data transmission
TRANSCRIPT
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Chapter 3Analog Transmission
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Fundamentals of Communication Chapter 3: Analog Transmission2
Chapter Outline3.1 Introduction3.2 Signal Conversion3.3 Analog Data, Analog Signal 3.3.1 Amplitude Modulation (AM) 3.3.2 Frequency Modulation (FM) 3.3.3 Phase Modulation (PM)3.4 Digital Data, Analog Signal 3.4.1 Amplitude Shift Keying (ASK) 3.4.2 Frequency Shift Keying (FSK) 3.4.3 Phase Shift Keying (PSK) 3.4.4 Quadrature Amplitude Modulation (QAM)3.5 Sideband3.6 Spread Spectrum 3.6.1 Basic Principle 3.6.2 Direct Sequence Spread Spectrum (DSSS) 3.6.3 Frequency Hopping Spread Spectrum (FHSS) 3.6.4 Time Hopping Spread Spectrum (THSS)3.7 Key Points3.8 Exercises
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Fundamentals of Communication Chapter 3: Analog Transmission3
3.1 Introduction
“Modulate” means to regulate or adjust. In communication, it means to regulate some parameter of a high-frequency carrier wave with a lower frequency information signal
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Fundamentals of Communication Chapter 3: Analog Transmission4
3.2 Signal Conversion
Encoder DecoderDigital
or Analog
Digital
g(t)
(a) Encoding onto a digital signal
x(t)
S(f)
fc f
t
x(t)g(t)
Modulator DemodulatorDigital
orAnalog
Digital
m(t)s(t)m(t)
(b) Modulation onto an analog signal
Figure 3.1: Encoding and modulation techniques
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Fundamentals of Communication Chapter 3: Analog Transmission5
Types of Data to Signal Conversion
1. Digital data to digital signal2. Analog data to digital signal3. Digital data to analog signal4. Analog data to analog signal
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Fundamentals of Communication Chapter 3: Analog Transmission6
3.3 Analog Data Analog Signal
Principal reasons for analog modulation of analog signals:1. A higher frequency may be needed for effective
transmission. For unguided transmission, it is virtually impossible to transmit baseband signals; the required antennas would be many kilometers in diameter.
2. Modulation permits frequency division multiplexing.
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Fundamentals of Communication Chapter 3: Analog Transmission7
The mathematical expression for a sinusoidal carrier wave is -
Obviously the waveform can be varied by any of its following three factors or parameter-
Ec - the amplitude fc - the frequency φ - the phase
Mathematical Expression of wave:
tEe cc sin tfEe cc 2sin
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Fundamentals of Communication Chapter 3: Analog Transmission8
3.3.1 Amplitude Modulation (AM)
Figure 3.2: Amplitude modulation
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Percent Modulation
Percent modulation, m, indicates the degree to which the AF signal modulates the carrier wave
100maxmax
wavecarrierofvalueimumwavesignalofvalueimumm
100amplitudecarrieramplitudesignal
100AB
100
minmax
minmax XEEEE
mcc
cc
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Fundamentals of Communication Chapter 3: Analog Transmission10
Effect of modulation Index
Figure 3.3: Amplitude modulation of various indexes
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Amplifier Analog Modulation
Figure 3.4: Methods of amplitude modulation
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Amplifier Analog Modulation
Figure 3.5: Block diagram of a typical AM transmitter
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Amplifier Detection
Rectified Signal
Selected Modulated Signal
C C1
CB
Diode
DCR
Output
L1 L
Figure 3.6: Amplitude detection
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AM Bandwidth
fcBWm BWm
BWt= 2 x BWm
BWm= Bandwidth of the modulating signal (audio)
BWt= Total bandwidth (radio)
fc= Frequency of the carrier
Frequency
Amplitude
Figure 3.7: Amplitude modulation bandwidth
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Amplitude modulation band allocation
Figure 3.8: Amplitude modulation band allocation
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3.3.2 Frequency Modulation (FM)
Figure 3.9: Frequency modulation
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Mathematical Analysis of FM Signal
The instantaneous frequency f for a frequency modulated signal is given by
tKEff mmc cos1
where, K = proportionality constant fc = unmodulated carrier frequency fm = carrier frequency of the modulating signal Em = maximum value of the modulating voltage ωc = unmodulated angular frequency of the carrier ωm = angular frequency of the modulating signal
Considering mf (modulation index) the a frequency modulated signal can be defined as - tmtEe mfcc sinsin
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FM Bandwidth
fc5 BWm 5 BWm
BWt= 10 x BWm
BWm= Bandwidth of the modulating signal (audio)
BWt= Total bandwidth (radio)
fc= Frequency of the carrier
Frequency
Amplitude
Figure 3.10: Frequency modulation bandwidth
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FM Bandwidth
Figure 3.11: Frequency modulation band allocation
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3.3.3 Phase Modulation (PM)
Phase modulation and frequency modulation are very closely related, and infact frequency modulation can be very easily obtained from phase modulation by the so called Armstrong method. Phase modulation and frequency modulation are basically two types of angle modula tion. The expression for a phase modulated wave will be-
ttEe mmcc sinsin
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3.4 Digital Data, Analog Signal
Figure 3.12: Digital to analog modulation
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Digital to analog modulation is the technique to convert digital data toan analog signal.
