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Bandpass ModulationBandpass Modulation
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WhyModulate?WhyModulate?Digital modulation is the process by which digital
symbols are transformed into waveforms that are
compatible with the characteristics of the channel
Modulation shifts the spectrum of a baseband signal tosome high frequency
In bandpass modulation the shaped pulses modulatea sinusoid called a carrier wave, or simply a carrier
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ExampleExample In radio transmission carrier is converted to an EM field
for propagation
Size of antenna depends upon wavelength
=c/f
c= 3x108 m/s , f=3000 Hz For cellular telephones, antennas are typically /4 in size
= 3x108 / 3000 = 100000
/4=2.5x104 m or 15 miles
15 miles span antenna is required without carrier wavemodulation
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But if the baseband information is first modulated on
higher frequency carrier e.g. 900 MHz carrier, diameter
of antenna would be about 8cm
=3x108 / 900x106 = 1/3 or 0.33
/4=0.333/4=8 cm
15 miles
8 cm
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Bandpass ModulationBandpass Modulation Bandpass modulation is the process by which an
information signal is transformed into a sinusoidal
waveform
Sinusoid has three features:
i. Amplitude
ii. Phase
iii. Frequency
Process by which the amplitude, phase, frequency of anRF carrier, or a combination of them, is varied in
accordance with the information to be transmitted
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WaveformWaveform The signal has the form:
s(t) =A(t) cos (t) =A(t) cos[ot + (t)]
The average power is
= =A2
/2 The average power of the signal may also be expressed
as the energy in timeT divided byT
A2/2= E/T
orA= (2E/T)1/2
Therefore the signal can be expressed as:
s(t) = (2E/T)1/2 cos[ot + (t)]
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Basic Digital Communication TransformationBasic Digital Communication Transformation
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BandpassBandpass Modulation & DeModulation & De--ModulationModulation
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Coherent DetectionCoherent Detection
The receiver uses the phase of the carrier todetect the signal
The receiver has a prototype of each of thesignals in the symbol and uses these replicasto correlate with the incoming signal
The receiver is phase locked to the incoming
signal so that these prototype waveformsduplicate all characteristics including phaseof the transmitted signal set
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NonNon--coherent Detectioncoherent Detection The receiver does not use such phase reference
information
The demodulators are designed to operate withoutknowledge of the phase reference of the signal
It results in reduced complexity because it does not do
phase estimation, but it also results in increaseprobability of error Pe
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Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)
ASK is a form of modulation that represents digital data
variations in the amplitude of a carrier wave
For BASK (also called ON-OFF Keying (OOK)), one
and zero are represented by two amplitude levels
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Phase Shift Keying (PSK)Phase Shift Keying (PSK) PSK is a digital modulation scheme that conveys data by
changing, or modulating, the phase of a reference signal
or the carrier wave
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FrequencyShift Keying (FSK)FrequencyShift Keying (FSK) FSK is a frequency modulation scheme in which digital
information is transmitted through discrete frequency
changes of a carrier wave
FSK conveys the data using distinct carrier frequencies
to represent symbol states