Feedback Pre-equalization in the Downlink for a Wireless MC-CDMA System

This paper addresses the dynamic preequalization problem in the downlink of MC-CDMA systems by following a distributed feedback perspective, and including explicit quality-of-service (QoS) restrictions. The proposal for the downlink is synthesized taking into account implementation considerations. The QoS requirement for each active user is translated into a prescribed signal to interference-noise ratio (SINR) after detection at the receiver. An independent pre-equalization factor per subcarrier is assumed which induce diversity in the problem formulation. In addition, a simple matched-filter at the mobile units (MUs) is assumed for signal detection. In this formulation, the pre-equalization factors that minimize the transmission power and achieve the objective SINR can be accomplished. A general closed-loop control structure is addressed that allows to use distributed control algorithm previously proposed for power control in CDMA systems. Also, stability conditions are analytically deduced to guarantee convergence of the iterative algorithms. A simulation evaluation is presented which shows that the QoS requirements are always satisfied for different loads conditions in the MC-CDMA system.