Note
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Types of digital data to analog modulation
Figure 3.13: Types of digital data to analog modulation
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3.4.1 Amplitude Shift Keying (ASK)
Figure 3.14: Amplitude shift keying
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Noise usually affects the amplitude; there fore, thus ASK is most affected by noise.
Note
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Amplitude Shift Keying (Cont.)
On/Off Keying (OOK) is a popular ASK technique. In OOK, logic 0 is represented by the absence of a carrier. This can save the required energy to transmit information.In mathematical terms the ASK modulated signal can be expressed as:
0 ,01,2cos
bitforbitfortfA
ts cc
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Bandwidth for ASK
Figure 3.15: Relationship between Baud rate and bandwidth in ASK
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3.4.2 Frequency Shift Keying (FSK)
Figure 3.16: Frequency shift keying
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Frequency Shift Keying (cont.)
In FSK system, two sinusoidal waves of the same amplitude but different frequencies f1 and f2 are used to represent binary bits 1 and 0 respectively.In mathematical terms the FSK modulated signal can be expressed as:
0,2cos1,2cos
2
1
bitfortfAbitfortfA
tsc
c
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Bandwidth for FSK
Figure 3.17: Relationship between Baud rate and bandwidth in FSK
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3.4.3 Phase Shift Keying (PSK)
Figure 3.18: Phase shift keying
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Phase Shift Keying (cont.)
In PSK system, two sinusoidal waves of the same amplitude and frequency fc but phases are 0 and are used to represent binary bits 1 and 0 respectively. In mathematical terms, the PSK modulated signal can be expressed as .
0,0,2cos1,0,2cos
)(forTttfAforTttfA
tscc
cc
Figure 3.19: PSK constellation
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Quadrature Phase-shift Keying (QPSK)
Figure 3.20: The 4-PSK method
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Quadrature Phase-shift Keying
In mathematical terms, the QPSK modulated signal can be expressed as-
01,0,4
72cos
00,0,4
52cos
10,0,4
32cos
11,0,4
2cos
)(
forTttfA
forTttfA
forTttfA
forTttfA
ts
cc
cc
cc
cc
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Quadrature Phase-shift Keying
The constellation diagram for the signal will be-
Figure 3.21 The 4-PSK characteristics
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QPSK and 8-PSK are 2 and 3 times as efficient as 2-PSK respectively.
Note
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π/4 Phase shift Keying (8-PSK)
Figure 3.22: The 8-PSK characteristics
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Bandwidth for PSK
Figure 3.23: Relationship between Baud rate and bandwidth in PSK
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3.4.4 Quadrature Amplitude Modulation (QAM)
Quadrature amplitude modulation is a combination of ASK and PSK such that a maximum contrast between each signal element (bit, dibit, tribit, and so on) is achieved.
Figure 3.24: The 4-QAM and 8-QAM constellations
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3.4.4 Quadrature Amplitude Modulation (QAM)
Figure 3.25: Time domain representation of 8-QAM signal
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Various 16 QAM Constellation
Figure 3.26: 16-QAM constellations
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3.5 Sideband
In Amplitude Modulation (AM), a band of frequencies higher than or lower than the carrier frequency, contains energy as a result of the modulation process. The frequencies above the carrier frequency constitute the upper side band (USB); and those below the carrier frequency constitute the lower side band (LSB).
Figure 3.27: Frequency domain representation of DSB-AM
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DSB-SC
Figure 3.28: Frequency domain representation of DSB-SC
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SSB-AM
Figure 3.29: Frequency domain representation of SSB-AM
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SSB-SC
Figure 3.30: Frequency domain representation of SSB-SC
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Vestigial Side Band (VSB)
Figure 3.31: Allocated frequency range for picture carrier and sound carrier
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3.6.1 Spread Spectrum Basic Principle
Figure 3.32: General model of spread spectrum digital communication system
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3.6.2 Direct Sequence Spread Spectrum (DSSS)
Figure 3.33: Direct Sequence Spread Spectrum
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3.6.3 Frequency Hopping Spread Spectrum (FHSS)
Figure 3.35: Time Hopping Spread Spectrum