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This study proposes a novel precoding scheme for multiuser multiple-inputmultiple-output (MIMO) relay systems in the presence of imperfect channel stateinformation (CSI). The base station (BS) and the MIMO relay station (RS) areboth equipped with the same codebook of unitary matrices. According to eachelement of the codebook, we create a latent precoding matrix pair, namely a BSprecoding matrix and an RS precoding matrix. The RS precoding matrix is formedby multiplying the appropriate unitary matrix from the codebook by a powerscaling factor. Based on the given CSI and a block of transmit symbols, theoptimum precoding matrix pair, within the class of all possible latentprecoding matrix pairs derived from the various unitary matrices, is selectedby a suitable selection mechanism for transmission, which is designed tominimize the squared Euclidean distance between the pre-estimated receivedvector and the true transmit symbol vector. We develop a minimum mean squareerror (MMSE) design algorithm for the construction of the latent precodingmatrix pairs. In the proposed scheme, rather than sending the completeprocessing matrix, only the index of the unitary matrix and its power scalingfactor are sent by the BS to the RS. This significantly reduces the overhead.Simulation results show that compared to other recently reported precodingalgorithms the proposed precoding scheme is capable of providing improvedrobustness against the effects of CSI estimation errors and multiuserinterference.

Robust MMSE Precoding for Multiuser MIMO Relay Systems using Switched Relaying and Side Information