New interpath interference model for DS-CDMA indoor transmissions with distributed antennas

Abstract

In channels where the delay spread is smaller than the chip interval, spread spectrum signals do not give rise to path diversity. In this situation, distributed antennas systems may be used to provide path diversity, significantly improving the system performance. 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 indoor systems with distributed antennas. The bit error rate (BER) at the output of a practical RAKE receiver is computed using an accurate approach for 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. Comparison of the BER obtained by the new approach for the IPI with those obtained by the GA shows that the latter method incurs in considerable error when used to evaluate performance of DS-CDMA indoor systems with distributed antennas.