Hi there,
I would say it requires linear amplification because QPSK contains information in the amplitude, not only because IQ trajectory crosses zero…
GSM/EDGE standard 3pi/8-PSK modulation does not cross zero, yet it does not use saturated PA. Crossing zero sets tougher requirements on linearity performance of PA.
Besides EVM, another difficult parameter to meet in the design of a linear PA is the ACPR spec (power in adjacent channels). Although the use of pre-distortion (analog or DPD) can somehow help, the most common way to get ACP spec is to optimize PA by sacrificing PAE performance (therefore class AB…)
Polar Modulation allows to use a saturated PA to transmit a linear modulation by shifting the linearity problem to the Drain/Collector modulator of the PA...
Mosieur_Oiso