Combined Multiuser Detection and Diversity Reception for Wireless CDMA Systems

Multipath fading is a major constraint on the performance of wireless code division multiple-access (CDMA) systems, with multipath propagation worsening the effects of multiple-access interference, and fading on propagation paths leading to the near-far problem. Multiuser detection, exploiting knowledge of other users to cancel multiple-access interference, has the capability of eliminating the near-far problem and providing a significant capacity increase in CDMA systems. On the other hand, diversity techniques effectively combat the fading effects of the channel. This paper investigates multiuser receivers that combine explicit antenna diversity, RAKE multipath diversity, and multipath decorrelat- ing detection. Both coherent reception with maximal-ratio combining and differentially coherent reception with equal-gain combining are analyzed. The results demonstrate a significant increase in up-link capacity over the conventional RAKE receiver, at the expense of complexity. In the case of limited receiver complexity, where the number of correlators is less than the number of resolvable paths at the RAKE front-end, antenna diversity is shown to be effective in reducing residual multiple-access interference.