1. 1 11 Lecture 11 Basic Modulation Techniques (V) Fall 2008 NCTU EE Tzu-Hsien Sang

2. 2 22 Outlines Linear Modulation Angle Modulation Interference Feedback Demodulators Analog Pulse Modulation Delta Modulation and PCM Multiplexing 2

3. 3 A number of data sources share the same communication medium. Q: How to avoid conflicts? (Independent channels) Frequency-Division Multiplexing (FDM): several message signals are translated, using modulation, to different spectral locations and added to from a composite signal.

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5. 5 Example: Stereophonic FM

6. 6 Quadrature Multiplexing (QM) Recall that cos and sin are orthogonal. Quadrature-carrier multiplexing: transmit two signals on the same carrier frequency. In digital communications, a similar technique: Quadrature Amplitude Modulation (QAM)

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8. 8 Time-Division Multiplexing (TDM) Each message signal occupies a small time slot in every T s second.

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10. 10 Example: Digital telephony system

11. 11 Comparison of multiplexing schemes FDM: simple to implement, inter-modulation distortion (crosstalk) due to nonlinear channels TDM: less crosstalk (in memoryless channels), difficult to keep synchronization (frame structure, header), “digital” (sampled) signals QM: efficient use of channel, crosstalk between I & Q channels (needs coherent demodulation)