## p/4 QPSK

#### A recent variant of QPSK, now widely used in the majority of digital radio modems is the p/4 QPSK format, so called because the four symbol set is rotated by p/4 or 45o at every new symbol transition. The reason for this rotation is to ensure that the modulation envelope of the filtered QPSK signal never passes through zero, and hence the peak to mean ratio of the modulation is minimized. |

| #### Comparing the vector diagrams for QPSK and p/4 QPSK, this property is clearly evident. The fact that the modulation envelope does not pass through zero is extremely important for the design of radio power amplifiers. Traditionally, RF power amplifiers are extremely difficult to design with linear response extending down to zero power output, hence the favoured status of p/4 QPSK over conventional QPSK for radio applications.

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