Multielement system capacities are usually thought of as limited only by correlations between elements. It is shown here that degenerate channel phenomena called "keyholes" may arise under realistic assumptions which have zero correlation between the entries of the channel matrix H and yet only a single degree of freedom. Canonical physical examples of keyholes are presented. For outdoor environments, it is shown that roof edge diffraction is perceived as a "keyhole" by a vertical base array that may be avoided by employing instead a horizontal base array.

Abstract—The analysis of the multiple-antenna capacity in the high- regime has hitherto focused on the high- slope (or maximum multiplexing gain), which quantifies the multiplicative increase as a function of the number of antennas. This traditional characterization is unable to assess the impact of prominent channel features since, for a majority of channels, the slope equals the minimum of the number of transmit and receive antennas. Furthermore, a characterization based solely on the slope captures only the scaling but it has no notion of the power required for a certain capacity. This paper advocates a more refined characterization whereby, as a function of �f, the high- capacity is expanded as an affine function where the impact of channel features such as antenna correlation, unfaded components, etc., resides in the zero-order term or power offset. The power offset, for which we find insightful closed-form expressions, is shown to play a chief role for levels of practical interest. Index Terms—Antenna correlation, channel capacity, coherent communication, fading channels, high- analysis, multiantenna arrays, Ricean channels.

For the analysis and design of multielement antenna systems in mobile fading channels, we need a model for the space-time cross correlation among the links of the multiple-input multipleoutput (MIMO) channel. In this paper we propose a general space-time cross correlation function for narrowband Rayleigh fading MIMO channels, where various parameters of interest such as angle spreads at the base station and the user, the distance between the base station and the user, mean directions of the signal arrivals, array configurations, and Doppler spread are all taken into account. The new space-time cross correlation function includes all the relevant parameters of the MIMO narrowband Rayleigh fading channel in a clean compact form, suitable for both simulation and mathematical analysis. It also covers many known correlation models as special cases. We demonstrate the utility of the new space-time correlation model by clarifying the limitations of a widely-accepted correlation model for MIMO fading channels.

This paper applies random matrix theory to obtain analytical characterizations of the capacity of correlated multiantenna channels. The analysis is not restricted to the popular separable correlation model, but rather it embraces a more general representation that subsumes most of the channel models that have been treated in the literature. For arbitrary signal-to-noise ratios @ A, the characterization is conducted in the regime of large numbers of antennas. For the low- and high- regions, in turn, we uncover compact capacity expansions that are valid for arbitrary numbers of antennas and that shed insight on how antenna correlation impacts the tradeoffs among power, bandwidth, and rate.

Abstract — We characterize the capacity-achieving input covariance for multi-antenna channels known instantaneously at the receiver and in distribution at the transmitter. Our characterization, valid for arbitrary numbers of antennas, encompasses both the eigenvectors and the eigenvalues. The eigenvectors are found for zero-mean channels with arbitrary fading profiles and a wide range of correlation and keyhole structures. For the eigenvalues, in turn, we present necessary and sufficient conditions as well as an iterative algorithm that exhibits remarkable properties: universal applicability, robustness and rapid convergence. In addition, we identify channel structures for which an isotropic input achieves capacity. Index Terms — Capacity, MIMO, input optimization, fading, antenna correlation, Ricean fading, keyhole channel.

This paper characterizes the capacity-achieving input covariance for multiantenna channels with (zero-mean) arbitrary distribution. The solution accommodates a wide range of correlation structures, not necessarily separate transmitreceive. Our characterization of the covariance encompasses both its eigenvectors, which are found explicitly, and its eigenvalues, for which we present necessary and sufficient conditions as well as an iterative algorithm. In addition, we identify the correlation structures for which an isotropic input achieves capacity. 1

In this paper we present multiple antenna transmitter optimization schemes that are based on linear transformations and transmit power optimization, while keeping the average transmit power conserved. We consider the downlink of a wireless system with multiple transmit antennas at the base station and a number of mobile terminals (i.e., users) each with a single receive antenna. We consider the maximum achievable sum data rates in the case of (1) zero-forcing spatial pre-filter and (2) modified zero-forcing spatial pre-filter. Using a multiple input single output (MISO) channel model with temporal and spatial correlations, we study the effect of delayed channel state information (CSI) on these schemes. It is seen that as the CSI delay increases, spatially uncorrelated channels perform worse than spatially correlated channels, which is in contrast to the case of zero delay CSI.

Abstract — This paper presents analytical expressions for the capacity of single-user multi-antenna channels, known instantaneously by both transmitter and receiver, at moderate and high signal-to-noise ratios. Fading channels with both uncorrelated and correlated antennas are encompassed. The characterization is conducted primarily in the limit of large numbers of antennas, with accompanying examples that illustrate the validity of the results for even small numbers thereof. In the absence of correlation, the capacity is also tightly bounded for fixed numbers of antennas with compact closed-form expressions. I.