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Linearly and quadratically constrained blind multiuser detection for quasisynchronous MC-CDMA uplink

We consider the problem of blind (i.e., without training sequences) mitigation of multiple-access interference in the uplink of quasisynchronous multicarrier code-division multiple-access (MC-CDMA) systems. We propose to improve the performance of the recently proposed linearly-constrained two-stage detector by adding a quadratic constraint in the first stage synthesis, which exploits the knowledge of the spreading codes of the active users within the cell of interest. It turns out that introduction of such a constraint improves the robustness of the two-stage receiver against errors in the estimated statistics of the received data, while it slightly reduces its interference suppression capabilities.