Characterizing the orthogonality factor in WCDMA downlinks

The loss of orthogonality between the spreading codes in a wideband code-division multiple-access downlink is modeled in analysis and simulations by means of the orthogonality factor (OF). The OF, which is an important parameter limiting downlink capacity, increases with the multipath dispersion in the channel. We utilize this relationship between the OF and the channel dispersion to show a novel, simple, and accurate way to determine the time-averaged OF directly from the channel delay profile. To this end, we define the diversity factor (DF) of a channel profile, derive the expression for it, and show that the DF and the OF are related by a simple mathematical relationship. We then verify our approach over an entire ensemble of channel profiles generated by the general COST259 macrocellular channel model.