Abstract-In this paper, we investigate spatial multiplexing at millimeter (mm) wave carrier frequencies for short-range indoor applications by quantifying fundamental limits in line-of-sight (LOS) environments and then investigating performance in the presence of multipath and LOS blockage. Our contributions are summarized as follows. For linear arrays with constrained form factor, an asymptotic analysis based on the properties of prolate spheroidal wave functions shows that a sparse array producing a spatially uncorrelated channel matrix effectively provides the maximum number of spatial degrees of freedom in a LOS environment, although substantial beamforming gains can be obtained by using denser arrays. This motivates our proposed mm-wave MIMO architecture, which utilizes arrays of subarrays to provide both directivity and spatial multiplexing gains. System performance is evaluated in a simulated indoor environment using a ray-tracing model that incorporates multipath effects and potential LOS blockage. Eigenmode transmission with waterfilling power allocation serves as a performance benchmark, and is compared to the simpler scheme of beamsteering transmission with MMSE reception and a fixed signal constellation. Our numerical results provide insight into the spatial variations of attainable capacity within a room, and the combinations of beamsteering and spatial multiplexing used in different scenarios.Index Terms-Millimeter wave communication, MIMO, channel capacity, antenna arrays.

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Abstract-The large amounts of bandwidth available in the millimeter (mm) wave band enable multiGigabit wireless networks with applications ranging from indoor multimedia networking to outdoor backhaul for picocellular networks. Carrier wavelengths in this band are an order of magnitude smaller than those for existing cellular and WiFi systems, resulting in a drastically different propagation geometry. Omnidirectional transmission is essentially infeasible because of the increased propagation loss at smaller wavelengths; on the other hand, highly directive transmission and reception with electronically steerable beams can be achieved using compact antenna arrays. Thus, in contrast to the rich scattering environment at lower carrier frequencies, a small number of paths are dominant for directional mm wave links. The small wavelength also implies that spatial multiplexing gains can be obtained even in Line of Sight (LoS), or more generally, sparse scattering, environments with antennas with moderate separation. In this paper, we examine the consequences of these observations for two scenarios. The first is a lamppost-based outdoor deployment, where we model fading due to ground and wall reflections, and examine MIMO techniques for combating fading. The second is for spatial multiplexing for an indoor link, where we model the number of eigenmodes as a function of form factor, and examine the effect of blockage. I. INTRODUCTIONMillimeter wave communication corresponds to the next big leap in wireless technology. The large amounts of unlicensed and semi-unlicensed spectrum in this band (especially the 7 GHz of unlicensed bandwidth in the 60 GHz band), together with the development of low-cost silicon realizations of mm wave radio frequency integrated circuits, imply that commercially viable multiGigabit wireless technology is now within reach. The carrier wavelengths in this band are an order of magnitude smaller than those in existing cellular and WiFi networks. As a result, the propagation and interference characteristics are drastically different from our current experience in wireless network design, and demand new models and design approaches, both at the physical and higher layers. Our purpose in this paper is to highlight how mm wave Multiple Input Multiple Output (MIMO) channels differ from their counterparts at lower carrier frequencies. To this end, we investigate channel models for two scenarios. The first is for a typical link in a lamppost-based outdoor deployment (e.g., for a mesh backhaul), where we model fading due to ground and wall reflections, and examine MIMO techniques for combating fading. The second is for spatial multiplexing for an indoor link (e.g., for streaming high-definition television from a settop box to a television set), where we model the number of eigenmodes as a function of form factor, and examine the effect of blockage.

We report first experimental results from a hardware prototype of a millimeter wave lineof-sight (LOS) 2x2 MIMO link. The proposed architecture uses antenna element spacing derived from the principles of diffraction limited optics to establish multiple parallel data channels. Operation at millimeter wave carrier frequencies reduces the antenna array size to reasonable dimensions. The proposed system architecture is scalable to larger one dimensional and two dimensional arrays supporting data rates >160Gbps. This paper presents the design and characterization of a hardware prototype 2x2 LOS MIMO link operating at 1.2Gbps.

Spatial multiplexing for millimeter (mm) wave line of sight (LOS) links potentially enables data rates of the order of 10-100 Gbps. Most prior work in this area has focused on uniform transmit and receive arrays, for which it is known that the spatial responses seen by different transmitters can be made orthogonal by choosing the antenna spacing appropriately as a function of range and wavelength. In this paper, we show that variations in range can cause significant degradation in performance for such uniformly spaced arrays optimized for a given range, due to the appearance of high correlations between the spatial responses for different transmitters (and hence rank deficiency in the MIMO channel matrix) as a function of range. We then demonstrate that optimized nonuniform arrays alleviate this problem by keeping correlations between spatial responses small over a significantly larger set of ranges than is possible with uniform spacing.