What is 16-QAM Modulation?

Figure 1: Constellation Plot for 16-QAM

16-QAM is a type of Quadrature Modulation (QAM) in which a carrier wave of a fixed frequency can exist in one of sixteen different states. This can be represented in a constellation diagram (Figure 1) where each state is a symbol that contains one of 16 different amplitude and phase levels. The constellation plot consists of two axis namely the in-phase (X-axis) and quadrature (Y-axis) where the two axes are orthogonal to each other i.e. they are separated by a phase of 90˚ from each other.

Each symbol in the constellation contains up to 4 bits (0s and 1s) in a 16-QAM scheme. Therefore, the number of combinations that are possible using 4 binary bits is 24 = 16, with each state or symbol containing a value from 0000 to 1111.

The below figure shows waveforms for four-bit symbols with each symbol containing a unique amplitude and phase level. For example, symbol 0000 and 0001 contains equal amplitude but different phase levels. Hence, they are separated in the quadrature axis. Similarly, two symbols 1001 and 1101 contain the same phase level but different amplitude levels. Therefore, they will be separated in the in-phase axis to allow distinction between the two.

Figure 2: Waveforms for all 16 combinations of symbols in 16-QAM

Using the 16-QAM technique, it is possible to modulate both the amplitude as well as the phase of the carrier signal unlike other lower order digital modulation schemes such as BPSK where only phase modulation is possible. As a result, it also supports a relatively larger amount of data to be transmitted. The maximum data rate supported by 16-QAM is 11.5 Mbps.

Thus, a high data rate with up to 4 bits per symbol modulation can be achieved. The number of bits supported by M-ary QAM is given by the following Equation. Where N represents the number of bits and M is the number of possible combinations corresponding to any given number of N bits i.e the number of constellation points.

Potential Applications

16-QAM (and 64-QAM) is generally used for digital terrestrial television that uses Digital Video Broadcasting (DVB). Systems designed to achieve a high spectral efficiency employ very dense QAM constellation schemes like 64-QAM, 256-QAM, 1024 QAM or 4096 QAM. These are used for applications such as wireless cellular and digital systems, video-on-demand, HD television and Internet Protocol (IP) cable TV broadcasting.

Click here to learn more about 64-QAM.