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).
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16-QAM constellation diagram is represented by 16 constellation points (24 = 16) giving 16 possible combinations of binary modulated (input) signal values.
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Fig. 1. 16-QAM constellation diagram
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64-QAM constellation diagram is represented by 64 constellation points (26 = 64) corresponding to 64 possible combinations of binary modulated signal values.
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Fig. 2. 64-QAM constellation diagram
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The main purpose of the constellation diagrams in DVB-T network maintenance is the ability to identify the mode and location of network failures.
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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.
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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 |
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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.
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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 |
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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.
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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).
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