Improved bit error probability estimation for DS-CDMA downlink equalizer-based receivers with small spreading factors

This paper provides insight into the statistical distribution of the interference at the output of receivers composed of a chip-rate linear transverse equalizer followed by a despreader for the downlink of direct-sequence code-division multiple access (DS-CDMA) systems with long scrambling sequences. It is shown that for small spreading factors, the probability density function (PDF) of the interference samples can be modeled accurately as a mixture of Gaussian PDFs. Each component of the mixture is associated with a particular realization of the scrambling sequence spanning the spread symbol to be retrieved. Based on this study, an analytical method allowing the computation of the bit error probability (BEP) for small spreading factors is derived. Numerical simulations show that the BEP estimation achieved by the proposed method is far more accurate than that obtained via the traditional Gaussian assumption on the interference statistics.