Robust linear precoding for MSE minimization in MIMO broadcast systems with channel Gram matrix feedback

In this paper we deal with amultiple-input multiple-output (MIMO) multiuser broadcast channel (BC) featuring linear precoding at the transmitter. The design of such precoder is performed according to the channel state information (CSI) sent by the different users through parallel feedback links. A general feedback framework based on the quantization of each user´s channel Gram matrix is presented. Once the channel Gram matrices are available at the transmitter, they can be used to compute equivalent triangular MIMO channels for which the optimum precoder is calculated. As an application of this framework, a robust precoder design that takes into account errors in the CSI that are originated, for example, from the finite-bit quantization process applied in the feedback, is also presented. Finally the benefits of the proposed robust strategy are evaluated by means of numerical simulations.