Quadrature amplitude modulation

QAM

1. Quadrature Amplitude Modulation Aanchal Kumari

2. What is QAM ? Quadrature Amplitude Modulation, QAM utilizes both amplitude and phase components to provide a form of modulation that is able to provide high levels of spectrum usage efficiency .

3. QAM is a signal in which two carriers shifted in phase by 90 degrees (i.e. sine and cosine) are modulated and combined . As a result of their 90° phase difference they are in quadrature and this gives rise to the name. Often one signal is called the In-phase or “I” signal, and the other is the quadrature or “Q” signal . The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations . It may also be considered as a mixture of amplitude and phase modulation .

4. 16 QAM: Basic signals exhibit only two positions which allow the transfer of either a 0 or 1 . In QAM there are many different points that can be used, each having defined values of phase and amplitude. This is known as a constellation diagram. The different positions are assigned different values, and in this way a single is able to transfer data at a much higher rate.

5. The constellation points are typically arranged in a square grid with equal horizontal and vertical spacing. Although data is binary the most common forms of QAM, although not all, are where there constellation can form a square with the number of points equal to a power of 2 i.e. 4, 16, 64 . . . . , i.e. 16QAM, 64QAM, etc.

6. ADVANTAGES: It increase the efficiency of transmission for radio. Communications systems by utilizing both amplitude and phase variations.

7. DISADVANTAGES: More susceptible to noise because the states are closer together. Level of noise is needed to move the signal to a different decision point. As QAM uses amplitude component of signal to represent binary data, linearity need to be maintained and hence linear amplifier is needed which consumes more power.

8. APPLICATIONS: QAM is used extensively as a modulation scheme for digital telecommunication systems, such as in 802.11 Wi-Fi standards . 64 and 256 QAM are often used in digital cable television and cable modem applications. Variants of QAM are used for many wireless and cellular technology applications.

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