Other quadrature amplitude modulation

When the data transmission rate is higher than the upper limit that 8-PSK can provide, QAM modulation is generally used. Because the constellation points of QAM are more dispersed and the distance between constellation points is larger than that of PSK, it can provide better transmission performance. However, the amplitudes of QAM constellation points are not exactly the same, so its demodulator needs to be able to detect the phase and amplitude correctly, unlike PSK demodulation, which only needs to detect the phase, which increases the complexity of QAM demodulator.

The relationship between bit error rate (including symbol error rate and bit error rate) and signal-to-noise ratio in digital communication measures the performance of modulation and demodulation methods. The symbols of some concepts are given below to get the expression of bit error rate in AWGN channel:

M = number of constellation points

Eb = average bit energy

Es = average symbol energy =

N0 = noise power spectral density

Pb = bit error rate

Pbc = error rate on each orthogonal carrier.

Ps = symbol error rate

Psc = symbol error rate on each orthogonal carrier.

The constellations of rectangular QAM are arranged in a rectangular grid. Because the minimum distance between rectangular QAM signals is not the largest at the same energy, its bit error rate performance is not optimal. However, considering that rectangular QAM is equivalent to the superposition of pulse amplitude modulation (PAM) on two orthogonal carriers, the modulation and demodulation of rectangular QAM is relatively simple. Although the non-rectangular QAM introduced later can achieve slightly better bit error rate performance, it is much more difficult to modulate and demodulate.

The earliest rectangular QAM is generally 16-QAM. The reason is that it is easy to see that 2-QAM and 4-QAM are actually binary phase shift keying (BPSK) and quaternary phase shift keying (QPSK), while 8-QAM has the problem of dividing the unit number into two carriers, and 8-PSK is much easier, so 8-QAM is rarely used.