TopTechnical DictionaryQuadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation (QAM)

QAM (Quadrature Amplitude Modulation) is a method of combining two of the simplest digital modulations: amplitude-shift keying (ASK) and phase-shift keying (PSK). This encoding mode involves simultaneous change in carrier signal amplitude and phase. The available number of binary bits per symbol is 2n. If n = 4 (24 = 16), the modulation is referred to as 16-QAM. If n = 6 (26 = 64), the modulation is 64-QAM. The higher the n value, the more data is “packed” in the channel; it increases network capacity and allows higher data transmission rates (this feature is used in LTE networks).

 

16-QAM modulation 

16-QAM constellation diagram is represented by 16 constellation points (24 = 16) giving 16 possible combinations of binary modulated (input) signal values.

 

Fig. 1. 16-QAM constellation diagram

 

64-QAM modulation 

64-QAM constellation diagram is represented by 64 constellation points (26 = 64) corresponding to 64 possible combinations of binary modulated signal values.

 

Fig. 2. 64-QAM constellation diagram

 

The main purpose of the constellation diagrams in DVB-T network maintenance is the ability to identify the mode and location of network failures.

 

QAM features high spectral efficiency (compared to PSK, FSK and related modulations) which means that high data transmission rates can be reached for a low-bandwidth channel provided that its SNR (Signal to Noise Ratio) - corresponding to the signal quality - remains at the high level.

 

Table 1. Comparison of theoretical spectral efficiencies of different modulations

 

Modulation type Theoretical spectral efficiency
[bits/Hz/s]
BPSK 1
QPSK 2
8PSK 3
16-QAM 4
32-QAM 5
64-QAM 6
256-QAM 8

The theoretical spectral efficiency of 16-QAM is 4 b/Hz/s, whereas the theoretical spectral efficiency of 64-QAM is 6 b/Hz/s. The higher the number of bits per second, the better the spectral efficiency.

 

Table 2. Spectral efficiency

 

n 2 4 8 16 32
n-QAM [bps/Hz] - - - 3,1 4
n-PSK [bps/Hz] 0,5 1 1,5 2 2,5
n-FSK [bps/Hz] 1 1 0,75 0,5 0,3

The spectral efficiency n n-QAM is determined by the transmission conditions. At lower n values (e.g. 4-QAM) the modulation is used at the edge of the broadcasting station range and shows low data transmission rates. At medium n values (e.g. 16-QAM) the transmission rates are average and the modulation is used in areas with moderate range conditions. 64-QAM or 256-QAM show high transmission rates at shorter distances from the broadcasting station.

 

The features discussed above are used in LTE networks. 64-QAM and 256-QAM are used in those networks for receiving data, and modulations up to 64-QAM are used for sending data. At high spectral efficiencies, both the network capacity and data transmission rates are higher (provided that good radio communication conditions are maintained), however, at a cost of higher probability of signal corruption (interference).

 

QAM applications: 

DVB-T/DVB-T2, Wi-Fi, LTE