2.5.3 Modulation

2.5.3 Modulation:

            Modulation is the process of encoding information from a message source in a manner suitable for transmission. It is generally involves translating a baseband message signal (called the source) to a bandpass signal at frequencies that are very high when compared to the base band frequency.

            The bandpass signal is called the modulated signal, and the baseband message signal is called the modulating signal. Modulation may be done by varying the amplitude, phase or frequency of a high frequency carrier in accordance with the amplitude of the message signal.

            Demodulation is the process of extracting the baseband message from the carrier so that it may be processed by the intended receiver.

2.5.3.1 Analog modulation methods:

1-Amplitude modulation (AM): The amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.

2-Frequency modulation (FM):The frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.

3-Phase modulation (PM):The phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.

2.5.3.2 Digital modulation methods:

1-Amplitude-shift keying (ASK):

 The most common ASK form is On-off keying (OOK).

2-Frequency-shift keying (FSK):

• Audio frequency-shift keying (AFSK).
• Multi-frequency shift keying (M-ary FSK or MFSK).
• Dual-tone multi-frequency (DTMF).
• Continuous-phase frequency-shift keying (CPFSK).

3-Phase-shift keying (PSK):

• Binary PSK (BPSK), using M=2 symbols.
• Quadrature PSK (QPSK), using M=4 symbols.
• 8PSK, using M=8 symbols.
• 16PSK, using M=16 symbols.
• Differential PSK (DPSK).
• Differential QPSK (DQPSK).
• Offset QPSK (OQPSK).