-PSK (also known as BPSK) system. However, M-ary signalling results in poorer error performance because of smaller distances between signals in the constellation diagram. Several commonly used M-ary signalling schemes are discussed below. QPSK: For more efficient use of bandwidth Quadrature Phase-S
b
s(t) = A cos
= A cos (2πfct + 180) for 10
= A cos (2πfct + 270) for 11
ase shift occurs in multiple of 90
Figu
The idea can be extended to have 8-PSK. Here the phase is shifted by
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Figure 2.6.10 Constellation diagram for 8-PSK signal
QAM (Quadrature Amplitude Modulation): Ability of equipment to distinguish small differences in phase limits the potential bit rate. This can be improved by combining ASK and PSK. This combined modulation technique is known Quardrature Amplitude Modulation (QAM). It is possible to obtain higher data rate using QAM. The constellation diagram of a QAM signal with two amplitude levels and four phases is shown in Fig. 2.6.11. It may be noted that M-ary QAM does not have constant energy per symbol, nor does it have constant distance between possible symbol values.
Figure 2.6.11 Constellation diagram for a QAM signal
Bit rate and Baud rate: Use of different modulation techniques lead to different baud rates (number of signal elements per second) for different values of bit rates, which represents the numbers of data bits per second. Table 2.6.1 shows how the same baud rate allows different bit rates for different modulation techniques. The baud rate, in turn, implies the bandwidth requirement of the medium used for transmission of the analog signal.
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Table 2.6.1 Bit rate for the same bit rate for different modulation techniques
Review Questions
Q1. What are the possible digital-to-analog modulation techniques?
Ans: Three possible digital-to-analog modulation techniques are:
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Amplitude Shift Keying (ASK)
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
Q2. Why PSK is preferred as the modulation technique in modems?
Ans: In PSK scheme it is possible to send a signal having more than one digital value. The approach is known as Quadrature PSK.
Q3. Out of the three digital-to-analog modulation techniques, which one requires higher bandwidth?
Ans: For a given transmission bandwidth, higher data rate can be achieved in case of PSK. In other words, in PSK higher channel capacity is achieved although the signaling rate is lower.
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