Analog vs Digital QAM Analog QAM
Analog QAMs are used to carry multiple signals on a single carrier. It’s similar to AM (Amplitude Modulation) but with two carrier signals at the same frequency, 90 degrees out of phase.
Analog QAM transmits color information in PAL and NTSC analog video TV systems. The I (in-phase) and Q (quadrature) signals carry the components of the color information.
PAL stands for Phase Alternating Line is the video standard which is mostly used in the European and Asian countries and NTSC stands for National Television Standards Committee is the analog color television standard which is mostly used in South America and North America.
Digital QAM
Digital QAM, also known as Quantized QAM, is commonly used in radio communication systems, including cellular technology and Wi-Fi. Digital QAM supports higher data rates than both amplitude and phase modulation schemes.
In digital QAM schemes, to define the values of phase and amplitude different points can be used. This is known as a constellation diagram. Thus a constellation diagram is the set of possible message points.
QAM can be realized by using a constellation diagram. In the constellation diagram, the constellation points are arranged in a square grid with equal horizontal and vertical distance. The minimum distance between the constellation points is known as a Euclidean distance.
In the digital communications, data is usually in a binary form and it has two states 0 or 1, so the number of constellation points in the grid is usually a power of 2 i.e. 2, 4, 8, 16, 32………… the most common formats of QAM are16-QAM (24), 32-QAM (25), 64-QAM (26), 128-QAM (27) and 256-QAM (28).
The bit sequence mapping for a 16-QAM is shown below in the constellation diagram. A diagram shows that binary values associated with different positions for a 16-QAM signal. It can be seen that a continuous bit stream may be represented as a sequence and divided into four groups in each of the four quadrants.
Normally a 16-QAM is considered as the lowest order QAM because 2-QAM is considered the same as for BPSK (i.e. Binary Phase Shift Keying) and 4-QAM is the same as QPSK (i.e. Quadrature Phase Shift Keying). In addition to the error-rate performance of 8-QAM is almost the same as that of 16-QAM hence it is not widely used.
