signals and systems -...

Spring 2012

Signals and Systems

Chapter SS-8Communication Systems

ShoushuiWeiSDU-BME

Sep08 – Dec08

Figures and images used in these lecture notes are adopted from“Signals & Systems” by Alan V. Oppenheim and Alan S. Willsky, 1997

SDUBME-SS8-Comm-2ShoushuiWei©2012

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Introduction

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SDUBME-SS8-Comm-3ShoushuiWei©2012Introduction

Modulation & Demodulation:– M: Embedding an information-bearing signal into a second signal

– D: Extracting the information-bearing signal

– Methods:

> Amplitude Modulation (AM)

> Frequency Modulation (FM)

Multiplexing & Demultiplexing:– Simultaneous transmission of more than one signal

with overlapping spectra over the same channel

– Methods:

> Time-Division Multiplexing (TDM)

> Frequency-Division Multiplexing (FDM)

SDUBME-SS8-Comm-4ShoushuiWei©2012Outline

Complex Exponential & Sinusoidal

Amplitude Modulation & Demodulation

Frequency-Division Multiplexing

Single-Sideband Sinusoidal Amplitude Modulation

Amplitude Modulation with a Pulse-Train Carrier

Pulse-Amplitude Modulation

Sinusoidal Frequency Modulation

Discrete-Time Modulation

SDUBME-SS8-Comm-5ShoushuiWei©2012Complex Exponential & Sinusoidal Amplitude Modulation & Demodulation

Signal Frequency Characteristics:

f (Hz)

200 4K 300M 300G100M 40G

Microwave LinkCommunication Satellite

Voice Signals

802.11 & Bluetooth

2.4G


AM with a Complex Exponential Carrier:


AM with Sinusoidal Carriers:


AM with a Sinusoidal Carrier:

SDUBME-SS8-Comm-11

ShoushuiWei©2012Complex Exponential & Sinusoidal Amplitude Modulation & Demodulation

AM with a Sinusoidal Carrier:

SDUBME-SS8-Comm-12ShoushuiWei©2012Complex Exponential & Sinusoidal Amplitude Modulation

Overlapping of AM with a Sinusoidal Carrier:


Not Synchronized in Phase:

SDUBME-SS8-Comm-14


Not Synchronized in Phase:


Asynchronous Demodulation:

– In audio transmission

over a RF channel

> wM: 15 - 20 Hz

> wc/2π: 500kHz – 2 MHz

Envelope Detector:

SDUBME-SS8-Comm-16


Asynchronous Demodulation:


Synchronous & Asynchronous Demodulation:










SDUBME-SS8-Comm-19ShoushuiWei©2012Frequency-Division Multiplexing (FDM)

FDM Using Sinusoidal AM:

SDUBME-SS8-Comm-20ShoushuiWei©2012Frequency-Division Multiplexing (FDM)

Demultiplexing and Demodulation:

SDUBME-SS8-Comm-23ShoushuiWei©2012Single-Sideband Sinusoidal Amplitude Modulation

SSB Modulation:


Retain Upper Sidebands Using Ideal Highpass Filter


Retain Lower Sidebands Using Phase-Shift Network

• Retain Upper Sidebands










SDUBME-SS8-Comm-27ShoushuiWei©2012Amplitude Modulation with a Pulse-Train Carrier

Modulation of a Pulse-Train Carrier:

SDUBME-SS8-Comm-28ShoushuiWei©2012Amplitude Modulation with a Pulse-Train Carrier

Time-Division Multiplexing (TDM):










SDUBME-SS8-Comm-30ShoushuiWei©2012Pulse-Amplitude Modulation

Pulse-Amplitude Modulated Signals:

TDM-PAM:


Intersymbol Interference in PAM Systems:


Avoiding Intersymbol Interference in PAM Systems:


General Form of Band-Limited Pulses with Time-Domain Zero-Crossing at kT1, k ∈ Z:










SDUBME-SS8-Comm-35ShoushuiWei©2012Sinusoidal Frequency Modulation

Frequency Modulation (FM):

• The modulating signals is used to

control the frequency of a sinusoidal carrier

• With sinusoidal AM, the peak amplitude of the envelope of the carrier

is directly dependent on the amplitude of the modulating signal x(t),

which can have a large dynamic range.

• With FM, the envelope of the carrier is constant

• An FM transmitter can always operate at peak power and

amplitude variations introduced over a transmission channel

due to additive disturbances or fading can be eliminated

at the receiver

• FM generally requires greater bandwidth than does sinusoidal AM


Angle Modulation:

• Phase Modulation:

– Use the modulating signal x(t) to vary the phase θc

• Frequency Modulation:

– Use the modulating signal x(t) to vary the derivative of the angle


Phase & Frequency Modulation:


Instantaneous Frequency:

• If y(t) is truly sinusoidal:

• Phase Modulation:

• Frequency Modulation:


Narrowband FM:

• Frequency Modulation with

• Instantaneous Frequency:


Narrowband FM:

• Modulation Index for FM:

• Which m is small ⇒ narrowband FM


Narrowband FM:

• When m is sufficiently small (<< π/2)


Narrowband FM:

Approximate spectrum for narrowband FM

Narrowband FM AM-Double Sideband/with carrier


Wideband FM:

• When m is large

Periodic signals with fundamental frequency ωm


Magnitude of Spectrum of Wideband FM:


Periodic Square-Wave Modulating Signal:


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