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On the Link-Level Performance of MIMO Systems in Line-of-Sight

Multiple-input multiple-output (MIMO) systems have recently appeared as candidates for future high-bandwidth wireless communications. Extensive research in that area has shown that MIMO systems offer significant capacity enhancements, especially in non line-of-sight (nLOS) environments, where rich scattering is present. In environments with a strong LOS signal, the capacity enhancement is thought to be limited due to correlation in that signal. In this paper, we investigate the effect of the LOS signal on the link-level performance of MIMO systems employing zero-forcing (ZF) or maximum-likelihood (ML) receivers. In detail, we derive expressions that relate the positioning of the antenna elements with the bit-error rate (BER). The validity of these expressions is confirmed by a link-level Monte-Carlo simulator. From these results we demonstrate the existence of optimal MIMO structures that can offer perfectly orthogonal signal transmission. Finally, a set of simple criteria for achieving the maximum performance in 2 x 2 MIMO systems is derived in terms of the separation distance, the inter-element spacing and the orientation of the transmit and receive arrays.