111 lecture 10 basic modulation techniques (vi) fall 2008 nctu ee tzu-hsien sang
TRANSCRIPT
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111
Lecture 10Basic Modulation Techniques (VI)
Fall 2008
NCTU EE
Tzu-Hsien Sang
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Outlines
• Linear Modulation
• Angle Modulation
• Interference
• Feedback Demodulators
• Analog Pulse Modulation
• Delta Modulation and PCM
• Multiplexing
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• One unfinished business: the Costas loop. It is very useful for carrier recovery in both analog and digital communication systems.
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Analog Pulse Modulation
• We now study the progression from techniques dealing with pure waveforms ones for “discrete” samples.
• Historically, these methods are the early attempts to achieve modern communications. They are in the twilight zone between analog and digital modulations.
• Today, their spirits can be still found in components such as ADC.
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• Analog pulse modulation: A pulse train is used as the carrier wave. Some characteristic feature of each pulse (e.g., amplitude, duration, or position) is used to represent message samples.
PAM – pulse amplitude
PDM – pulse duration
PPM – pulse position
• Digital Pulse Modulation: Messages are discrete-amplitude (finite levels) samples.
DM – delta modulation
PCM – pulse-code modulation 5
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PAM
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• The “conceptual” demodulation scheme:
• The idea of equalizer: Anything that goes through a known distortion can be recovered.
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1/H(f)
LPF
equalizer
mc(t) m(t)
m(t)
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PWM• Spectrum: complicated (Fourier-Bessel spectra)
• Demodulation: area of “pulse.” Low-pass filtering (integration)
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PPM
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Delta Modulation
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• Slope overload: The message signal m(t) has a slope greater than can be followed by the stair-step approximation ms(t). Assume the step-size = 0 slope (max) = 0/Ts.
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• Solution (may not be perfect): adaptive delta modulation -- adjust the step-size 0 based on xc(t). Idea: If m(t) constant, xc(t) alternates in sign get 0 . If m(t) ( or ) rapidly, xc(t) has the same polarity get 0.
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• Question: How does the receiver knows the time-varying step size?
• Answer: Regenerate the step-size rule. Many so-called “adaptive” schemes, for example, in adaptive video coding, rely on this type of regeneration mechanism.
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PCM
• m(t) samples (analog amplitude) quantized samples binary representation binary modulated waveform (ASK (AM), PSK (PM), FSK (FM) )
• Main advantages of digital communication
– more reliable communication
• Main disadvantages of digital communication
– wide BW (reduced by “compression”)
complicated circuits (cost reduced by VLSI)15
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A rough discussion on BW
Assume the number of quantization levels 2 .
Message bandwidth .
Sampling rate 2 . 2 binary pulses/sec.
1Assume the maximum width of a pulse .
2Transmission band
nq
W
W nW
nW
width . (proportional to )
Now, the recovered message error is mainly due to quantization.
Thus, error bandwidth .
knW nW
q
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