PSK and QAM Modulation Schemes

Daniel Weibel

Created 5 Feb 2016

Illustration of the different types of Phase-Shift Keying Modulation (PSK) and Quadrature Amplitude Modulation (QAM) schemes.

PSK Modulation Schemes

The distinct states of the output signal are achieved by varying the phase of the carrier signal.

Binary PSK (BPSK)

Other names:
- 2-PSK
2 distinct phases $\rightarrow$ 2 symbols $\rightarrow$ 1 bit/symbol

Quadrature PSK (QPSK)

Other names:
- 4-PSK
- 4-QAM
4 distinct phases $\rightarrow$ 4 symbols $\rightarrow$ 2 bits/symbol
4-PSK and 4-QAM are based on different concepts, however, they result in the same modulated signal.
Used in LTE

8-PSK

8 distinct phases $\rightarrow$ 8 symbols $\rightarrow$ 3 bits/symbol
Highest-order PSK modulation scheme that is used in practice. In e.g. 16-PSK, with 16 distinct phases, the error rate would be too high. If a higher bits/symbol rate is needed, then another modulation scheme (such as QAM) is used.

QAM Modulation Schemes

Combination of phase-shift keying (PSK) and amplitude-shift keying (ASK). The distinct states of the ouptut signal are achieved by the combinations of a number of distinct phases and a number of distinct amplitudes.

4-QAM

4 phases $\times$ 1 amplitude $\rightarrow$ 4 symbols $\rightarrow$ 2 bits/symbol
In effect, identical to QPSK

16-QAM

E.g.: 8 phases $\times$ 2 amplitudes $\rightarrow$ 16 symbols $\rightarrow$ 4 bits/symbol
Other combinations of phases and amplitudes are possible. There must be at least 16 combinations to accommodate the 16 symbols.
Used in LTE

64-QAM

E.g.: 16 phases $\times$ 4 amplitudes $\rightarrow$ 64 symbols $\rightarrow$ 6 bits/symbol
Other combinations of phases and amplitudes are possible. There must be at least 64 combinations to accommodate the 64 symbols.
Used in LTE

256-QAM

E.g.: 32 phases $\times$ 8 amplitudes $\rightarrow$ 256 symbols $\rightarrow$ 8 bits/symbol
Other combinations of phases and amplitudes are possible. There must be at least 256 combinations to accommodate the 256 symbols.