Digital Signal Modulations

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In general modulation is defined as a process of varying one or more properties of a signal that carries information. We basically have two types of signals,ie analog signals and digital signals.
Analog Signals are continuous time varying signals where as digital signals are discrete time signals.
fig: Analog and digital signals
In olden days Analog signals were used in for transmitting information from one place to another.But it has many disadvantages like information loss and high noise while transmitting, even after demodulation the complete transmitted information could not be retrieved. So there has been a shit from analog to digital signals. with digital signals there are many advantages and because of this we see all our electronic devices relay only on digital signals and not on analog.

The digital signals are transmitted using various modulation techniques according to the requirements.In modulation the major components are Signal that carries information, modulator,transmitting medium  and demodulator.

Common modulation methods are:
  • Amplitude modulation (AM), in which the height (i.e., the strength or intensity) of the signal carrier is varied to represent the data being added to the signal.
  • Frequency modulation (FM), in which the frequency of the carrier waveform is varied to reflect the frequency of the data.
  • Phase modulation (PM), in which the frequency of the carrier waveform is varied to reflect changes in the frequency of the data (similar but not the same as FM).
  • Polarization modulation, in which the angle of rotation of an optical carrier signal is varied to reflect transmitted data.
  • Pulse-code modulation in which an analog signal is sampled to derive a data stream that is used to modulate a digital carrier signal.
The main aim of modulation today is to squeeze as much data into the least amount of spectrum possible. That objective, known as spectral efficiency, measures how quickly data can be transmitted in an assigned bandwidth. The unit of measurement is bits per second per Hz (b/s/Hz). Multiple techniques have emerged to achieve and improve spectral efficiency.

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