1. 8. Communication Systems
8.1 Complex Exponential and Sinusoidal
Amplitude Modulation
Modulating system model:
x(t)
c(t)
y(t)=x(t)c(t)
x(t) --- modulating signal
c(t) --- Carrier signal

2. 8.1.1 Amplitude Modulation with Complex Exponential Carrier
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8.1.1 Amplitude Modulation with Complex
Exponential Carrier
(1) Modulation Theory
Exponential carrier:
For convenience, let c=0, so
Output signal(modulated signal):

3. 8 Communication systems

4. 8 Communication systems
(1) Implementation

5. 8.1.2 Amplitude Modulation with Sinusoidal signal
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8.1.2 Amplitude Modulation with Sinusoidal signal
For convenience, choose c=0, so

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7. 8 Communication systems

8. 8.2 Demodulation for Sinusoidal AM
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8.2 Demodulation for Sinusoidal AM
8.2.1 Synchronous demodulation
(1) Demodulation process

9. In time domain: In frequency domain: Expected signal:
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In time domain:
In frequency domain:
Expected signal:

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11. (2) Synchronous problem
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(2) Synchronous problem

12. The output of lowpass filter:
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Time domain:
The output of lowpass filter:
Ideal output: x(t)
When , it is referred to as synchronous
demodulation.

13. 8.2.2 Asynchronous demodulation
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8.2.2 Asynchronous demodulation
Amplitude-modulated signal:

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15. 8 Communication systems
Problem: