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

For digital communication in general:

• Amplitude Shift Keying (ASK) is very inefficient and susceptible to interference for wireless (but can be useful over fiber optics or copper).
• Frequncy Shift Keying (FSK) is less susceptible to errors and interference than ASK because interference is often confined to a specific frequency.
• Phase Shift Keying (PSK) is the least susceptible to error of these three. In terms of bit error rate (BER), binary PSK is about twice as good as ASK and FSK in the presence of noise, and the BER can be significantly improved through other techniques.
• Orthogonal frequency-division multipleing (OFDM) can also be used. It is very spectrally efficient.

For analog communication in general:

• Amplitude Modulation (AM) is efficient in terms of bandwidth and has simple receiver circuit but subjected to amplitude variations due to noise and interference
• Frequency Modulation (FM) is less susceptible to interference and noises. But has complex circuits and requires large bandwidth.
• Phase Modulation (PM) is not affected by channel noise but on the other hand it has complex circuit as compare to AM and FM.

For in-depth detail of various modulation schemes you can refer to this document.

Demodulation

Demodulation is technique to obtain message signal from the receive signal. Modulation and demodulation go hand in hand. Following block diagram gives a general idea:

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