Closed-loop pre-equalization for wireless MC-CDMA systems under QoS requirements

This paper addresses the dynamic pre-equalization problem in the downlink and uplink of MC-CDMA systems by following a distributed feedback perspective, and including explicit quality-of-service (QoS) restrictions. The proposals for the downlink and uplink are synthesized taking into account implementation considerations. The QoS requirements for each active user are translated into a prescribed signal to interference-noise ratio (SINR) after detection at the receiver. In the downlink, an independent pre-equalization factor per subcarrier is suggested which induces diversity in the problem formulation, where a simple matched-filter at the mobile units is assumed for signal detection at the mobile units (MU). In this formulation, the pre-equalization factors that minimize the transmission power and achieve the objective SINR can be accomplished. Meanwhile, in the uplink, a uniform pre-equalization scaling for all subcarriers is proposed, where now multiuser detectors are considered at the base station (BS) in order to achieve the objective SINR's. Moreover, a general closed-loop control structure is addressed which could simultaneously consider our proposals in the downlink and uplink. This framework 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. An extensive simulation evaluation is presented which shows that the QoS requirements are always satisfied for different load conditions in the MC-CDMA system.