New results on the effects of nonlinear amplifiers on DOQPSK and π/4-DQPSK signals

One important feature of a modulation format for digital mobile radio should be the generation of an envelope, which shows only small fluctuations and therefore supports power efficient, nonlinear amplifiers. The bandwidth efficient linear modulation formats offset-QPSK (OQPSK) and π/4-QPSK do fulfil this requirement, where OQPSK has further reduced envelope fluctuations compared to π/4-QPSK. The latter modulation format is attractive and has been implemented in several current mobile radio systems, since it can be demodulated differentially, which is a simple and efficient measure for fast phase synchronisation and tracking on time selective channels. However, a new receiver structure for the differential demodulation of differentially encoded offset-QPSK (DOQPSK) signals has been presented previously. In this paper the effects of nonlinear amplifiers on both modulation formats applying differential demodulation are investigated by computer simulations. The different effects of nonlinear distortions from power amplifiers and the resulting adjacent channel interference are investigated by introducing a generic nonlinear amplifier model. With the aid of the generic model, the out-of-band powers of the two linear modulation formats are compared as well as the BER in an additional AWGN and Rayleigh fading environment