Hybrid Modulation

What is Modulation
Modulation is a technique used to send information by modifying the characteristics of a basic electromagnetic signal. The basic signal is called the carrier signal.
The characteristics of a signal are amplitude, frequency and phase.
There are two types of modulations as:
·         Analog Modulation
1.      Frequency Modulation  (FM)
2.      Amplitude Modulation (AM)
3.      Phase Modulation (PM)
·         Digital Modulation
1.      Frequency Shift Keying (FSK)
2.      Amplitude Shift Keying (ASK)
3.      Phase Shift Keying (PSK)


What is Hybrid Modulation?
Hybrid modulation is a combination of ASK and PSK. This method of modulation is called Amplitude Phase Shift Keying (APSK) or Quadrature Amplitude Modulation (QAM). It generates by quadrature components with two independent carrier signals with same frequency and different amplitude which are transmitted simultaneously in same medium and phase by shifting 90 degrees with respect to the other. This is mainly used in as a modulation scheme for digital telecommunication signal and in wireless standards. Some examples are microwave digital radio, DVB-C (Digital Video Broadcasting – Cable) and modems.

Amplitude Shift Keying (ASK)
Amplitude Shift Key is a form of amplitude modulation that represents digital data as variation in the amplitude of a carrier wave. In an ASK system, the binary symbol 1 is represented by transmitting fixed amplitude carrier wave and fixed frequency for a bit duration of t seconds.


Phase Shift Keying (PSK)
Phase Shift Keying is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data.


Ø  The PSK has different versions,
·         BPSK (Bipolar Phase Shift Keying)                    
·         QPSK (Quadrature Phase Shift Keying)
·         8PSK
·         16PSK


QAM is be being used in optical fiber systems as bit rates increase; QAM16 and QAM64 can be optically emulated with a 3-path.

Diagrams of Hybrid Modulation
Encode a big bit stream:
                                001010100011101000011110
Break it up into 3-bit triads:
                                001-010-100-011-101-000-011-110




Advantages and Disadvantages of QAM
v  Advantages
1.      Allows for more data to be transmitted over roughly the same bandwidth as simple AM.
2.      It increases the efficiency of transmission.
v  Disadvantages
1.      Lower levels of noises needed to move the signal to a different decision point and this noise can be problem with QAM.


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