Lecture 10 Basic Modulation Techniques (VI)

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Presentation transcript:

Lecture 10 Basic Modulation Techniques (VI) Fall 2008 NCTU EE Tzu-Hsien Sang 1 1

Outlines Linear Modulation Angle Modulation Interference Feedback Demodulators Analog Pulse Modulation Delta Modulation and PCM Multiplexing 2 2

One unfinished business: the Costas loop One unfinished business: the Costas loop. It is very useful for carrier recovery in both analog and digital communication systems.

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.

Analog pulse modulation: A pulse train is used as the carrier wave 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

PAM

The “conceptual” demodulation scheme: The idea of equalizer: Anything that goes through a known distortion can be recovered.

PWM Spectrum: complicated (Fourier-Bessel spectra) Demodulation: area of “pulse.” Low-pass filtering (integration)

PPM

Delta Modulation

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.

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.

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.

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)