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Presented By:-
Prachir Beriwal
08EC000325
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MODULATION
We need to transform the signal into a formsuitable for transmission through a givencommunication channel.
Modulation may be defined as the process bywhich some characteristics of a signal calledcarrier is varied in accordance with theinstantaneous value of another signal calledmodulating signal.
Characteristics of carrier varied are Amplitude,frequency, phase and polarization.
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Need for Modulation
Modulation is needed for:
Practicality of antenna.
To remove interference. Reduction of noise.
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Optical Communication
Fiber-optic communication is a method oftransmitting information from one place to anotherby sending light through an optical fiber.
The main benefits of fiber are :
Exceptionally low loss allowing long distancescommunication between amplifiers or repeaters.
Inherently high data-carrying capacity, such thatthousands of electrical links would be required toreplace a single high bandwidth fiber.
No crosstalk.
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Modulation formats
In digital communications, the modulatingsignal consists of Binary data.
This data is used to modulate carrierwave(usually sinusoidal) with fixed
frequency. The channel may be telephone channel,
microwave radio link, satellite channel oroptical fiber.
The modulation process involves switchingor keying the amplitude, frequency, phaseor polarization of the carrier in accordancewith the input data.
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The modulation formats of
optical communication are:
1. Amplitude Shift Keying (ASK).
2. Frequency Shift Keying (FSK).
3. Phase Shift Keying (PSK).
4. Polarization Shift Keying (PolSK).
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Amplitude Shift
Keying(ASK) Also known as ON-Off Keying (OOK).
Modulate the intensity of the carrier signal.
Carrier is switched between On and Off
states depending on the input binary
sequence.
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Amplitude Shift Keying
Digitalinformation
Carrier wave
ASK
modulated
signal
Carrier present Carrier absent
Amplitude varying-
frequency constant
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Advantage:
The ASK-based modulation formats arecharacterized by simple signalgeneration and detection, due to whichall currently deployed optical
transmission systems employ ASK-basedmodulation formats.
DISADVANTAGE:
It has not got a constant envelope. Thismakes its processing (e.g., poweramplification) more difficult.
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Frequency shift keying(FSK)
It is realized by switching the laser light
frequency between two frequency
values.
Carrier frequency is switched depending on
the input binary sequence.
If binary data=0; low frequency
binary data=1; high frequency
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Frequency shift keying
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Advantages:
Frequency shift keying has a better signalto noise ratio than on off keying.
High Data Rates/Long Range.
Excellent Immunity to Harsh Environments.
DISADVANTAGE:
The optical signal envelope remainsunchanged and the complexity of signalgeneration and detection increasescompared to ASK modulation.
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Phase shift keying(PSK)
It uses the phase of the signal to
encode information.
Phase of the carrier is switched
depending on the input binary
sequence.
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Advantages:
Widely used these days within a wholeraft of radio communications systemsmore efficiently than FSK.
Improved receiver sensitivity than ASK.
DISADVANTAGES:
Noise interference is more. Higher probability of error.
Regeneration of carrier is difficult.
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Polarization shift keying(PolSK)
It is the most exotic modulation
format.
The optical PolSK signals are
generated by switching the signal
polarization between two orthogonal
states of polarization.
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Advantages:
The PolSK is characterized by a Constant signalenvelope enabling an improved nonlinear tolerance,an improved sensitivity compared to ASK-basedmodulation.
Enable a better utilization of the system
bandwidth by the use of orthogonal polarizationas an additional degree of freedom.
DISADVANTAGES:
Increased complexity of signal generation and
detection, as well as the sensitivity to polarization.
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Modulators
1. Internal Modulators(On-Off keying)
2. External Modulators
i. Electro-Optic Modulator(Phase
Modulator)
ii. Mach-Zehnder modulator(Amplitude
Modulator)
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