Performance analysis of QAM modulations applied to the LINC transmitter

Future mobile radiocommunications systems will use linear modulations because they show a higher spectrum efficiency than classical FM modulations. Furthermore, in order to use these modulations in hand-portable equipment, power efficiency is also requested for the power amplifiers. To obtain both power and spectrum efficiency, a LINC transmitter can be considered. The authors present an analysis of the effect of different types of imbalances between the parallel signal paths in a LINC transmitter. The system degradations are described in terms of adjacent channel rejection, (U_R). Classical raised cosine (Nyquist filtered) 4, 16, and 64 QAM modulation patterns are taken into account, and in all cases, upper bounds for adjacent channel rejection as function of the gain and phase imbalances as well as of the guard band between adjacent channels are presented. Moreover, the impact of these imbalances in the system performance, characterized by means of the signal-to-noise ratio (SNR) increment needed to maintain a fixed error rate, is also considered. The results show that gain and phase imbalance between both RF paths could be a serious limitation for the LINC transmitter performance