Transmitter Diversity and Multiuser Precoding for Rayleigh Fading Code Division Multiple Access Channels

Abstract

Transmitter diversity in the downlink for Code Division Multiple Access (CDMA) systems provides a means to achieve similar performance gains as for the mobile station (MS) receiver diversity without the complexity of a MS receiver antenna array. Transmitter based methods enable to shift signal processing to the transmitter where power and computational complexity are more abundant, thus simplifying receiver units. We examine feasibility of several transmitter diversity techniques for the Wideband CDMA (W-CDMA) systems. We also investigate an optimal method to combine transmitter diversity and precoding that achieves the gain of maximum ratio combining of all space and frequency diversity branches.

Under severe channel conditions (i.e. multipath), the multiple access interference (MAI) becomes the major source of performance degradation for direct sequence CDMA (DS/CDMA) systems. This is because of the loss of orthogonality between the spreading codes used by each user due to the multipath channel effects. To overcome this problem, many receiver based multiuser detection (MUD) techniques have been proposed. These techniques demand high computational complexity, power and knowledge of spreading codes of all users. As a result, in the downlink of a CDMA system it is not feasible to employ such methods at the MS. Alternatively, transmitter based techniques were proposed to shift computational complexity and power consumption to the BS, where they can be afforded. It was shown that these methods are very effective in removing the MAI. Although these methods are powerful, they are high in complexity and require the accurate knowledge of the channel. since MAI cancellation filters need to be updated continuously as fading coefficients vary. We propose a less complex method with similar performance improvements. In the proposed method, the functions of multipath combining and MAI cancellation are separated. Thus the MAI cancellation matrix does not depend on rapidly time-varying fading coefficients. Transmitter diversity and multiuser precoding can be combined to further improve the performance. Multiuser precoding preserves the multipath diversity while removing the MAI. Extending multiuser precoding to multiple antennas results in space diversity in addition to multipath diversity.

Both transmitter diversity and multiuser precoding require the knowledge of the channel state information (CSI). The CSI can be estimated at the receiver and sent to the transmitter via a feedback channel. To enable the studied adaptive techniques for practical systems, we employ the long range prediction (LRP) algorithm, which characterizes the fading channel using the autoregressive (AR) model and computes the Minimum Mean Squared Error (MMSE) estimate of a future fading coefficient sample based on a number of past observations.

Numerical, simulation and theoretical results are presented to show that transmitter diversity and multiuser precoding can be used to remove MAI and achieve frequency and space diversity through multipath channels and multiple antennas.