modulation (1): amplitude modulation
TRANSCRIPT
Modulation (1): Amplitude Modulation
Luiz DaSilva Professor of Telecommunications [email protected] +353-1-8963660
What is modulation and what is it for? q Something we do to an information signal to
make it suitable for transmission
q The process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted
q It typically involves translating a baseband message signal to a bandpass signal at frequencies that are very high compared to the baseband frequency
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Types of modulation q Modulation can be performed by varying the
Ø Amplitude Ø Phase Ø Frequency of a high frequency carrier in accordance with the amplitude of the message signal
q Demodulation is the inverse operation: extracting the baseband message from the carrier so that it may be processed at the receiver
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Analog and digital modulation q Analog Modulation
Ø Continuous signal Ø Used in first generation mobile wireless systems such as AMPS (Advanced Mobile Phone System) in USA
q Digital Modulation Ø Time sequence of discrete-valued symbols or pulses Ø Used in current and future mobile radio systems, WiFi, etc.
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Amplitude modulation: characteristics q Changes the amplitude of the carrier signal
according to the amplitude of the message signal q All info is carried in the amplitude of the carrier q There is a linear relationship between the
received signal quality and received signal power q AM systems usually occupy less bandwidth than
FM systems q AM carrier signal has a time-varying envelope
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AM signal The amplitude of the carrier signal is varied according to the instantaneous amplitude of the modulating message signal m(t) Carrier signal: AM signal:
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Ac cos(2π fct)
AMModulatorm(t) sAM(t)
sAM (t) = (Ac +m(t))cos(2π fct)
AM signal (cont’d) We can also express sAM(t) as where g(t) = Ac + m(t) g(t) is called the envelope of the AM signal
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sAM (t) = Re{g(t)ej2π fct}
Modulation and demodulation
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Modulator Demodulator
Baseband Signal with frequency
fm (Modulating Signal)
Bandpass Signal with frequency
fc (Modulated Signal)
Wireless Channel
Original Signal with frequency
fm
Source Sink
fc>>fm
AM example Message signal: m(t) = 2 + 2 cos(t) Carrier signal: 4 cos(10t)
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-20
-15
-10
-5
0
5
10
15
20
0 2 4 6 8 10 12 14
AM Double Side Band (DSB) The unmodulated carrier is described as: where
q Ac = peak amplitude of the carrier
q ωc = carrier (angular) frequency
q θ = carrier phase at t=0 (we usually set it to 0)
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c(t) = Ac cos(ωct +θ )
AM DSB example in frequency domain
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-W W 0
0 fc -fc
LOWER SIDEBAND
UPPER SIDEBAND
CARRIER PRESENT
δ(f-fc)δ(f+fc)
modulated signal
message signal M(f)
AM power Consider a single tone modulating wave Power in carrier: Power in upper and lower sidebands:
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m(t) = Am cos(ωmt +θ )