An Improved Approach for Performance Evaluation of the Downlink of DS-CDMA PCS Indoor Systems with Distributed Antennas

Abstract

This paper studies the performance of the forward link of a DS-CDMA system with distributed antennas over indoor radio channels. The interference component caused by the same cell-site users, or interpath interference (IPI), is a dominant factor in the performance of these systems. In their analysis it is typically assumed that the probability density function (pdf) of the IPI can be approximated by the Gaussian distribution. This is called the Gaussian approximation (GA). We show that the GA for the IPI does not provide an accurate characterization of the performance of the downlink of DS-CDMA PCS indoor systems with distributed antennas. The bit error rate (BER) at the output of a practical RAKE receiver is computed using an accurate description for the pdf of the IPI. We show that when the IPI density is thus modeled, the signal to noise ratio (SNR) at the output of the combiner is well approximated by a random variable with a Weibull distribution. We also propose an analytical expression for the frame error rate (FER) of convolutional codes (CC) over slowly fading channels. Results based both on the analytical model and computer simulations for cases involving different numbers of antenna elements and different levels of interference from other cells are presented. The analytical model is shown to be highly efficient in terms of computational cost while preserving excellent accuracy. Comparison of the FER obtained by the new approach for the IPI with those obtained by the GA show that the latter method incurs considerable error when used to evaluate performance of DS-CDMA PCS indoor systems with distributed antennas.KeywordsCDMAPCSconvolutional codesdiversity combining