111 Lecture 5 Basic Modulation Techniques (I) Principles of Communications Fall 2008 NCTU EE Tzu-Hsien Sang

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

3 To modulate or not to modulate, that is the question… Recall the simple digital modulation for transmitting 0s and 1s with fiber optics. In some aspects, digital modulation is much simpler than analog modulation. Modulator alters a carrier’s shape so that the alteration the intended message (information). Modulation, historically, is done on the RF carrier wave. Thus the conversion from message signals to RF signals is also called modulation.

4 Linear Modulatoin General form: The envelope A(t) has a one-to-one correspondence with the message m(t). Example I: DSB (Double SideBand) Suppressed Carrier (SC)

5 Coherent (Synchronous) Demodulator (Detector): The receiver knows exactly the phase and frequency of the received signal.

6 What if the receiver reference is not coherent? ---- A phase error case (  (t), unknown, random, time-varying, …)

7 Let’s synch up! Carrier recovery: Regenerate the carrier at the receiver ( f c and  (t) ) Example: Square circuit

8 What are we expecting? It will become much clearer if we look in the frequency domain. FT of x r 2 (t):

9 Side Show: The importance of synchronization In communications, many things heavily depend on the synchronization between the Rx and the Tx. There are many usages of the word synchronization. So be careful … Example: The French composer Pierre Schaeffer ( ) had done probably the first tele- synchronized act of art in 1944 to celebrate the liberation of Paris.

10 Example: Global Positioning System: Use the time differences of received signals from satellites to determine the location. Q: Can you do a coarse estimation of what it takes to have a 10m precision? The speed of light: The modulation scheme

11 Example II: Amplitude Modulation (AM), it’s simply DSB with carrier

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13 Demodulation Coherent detection: precise but requires carrier recovery circuit. Incoherent detection: envelope detection Simple receiver (LPF) but requires sufficient carrier power (a > W. ( In theory, f c > W is ok, but a “good” LPF is needed.) Think about the advantages and disadvantages of analog signal processing (against DSP).

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16 This is the way to figure out the power…

17 How much power is for the message?

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19 Single Sideband (SSB) modulation: A good example of signal manipulation Method one of generation of SSB signals: Sideband filtering, easy to understand, but difficult to implement. Why?

20 Method 2: Phase-shift modulation: