Modulation and Demodulation

Modulation is very important step in the transmission of signal. Our message signal is generally a low frequency signal and path loss of signal is proportional to the square of the wavelength (and hence inversely proportional to the square of the frequency).
So, we have to find a method to devise a way to encode our message signal to higher frequency signal. We can tinker with different properties of carrier signal like frequency, phase, amplitude to encode our message signal on carrier signal.

Reasons for Modulation

• Modulation of signal is important to prevent mixing of message signals from different transmitters and makes it easier to tune to the desired signal.
• Without modulation, size of the antenna will become very large and it will make antenna size in order of km for message signal of kHz order.
• For reliable and effective communication over long distance i.e. minimizing the signal losses during transmission.
• As signal band is limited (from few MHz to maybe some GHz) we have to use as low bandwidth of signal as low as possible. (i.e. maximizing the use of bandwidth allocated)
  

How to decide type of Modulation?

Basically selection of modulation technique comes from your requirements and constraints. There are many parameters that we have to look into, some are mentioned below:

• Transmission Rate required/ Spectral efficiency
• Acceptable Bit error rate (BER)
• Hardware availability / Complexity at reception side (if we want most people to receive our signal, we'll use a popular modulation, like Continuous Wave)
• Bandwidth available

Some Pros and Cons of General Modulation Schemes

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