Digital Modulation Schemes ( QAM)

Quadrature Amplitude Modulation (QAM)

So in this post I shall continue with digital modulation schemes. in the last post I intentionally missed out one of the most widespread, well known and frequently used digital modulation scheme i.e Quadrature Amplitude Modulation. AM can be thought of as a combination of ASK and PSK. In QAM we either vary amplitude, or phase or both of a high frequency analog signal to send digital symbol. In QAM each digital symbol is assigned a combination of an amplitude and a phase from the available set. the combination set depends on the number of bits that we wish to transmit per symbol. lets say I wih to transmit 3 bits per symbol then /i shall have 2^3= 8 different combination ( 2 amplitudes {A1,A2} and four phases {0,90,180 and 270} ). such a QAM will be called 8-QAM. And if I want to transmit 4 bits per symbol I can have 16 different combinations which may include 2 Amplitudes and 8 phases.

QAM symbols are usually shown on a constellation map. now the question arises what is constellation map. A constellation map is used to express/ show the exact assignment of data bits to specific phase and amplitude changes on a two dimensional plane.

8QAM constellation map and symbol mapping table are shown in the figures below

the figure below shows the constellation map for 16-QAM

Practically in a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave.  

Individually each of these signals can be represented as:

I = A cos(φ) and Q = A sin(φ).

In the above shown constellation diagram, In-phase or I signal is mapped to y-plane and Qudrature or Q-signal is mapped to x-plane. 

The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information.

Advantages of QAM

QAM uses the signal space more efficiently which results in increased data rate, lowers the bit rate, and enhances system efficiency. Thus QAM is mostly used in systems that are designed to transmit high data rate at low energy level.