Imperfect orthogonal multiplexing in the forward link of CDMA networks

The Universal Mobile Telecommunications System (UMTS) will comprise both a terrestrial part (T-UMTS) and a satellite part (S-UMTS). The air interface will include code division multiple access as one of the operating modes, w ith direct sequence spreading based on scrambling and channelization codes. The latter are used for discriminating among different users/services, and play a fundamental role in the overall system performance. We consider Ort hogonal Variable Spreading Factor codes for channelization, and we focus ou r attention on the for-ward link from the base station to the mobile termin al. Our aims are to describe analytically the transmitted multiplex signal passed through a high power amplifier (inevitable in S-UMTS), before and af ter the non-linear amplification, and to analyze the performance loss which derives from imperfect orthogonality induced by the non-linear device. To this end, we introduce a semi-analytic procedure based on the notion of cha nnelization code power ratio, and we validate it through computer simulatio n. The major finding in our numerical results is that due to the orthogonal ity loss, it is impossible to fully exploit the forward link power control dynamics, with consequent capacity loss. (C) 2001 Elsevier Science B.V. All rights reserved.