Terminology » Modulation » 16 QAM vs 64 QAM vs 256 QAM Modulation: Differences and Applications

16 QAM vs 64 QAM vs 256 QAM Modulation: Differences and Applications

qam modulation digital modulation signal amplitude phase

The full form of QAM is Quadrature Amplitude Modulation. It’s a digital modulation technique, specifically a combination of both Amplitude and Phase Modulation. QAM excels over QPSK in terms of data carrying capacity.

QAM leverages the concept of transmitting two signal frequencies – one shifted by 90 degrees relative to the other – on the same carrier.

For QAM, each carrier is ASK/PSK modulated, meaning data symbols have varying amplitudes and phases. Mathematically, this can be represented as:

$S(t)= d_1(t) \cos(2 \pi f_c t) + d_2(t) \sin(2 \pi f_c t)$

QAM Figure

The image above illustrates the constellation points and encoding rules for 16-QAM, based on IEEE standard 802.16-2004.

Let’s compare 16 QAM, 64 QAM, and 256 QAM to highlight the key differences.

16-QAM vs 64-QAM vs 256-QAM

As mentioned earlier, both phase and amplitude are varied for each symbol to represent different bits. There are generally two amplitude levels (d1 and d2) for each phase. Several variations of QAM exist, with 16-QAM, 64-QAM, and 256-QAM being the most common.

Let’s take 16-QAM as an example.

In 16-QAM, each symbol represents 4 bits. This is evident in the 16-QAM constellation diagram shown above. For instance, if the input bits are “1010,” the output might be (-3 - j*3)*KMOD. KMOD, in this case, is typically 1/ $\sqrt{10}$ for 16-QAM.

In digital modulation, the baseband signal is separated into in-phase (I) and quadrature-phase (Q) components. The combination of I and Q forms the baseband modulating signal, often represented in an IQ diagram.

The constellation diagram visually represents all possible modulated symbols used by the modulation technique to map information bits. These symbols are depicted on the complex plane, showing their amplitude and phase information.

64-QAM: Bits per symbol are 6. Each symbol is represented by 6 bits.
256-QAM: Each symbol is represented by 8 bits.

As the level increases, QAM becomes more bandwidth-efficient. However, it demands more sophisticated algorithms at the receiver to accurately decode the complex symbols back into bits. Therefore, 256-QAM is inherently more complex than 16-QAM.

QAM offers better bandwidth efficiency compared to BPSK, but it is less robust. Hence, QAM is preferred in systems with better Carrier to Interference and Noise Ratio (CINR), resulting in higher data rates. Conversely, BPSK is employed when the CINR is poor.

Tabular Difference Between 16-QAM, 64-QAM and 256-QAM

The table below summarizes the differences between 16-QAM, 64-QAM, and 256-QAM modulation techniques. The purpose of KMOD here is to achieve the same average power for all the mapped symbols (i.e. average power of 1).