Analyzing spectral regrowth of QPSK and OQPSK signals

A comparison is made between the spectral regrowth of quadrature phase shift keyed (QPSK) and offset QPSK (OQPSK) signals as they go through nonlinear amplifications. Contrary to existing approaches that assume the power amplifier input is Gaussian, our analysis is carried out without the Gaussian assumption, by using higher-order statistics. We show that it is possible to assess quantitatively whether and how much OQPSK is beneficial in reducing spectral regrowth. Simple closed form formulas are obtained when the pulse shape filter is time-limited. A particular measure of spectral broadening is also provided.