Channel-dependent adaptation scheme for spreading codes in DS-CDMA

Abstract

DS-CDMA is currently the most widely used multiple-access scheme in mobile communications. It uses spreading sequences to separate different links, e.g., the base-station to user links in the downlink or the user to base-station links in the uplink. These spreading codes are usually allocated when the link is set up and kept fixed for the duration of the transmission. Spreading sequences are designed to have good cross-correlation properties in order to avoid interference and to be easy to generate in order to reduce the implementation complexity. In addition, they are designed to have good autocorrelation properties or equivalently a flat spectrum since this guarantees the anti-fading capability of DS-CDMA links. This paper presents a new design paradigm where spreading codes are not kept fixed anymore but are adapted to channel variations. It will be shown that this scheme can significantly improve the DS-CDMA link performance. The resulting implementation problems and complexity will be discussed.