A four-phase and an eight-phase signal constellation are shown in the figure below.
Assuming high $\text{SNR}$ and that all signals are equally probable, the additional average transmitted signal energy required by the $8$-$\text{PSK}$ signal to achieve the same error probability as the $4$-$\text{PSK}$ signal is
- $11.90 \mathrm{~dB}$
- $8.73 \mathrm{~dB}$
- $6.79 \mathrm{~dB}$
- $5.33 \mathrm{~dB}$