Non-linear effects on orthogonal multiplexing for the forward link of CDMA networks

The Universal Mobile Telecommunication 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 (CDMA) as one of the operating modes, with 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 the orthogonal variable spreading factor (OVSF) codes for channelization, and we focus our attention on the forward link from the base station to the mobile terminal. Our aims are to describe analytically the transmitted multiplexed signal passed through a HPA (inevitable in S-UMTS), before and after 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 CCPR (channelization code power ratio), and we validate it through computer simulation. The major finding in our numerical results is that due to the orthogonality loss, it is impossible to fully exploit the forward link power control dynamics, with consequent capacity loss.