Interference factor evaluation for CDMA system capacity analysis

This paper deals with the correlation properties of spreading sequences used in CDMA systems. The focus of this analysis is to evaluate the correlation factors between sequences in the uplink or in the downlink. This system parameter is a key factor in the evaluation of the system capacity. Although many studies have been devoted to find out the FDD-WCDMA capacity, the interference factor between users is usually taken equal to 0.4. Nevertheless this interference factor deeply depends on the sequences we use in the system. The orthogonality is lost because of multipath interference in the downlink and of the asynchronous transmission in the uplink. We investigate the distribution of this factor for various spreading sequence families used in W-CDMA and TD-CDMA. The variance of the factor is sensitive to the channel delay distribution and to the chosen sequences using different spreading factors. This study is a first step in order to evaluate the capacity combined with resource allocation. It is also useful for evaluating the benefits of RAKE receivers or joint detection.