ON THE BEHAVIOR OF THE LINC TRANSMITTER zyxwv Fernando CASADEVALL, Juan J. OLMOS Departament de Teoria del Senyal i Comunicacions Universitat Politecnica de Catalunya Apdo. 30002, 08080 Barcelona, SPAIN In this paper we present a theoretical analysis of the effect of three different kinds of imbalances between the parallel signal paths in a real LINC transmitter. The system degradations are described in terms of the undesired response rejection at the system output (URR). A classical two tones test has been used. INTRODUCTION In mobile radio systems, the relatively inefficient use of the spectrum by the present FM-type modulations, like MSK, TFM, etc zyxwvutsrqp ... has manifested itself as crowding on the available channels. However, they are still widely used because their constant envelope property is appropriate for using power efficient non linear amplifiers. In the most recent studies of digital mobile radio systems, the introduction of linear modulation methods, such as QPSK and M-QAM, is considered in order to improve spectrum efficiency. However, as M-QAM has a non-constant envelope, it will be necessary to consider the linear power amplifiers, which are less efficient that the classical class-C power amplifiers used with the FM-type modulations. In order to achieve both spectrum and power efficiency, several techniques of linearizing power amplifiers have been presented in the literature, [l], [2], [3], [41. Among all of them, one of the more promising appears to be the so called LINC transmitter. The basic principle of the LINC transmitter is to represent any arbitrary bandpass signal, which may have both amplitude and phase variations, by means of two signals which are of constant amplitude and have only phase vanations. These two angle modulated signals can be amplified separately using efficient high-power non linear devices. Finally the amplified signals are passively combined to produce an amplitude modulated signals. 29 In a practical LINC transmitter there are three main mechanisms that degrade the overall performance: the power gain and delay (or phase) imbalance between the two RF paths and the different non-linear characteristics of both RF power amplifiers. Even though some practical implementations have pointed out the effects of these degradations in the system performances, with respect to the theoretical behavior there is not yet, to our knowledge, a closed expression providing a characterization of these effects. In this paper we present a theoretical analysis of the effect of the above mentioned imbalances between the parallel signal paths in the system performance. The system degradations are described in terms of the undesired response rejection at the system output (URR). A classical two tones test has been used. SYSTEM DESCRIPTION Figure 1 shows the schematic drawing of LINC system, where: S(t)= a(t).cos[wot+ O(t)+ $1 (1) =SIl(t) cos[wot+$]+ SQl(t)sin[wot+ $1 (2) S2(t)= V/2.sin[wot+ e(t)- w(t)+ $]= = S12(t) cos[wot+ $]+ SQz( t) sin[wot+ $1 (3) max[ a(t)]IV, with: v(t)=sin-'[a(t)/~], being and: CH2846-4/90/0000-0029 $1.00 ' 1990 IEEE