2 In the DSP-SC demodulation, a receiver must generate a local
carrier in frequency and phase synchronism with the carrier used
for modulation. This call for a sophisticated receiver and could be
quit costly. The other alternative is for the transmitter to
transmit a carrier A c cos ( c t) along with the modulated signal
m(t) cos ( c t). so there is no need to generate a carrier at the
receiver and this is the idea of the standard AM.
Slide 3
Modulation 3 The modulating signal (information or baseband
signal) The carrier signal is The modulated signal
Slide 4
Modulation 4
Slide 5
Modulation case 1 5 When ( is nonnegative) for all value of t.
The envelop has the same shape as m(t). Hence, we can recover m(t)
from this envelop. At the receiver, the detection is extremely
simple and inexpensive operation, which doesnt require generation
of a local carrier for demodulation.
Slide 6
Modulation case 2 6 The condition is not satisfied. The envelop
shape is not as m(t). So, we cant recover m(t) from this envelop.
We cant build a simple receiver.
Slide 7
Modulation Index 7 Let A m the peak amplitude of m(t) and A c
is the carrier amplitude. We define modulation index as: When :
this mean that for all t and the S AM (t) (modulated signal) can be
demodulated by the envelop detector. When (overmodulation): this
mean that A + m(t) is not 0 for all t and the option of envelop
detection is no longer viable.
Slide 8
Spectrum 8 The spectrum of the modulated signal S AM (t) is the
same as that of m(t) cos ( c t) plus tow additional impulses at + c
and - c. If then
Slide 9
Spectrum 9 USB LSB
Slide 10
Spectrum 10 The modulated signal spectrum centered at c is
composed of three parts: An impulse at the carrier frequency c The
upper sideband (USB), a portion that lies above c whose highest
frequency component is at c + M The lower sideband (LSB), a portion
that lies below c whose lowest frequency component is at c - M The
bandwidth of the modulated waveform is twice the information signal
bandwidth.
Slide 11
Transmission Power 11 The advantage of envelop detection in the
standard AM has its price. Recall, the transmission power and the
channel bandwidth are the two primary communication resources and
should be used efficiently. The transmission of the carrier wave
represent a waste of power. In the standard AM, only a fraction of
the total transmitted power is actually for m(t).
Slide 12
Demodulation 12 In the standard AM modulation, the modulation
index should be in order to demodulate the received modulated
signal by using an envelop detector.
Slide 13
Single-Sideband (SSB) Modulation 13
Slide 14
Introduction 14 Standard AM and DSB-SC modulations has two
sidebands: LSB and USB. These two methods waste the channel
bandwidth because they both require a transmission bandwidth equal
to twice the message bandwidth.
Slide 15
Introduction 15 Note that the USB and the LSB are symmetric
about the carrier frequency. Hence, given the spectra of either
sideband, we can determine the other. So, the transmission of
either sideband is sufficient to reconstruct the message signal
m(t) at the receiver. Thus, the bandwidth of the transmitted signal
( modulated signal) will be the bandwidth of the modulating signal
( baseband signal)
Slide 16
Single-Sideband (SSB) Modulation 16 In single-sideband (SSB)
modulation just only one sideband is transmitted.
Slide 17
Generation of SSB Signals 17 One way to generate an SSB signal
is to: generate a DSB signal first, and then suppress one of its
sidebands by filtering ( band-pass filter designed to pass one of
the sidebands of the modulated signal)
Slide 18
Spectrum of SSB Signals 18 The filter must have sharp cutoff
characteristics to eliminate the undesired sideband
Slide 19
Demodulation 19 Demodulation of SSB signals can be achieved
easily by using the coherent detector as used in the DSB
demodulation, that is, by multiplying by a local carrier and
passing the resulting signal through a low-pass filter.
Slide 20
Demodulation 20
Slide 21
Vestigial-Sideband (VSB) modulation 21
Slide 22
Vestigial-Sideband (VSB) modulation 22 Vestigial-Sideband (VSB)
modulation is a compromise between SSB and DSB modulations. In this
modulation scheme, one sideband is passed almost completely,
whereas just a trace, or vestige, of the other sideband is
retained. The typical bandwidth required to transmit a VSB signal
is about 1.25 that of SSB. VSB is used for transmission of the
video signal in commercial television broadcasting.
Slide 23
Generation of VSB Signals 23 A VSB signal can be generated by
passing a DSB signal through a sideband-shaping filter [VSB
filter].
Slide 24
24
Slide 25
Demodulation of VSB 25 The m(t) can be recovered by synchronous
or coherent demodulation, that is, by multiplying by a local
carrier and passing the resulting signal through a low-pass
filter.