Effects of nonlinear power amplifiers on the spectrum of CPFSK signals

The nonlinear behaviour of high-power amplifiers (HPAs) constitutes a serious, conditioning performance impairment in digital radio, whenever great spectrum efficiency is required such as in modern satellite communications, terrestrial radio links or radio mobile systems. In fact, the effects of filtering followed by amplifying in transmission give rise to power spectrum spreading on account of nonlinear distortion, and consequently to an increase of adjacent channel interference. When the HPA must work at saturation in order to give out its maximum power, angle modulation systems, with constant envelope, seem to be more attractive than amplitude - and phase - modulated ones, which require linearization [1]. In this paper, the CPFSK signal with modulation pulses assumed to be rectangular, with a length ¿ less than the symbol time, T, is tested in the above-mentioned conditions. The power spectral density in the various points of the transmitter is displayed for fixed HPA characteristics (AM/AM and AM/PM conversions); its behaviour is analyzed for varying filter bandwidths and modulation parameters (duty-cycle of the modulation pulses, ¿/T, and maximum angular frequency deviation, ¿d). The results obtained, experimentally checked, show that, unlike other angle modulation systems (PSK, offset PSK, MSK), in the CPFSK case the HPA output has a power spectrum which can be efficiently controlled by means of filtering at the intermediate frequency. For this purpose, only a suitable choiche of ¿/T and ¿d is required.