Pre-rake Diversity with Generalized Orthogonal Codes and Imperfect Channel Conditions for Fdd/ds-cdma Systems

The Pre-Rake diversity combining technique decreases the complexity, size and cost of the mobile unit while achieving the same inter symbol interference (ISI) mitigation effects of conventional Rake receiver for direct sequence code division multiple access (DS-CDMA) systems. The technique is based on preprocessing of signal at the transmitter relying on knowledge of the channel state information. This a priori information is available in time division duplex (TDD) mode due to channel reciprocity. In frequency division duplex (FDD) mode the channel has to be estimated at mobile unit (MU) for the downlink and fed to base station to predict the channel for downlink time slot. In this paper, we compare the performance of Pre-Rake system with different spreading codes. We will also propose a method for channel prediction in FDD mode and evaluate the system under ideal and predicted channel conditions using generalized orthogonal (GO) and Walsh codes through analytical and computer simulations for DS-CDMA downlink.