4 NOVEL QUADRATURE OVERLAPPING MODULATIONS AND THEIR SPECTRAL EFFICIENCY ANALYSIS OVER BAND-LIMITED NONLINEAR CHANNELS

The power spectral density (PSD) analysis considering non-linearity is important in studying spectral efficiency of a digital modem for sate llite systems. A general closed formula for numerical PSD calculation of quadrature overlapping modulations is derived in this paper. Four n ew quadrature overlapping modulation schemes are proposed because they have better spectral characteristics than those of conventional modul ations, such as offset QPSK, MSK and staggered QORC, in both linear an d nonlinear channels. The four new schemes are; overlapped minimum shi ft keying (OMSK), minimum shift keying triangular cosine (MSKTC), rais ed cosine triangular cosine (RCTC) and sinusoidal quadrature overlappe d triangular cosine (SQOTC). The results show that for these new modul ations, PSD side lobe regeneration caused by the channel non-linearity , is much less than that for the OQPSK, MSK and staggered QORC, a desi rable feature enabling even tighter frequency band allocation in satel lite communication systems. (C) 1997 by John Wiley & Sons, Ltd.